ORIGINAL PAPER

Thyroid hormones and deiodinase activities in plasma and tissuesfrom East Greenland polar bears (Ursus maritimus) during winterseason

Kristin Møller Gabrielsen1• Julie Stene Krokstad1

• Maria-Jesus Obregon2•

Gro Dehli Villanger1,4• Christian Sonne3

• Rune Dietz3• Bjørn Munro Jenssen1

Received: 30 June 2014 / Revised: 6 April 2015 / Accepted: 6 April 2015

� Springer-Verlag Berlin Heidelberg 2015

Abstract Thyroid hormones (THs) are essential for

metabolism and thermoregulation in arctic animals. Still,

there is a lack of deeper basic knowledge regarding the

regulation and functioning of THs in the environmental

physiology of polar bears (Ursus maritimus). This is nec-

essary in order to understand the true impact and conse-

quences of the combination of stressors such as depletion

of sea ice and endocrine-disrupting contaminants for the

polar bear species. As a first step to gain insight into TH

physiology in polar bears, TH concentrations in liver,

kidney and muscle in East Greenland polar bears sampled

February–March 2011 were analysed and their associations

with circulating levels of THs were investigated. In addi-

tion, type 1 deiodinase (D1) activities in liver, kidney and

muscle and type 2 deiodinase (D2) activities in muscle

were analysed. Concentrations of 3,5,30,50-tetraiodothy-ronine (T4) were highest in plasma, followed by liver,

kidney and muscle, whereas concentrations of 3,5,30-tri-iodothyronine (T3) were highest in kidney followed by

liver, plasma and muscle. D1 activities in the tissues varied

in the order liver[ kidney � muscle, while D2 activity

was only analysed in muscle. There were significant posi-

tive relationships between T4 in plasma and liver as well as

between T4 in plasma and D1 activity in liver. This implies

that liver is an important non-thyroidal organ for deiodi-

nation of T4, thus providing T3 to the plasma pool, in polar

bears. The polar bears with the lowest body condition also

had the lowest concentrations of free T3 in plasma and D2

activities in muscle.

Keywords Polar bear � Thyroid hormones � Deiodinaseactivity � Tissue levels � East Greenland

Introduction

The polar bear (Ursus maritimus) experiences many of the

most extreme conditions mammals can be subjected to.

Unlike brown bears (U. arctos) and black bears (U. amer-

icanus), polar bears do not enter the dormant state referred

to as hibernation or winter sleep during the winter season

(Stirling 2002; Robbins et al. 2012). The exception is the

pregnant female, which enters her den in the fall and stay

there until spring, whereas other females and males actively

search for food during the winter (Stirling 2002). The long,

dark arctic winters with temperatures below-40 �C require

adequate thermoregulatory adaptations (Scholander et al.

1950a, b; Øritsland 1970; Best 1982). During the summer

periods, ice-free areas result in several months of sparse and

unpredictable availability of seals as the major prey, re-

sulting in long periods of fasting as a consequence (De-

rocher et al. 1990; Stirling 2002). To conserve energy and

heat in the Arctic during periods of low temperatures and/or

food shortage, the polar bear have evolved fine-tuned

bioenergetic adaptations (Welch et al. 2014). The polar bear

& Kristin Møller Gabrielsen

kristin.m.gabrielsen@gmail.com

& Bjørn Munro Jenssen

bmj@bio.ntnu.no

1 Department of Biology, Norwegian University of Science

and Technology, 7491 Trondheim, Norway

2 Instituto de Investigaciones Biomedicas, Centro Mixto from

CSIC-UAM, 28029 Madrid, Spain

3 Department of Bioscience, Arctic Research Centre, Aarhus

University, P.O. Box 358, 4000 Roskilde, Denmark

4 Department of Child Development and Mental Health,

Norwegian Institute of Public Health, P.O. Box 4404,

0473 Nydalen, Oslo, Norway

123

Polar Biol

DOI 10.1007/s00300-015-1694-z

is believed to be able to switch between a fasting and a

feeding metabolism, often referred to as walking hiberna-

tion (Derocher et al. 1990). The energy cost during these

periods of fasting is, however, still larger than for hiber-

nating bears, indicating a high net cost of staying active

during cold winters and ice-free conditions (Robbins et al.

2012). Recent evidence suggests changes in annual patterns

of sea ice distribution and subsequent loss of sea ice, the

primary habitat of polar bears, and thus decreased avail-

ability of seals, due to climate warming (Stirling and De-

rocher 2012). For the polar bears, this will inevitably mean

longer periods of fasting and a consequently lower body

condition.

Thyroid hormones (THs) are essential in numerous

physiological processes in mammals, including metabolism

and thermoregulation (McNabb 1992; Hulbert 2000; Silva

2006). Basal metabolic rate (BMR) and daily energy ex-

penditure in humans (Homo sapiens) have both been linked

to THs (Hulbert 2000). Studies have also shown that

hibernation or winter sleep is associated with decreased

plasma concentrations of THs in American black bears and

European brown bears (Azizi et al. 1979; Hissa et al. 1994;

Tomasi et al. 1998). Furthermore, reduced levels of THs

have been reported in fasting mammals, such as the Arctic

fox (Vulpes lagopus) (Fuglei et al. 2000; Nieminen et al.

2004). The lowered concentration of circulating THs dur-

ing hibernation and fasting is thought to conserve energy

due to a decrease in metabolic rate and increased protein

conservation (Carter et al. 1975; Cox et al. 1984; Hulbert

2000). The THs are highly conserved among vertebrates,

and their production is regulated via a negative feedback

system controlled by the hypothalamus–pituitary–thyroid

(HTP) axis (Zoeller et al. 2007). The thyroid gland syn-

thesises, stores and secretes both 3,5,30,50-tetraiodothy-ronine (thyroxine [T4]) and 3,5,30-triiodothyronine (T3)

(Hulbert 2000; Zoeller et al. 2007). Since T3 has the

highest affinity towards the nuclear TH receptors (TRs), it

is regarded as the active hormone (Cheng et al. 2010).

However, TH action can also be mediated through non-

genomic mechanisms.

Approximately 80 % of the circulating T3 is produced

by deiodination enzymes in extrathyroidal tissues, although

the percentage varies among species and physiological

conditions (Kohrle 2000; Orozco et al. 2012). The

deiodination of T4 to T3 occurs via outer (50) ring

deiodination performed by type 1 (D1) and type 2 (D2)

deiodinases (Kohrle 2000; Orozco et al. 2012). In mam-

mals, liver, kidney and thyroid gland are the primary tis-

sues where D1 is expressed and has high activity, while D2

functions in, e.g., the brain, skeletal muscle, brown adipose

tissue (BAT) and heart. Both D1 and D2 can inactivate T4

or T3 by producing, e.g., reverse T3 (rT3) or other

iodothyronines, although the major inactivator of T3 is type

3 (D3) deiodinase (Bianco et al. 2002). D1 activity in all

tissues, except for in the thyroid gland, is increased by

excessive levels of THs (hyperthyroidism) and decreased

by insufficient levels of THs (hypothyroidism), while the

opposite is true for D2 activity (Kohrle 1999). Altogether,

the deiodinases are very important for regulating plasma

and intracellular levels of THs and especially levels of T3,

thus subsequently regulating the biological action of THs

(Kohrle 1999, 2000; Bianco and Kim 2006).

In plasma, T4 and T3 are transported bound to transport

proteins and\0.01 % of T4 and 0.1 % of T3 exist as free

hormones (McNabb 1992; Zoeller et al. 2007). Transport of

THs from plasma into tissues occurs through carrier-me-

diated membrane transport systems (Hennemann et al.

2001). The transport and uptake of THs as well as available

intracellular binding sites all contribute to regulate intra-

cellular levels of THs together with deiodinases and other

enzymes involved in TH metabolism (Kohrle 1999; Hul-

bert 2000; Hennemann et al. 2001). The deiodinase en-

zymes, involved in activating and deactivating THs, have

the last few years received renewed scientific attention

(Bianco et al. 2002; Gereben et al. 2008; Arrojo e Drigo

et al. 2013; Dentice et al. 2013). This is due to their ability

to regulate tissue levels of THs in response to tissue-

specific needs, without necessarily altering circulating

levels.

Concentrations of THs in vertebrates will vary with life

history events such as development, growth and repro-

duction (McNabb 1992; Hulbert 2000; Santisteban and

Bernal 2005) and seasonally with annual events such as

moulting, hibernation and periods of low food availability

(Hissa et al. 1994; Boily 1996; Tomasi et al. 1998; Ebling

and Barrett 2008). The diversity of adaptations evolved for

life strategies in wildlife inhabiting extreme environments

gives reasons to believe that TH responses, functioning and

regulation might differ between species. In pinnipeds,

where the TH system has been widely studied, TH re-

sponses to fasting, in addition to varying widely with sex,

life history stage and breeding status as well as individual

needs for energy expenditure (Crocker et al. 2012; Kelso

et al. 2012), also show atypical response to fasting during

development. Martinez et al. (2013) described an atypical

TH response to fasting in fasting-adapted elephant seal

pups (Mirounga angustirostris) as compared to, e.g., hu-

mans and rodents. There are, thus, reasons to believe that

there is a lack of knowledge regarding the functioning and

regulation of the TH system in wildlife adapted to extreme

environmental conditions. Despite the suggested involve-

ment of THs in explaining the seasonal rhythms of food

intake and energy expenditure in mammals experiencing

alternating external environments (Ebling and Barrett

2008), the knowledge concerning the role of THs in arctic

mammals, such as the polar bear, is scarce.

Polar Biol

123

Although levels of T4 and T3 have been reported in

polar bears previously, these measurements have been

constrained to circulating levels (Leatherland and Ronald

1981; Cattet 2000; Skaare et al. 2001; Braathen et al. 2004;

Villanger et al. 2011a; Bytingsvik 2012). However, circu-

lating levels of THs are thought not to reflect intracellular

TH status and do therefore not give information about TH

status in tissues (Bianco 2013; Dentice et al. 2013). Levels

of THs and deiodinase activities in various tissues have

been reported in studies using experimental animals

(McCann et al. 1984; Morreale de Escobar et al. 1985; van

Doorn et al. 1985; Raasmaja et al. 1996; Viluksela et al.

2004; Kunisue et al. 2011; Lavado-Autric et al. 2013), but

there are to our knowledge no reports of levels of THs or

deiodinase activities in tissues of free-ranging polar bears

or other arctic wildlife species.

The polar bear species is facing numerous threats. In

addition to loss of suitable habitat for obtaining prey and

reproduction due to climate warming, polar bears also

carry high amounts of persistent organic pollutants (POPs)

(Letcher et al. 2010; McKinney et al. 2011; Stirling and

Derocher 2012; Jenssen et al. 2015). Several health effects

have been associated with high levels of these anthro-

pogenic contaminants in polar bears, including effects on

the TH concentrations (Sonne 2010; Villanger et al.

2011a). Lipophilic POPs accumulate in lipid-rich tissues

such as adipose tissue, and mobilisation of lipid stores

during periods of fasting can result in redistribution of

POPs from adipose tissue to blood and other organs that

are targets of POP-mediated toxicity (Lydersen et al.

2002; Polischuk et al. 2002). There does, however, seem

to be a lack of deeper basic knowledge regarding the

regulation and functioning of THs in the environmental

physiology of polar bears. This is necessary in order to

understand the true impact and consequences of the

combination of stressors climate warming and con-

taminant threats for the polar bear species. Thus, as a first

step to gain insight into TH physiology in polar bears, we

analysed levels of T4 and T3 in liver, kidney and muscle

in polar bears from East Greenland sampled during winter

conditions and investigated their associations with circu-

lating levels of THs measured in plasma. In addition, D1

activities in liver, kidney and muscle and D2 activity in

muscle were analysed, and associations between levels of

THs and D1 and D2 activities were investigated.

Depending on the polar bears hunting successes, the polar

bears may have experienced long periods of food depri-

vation during winter. Body condition is an important

measure for individual animal health and nutritional sta-

tus, so associations between body condition of the polar

bears and the thyroid hormone levels and deiodinase ac-

tivities were therefore also assessed.

Materials and methods

Sampling

The polar bears (N = 7) investigated in the present study

were legally harvested by local Inuit hunters in February to

March 2011(*4 weeks) as part of the annual subsistence

quota of 35 polar bears in the Scoresbysund (Ittoqqor-

toormiit) area, 70�290N 21�580W, East Greenland (Fig. 1).

The map of the sampling area was produced using ArcGIS

(ArcGIS 10.1, ERSI, Redlands, CA, USA). The two males

and five females were 2–4 years of age and defined as

sexually immature and hence classified as sub-adults

(Rosing-Asvid et al. 2002). The present study was part of

the AURORAE programme led by Prof. DSc Rune Dietz at

Aarhus University.

The animals were as quickly as possible subjected to

post-mortem necropsy and sampling (\1–3 h after death).

Sex, standard length, zoological length and circumference

were noted. Blood samples were collected primarily from

the heart using a 50-mL syringe, transferred into Vacuet-

te� heparinised tubes and then centrifuged in order to

collect plasma. The plasma samples were kept frozen at

-10 to -20 �C during the field period (*4 weeks). Tissue

samples of liver, kidney and muscle were sampled and then

stored in two different manners. Samples of 20–40 g were

taken and stored frozen at -10 to -20 �C during the field

period (*4 weeks) and then transferred to and stored at

-20 �C. Plasma and these tissue samples were shipped

frozen and stored at -20 �C until analysis. Additional

tissue samples (2–5 g) were snap-frozen during the field

period and subsequently stored and shipped in liquid ni-

trogen (-196 �C). These samples were then transferred to

and stored at -70 �C until analysis. The nitrogen-stored

tissues were the preferred tissues for analysing levels of TH

and deiodinase activity in tissues. However, snap-frozen

samples were not available for all specimens, and there-

fore, analyses of regular frozen tissue from liver/kidney

and muscle were conducted on two and three specimens,

respectively.

Analysis of thyroid hormones in plasma

Analyses of total T3 and T4 (i.e. free and protein-bound)

and free T4 and T3 in plasma (T4P, fT4P, T3P, fT3P, re-

spectively) were conducted at the Norwegian University of

Science and Technology, Trondheim, Norway, using

commercially available solid-phase 125I radioimmune as-

say (RIA) kits (Coat-A-Count Free T3; Coat-A-Count Free

T4; Coat-A-Count Total T3; Coat-A-Count Total T4, Sie-

mens Medical Solutions, CA, USA) without modifications.

Although developed for human use, commercial kits have

Polar Biol

123

been validated to be applicable to polar bear (Braathen

et al. 2004; Bytingsvik 2012) as well as other marine

mammals’ plasma (Gabrielsen et al. 2011; Villanger et al.

2011b) by running a parallelism assay of diluted samples

with the standards curve. This is due to the fact that THs

are highly conserved among vertebrate species (Zoeller

et al. 2007). The procedure was similar to that reported for

TH analysis by Gabrielsen et al. (2011). The bound ra-

dioactive antigen was quantified using a Cobra Auto-

Gamma gamma counter (Packard Instruments Company,

Dowers Grove, IL, USA). Calibration curves from analysed

calibrators were generated from the gamma counter soft-

ware (Spectra Works Spectrum Analysis Software,

Meriden, USA) and used to calculate the TH concentra-

tions in the samples. The samples were analysed in du-

plicates (T3P and fT3P) and triplicates (T4P and fT4P), and

the means were reported. Instrumental detection limits

were given for each RIA run and were 0.016 and

2.048 nmol/L for T3P and T4P, respectively, and 0.012 and

0.425 pmol/L for fT3P and fT4P, respectively. All samples

were above detection limits.

The coefficients of variation (CVs) were acceptable

(B15 %) except for two samples with CV values of 51.5 and

51.7 %. As the intra-assay control parameters were all ac-

ceptable, the high variation in these two samples was most

likely caused by the very low hormone concentrations. These

Fig. 1 Map showing the

location of the study area where

the polar bears (Ursus

maritimus) included in the study

(n = 7) were captured

Polar Biol

123

CV values were nevertheless accepted as the fT3 levels were

within physiological fT3 values previously reported in polar

bears (Braathen et al. 2004). All hormone assays were single

runs except for fT3, where results from two runs were used.

Standard reference material (SRM, Lypohchek� Im-

munoassay Plus Control, Levels 2, BIO-RAD, CA, USA),

bovine plasma and/or repeated measures of individual

samples were used to test intra- and interspecific variation.

Intra-assay variation ranged from 2.2 to 13.2 %, while inter-

assay variation ranged from 3.8 to 14.1 % (fT3 only). TH

concentrations in plasma were converted to per gram (1 mL

plasma = 1.025 g plasma) and reported in pmol/g (total

hormone) and fmol/g (free hormone) in order to compare

levels across the investigated tissues.

Analysis of thyroid hormones and deiodinase

activity in liver, kidney and muscle

Analyses of TH concentrations and D1 and D2 activities in

liver, kidney and muscle were performed at Instituto de

Investigaciones Biomedicas (IIBM), Madrid, Spain. The

high specific activity ([3000 mCi/mg) labelled iodothy-

ronines ([125I]-T4, [131I]-T4, [125I]-T3, [125I]-rT3) used in

the analyses were synthesised at the laboratory using

chloramine-T and T3, 3,5-T2 and 30,3-T2 as substrates,

respectively, as described by Weeke and Orskov (1973).

[125I]-T3 and [125I]-T4 were used for the determination of

TH concentrations in RIAs, while [131I]-T4 and [125I]-T3

were used as recovery tracers during the extraction. Fur-

ther, [125I]-rT3 was used as substrate for the determination

of D1 and D2 activities.

Thyroid hormone concentrations in liver, muscle

and kidney

Total T4 and T3 in liver (T4L, T3L), kidney (T4K, T3K) and

muscle (T4M, T3M) samples were analysed by RIAs after

extraction and purification of the tissue samples, as

originally described in Morreale de Escobar et al. (1985)

and later in Lavado-Autric et al. (2013). In short, frozen

tissue samples were homogenised in methanol, and tracer

amounts of [131I]-T4 and [125I]-T3 were added to each

sample. The extraction of the iodothyronines was done

twice using chloroform–methanol (2:1) before back-ex-

traction into an aqueous phase (Cl2Ca at 0.05 %). Bio-Rad

AG1x2 resin columns (Bio-Rad Laboratories, Hercules,

CA) were then used to purify the aqueous extracts. The

acetic extracts obtained in the last elution were evaporated

to dryness and dissolved in RIA buffer. Recovery of

[131I]-T4 and [125I]-T3 was determined in each sample.

Aliquots of the samples were then analysed by sensitive

RIAs to determine T4 and T3 concentrations. Limits of

sensitivity were 2.5 pg T4 and 0.75 pg T3 per tube.

Samples were analysed in duplicate. Calculations of T4 and

T3 concentrations were done using the amount of hor-

mones detected by RIAs, the individual recovery of the

added tracers and the weight of the extracted tissue sample.

Corrections for the amounts of THs in the blood trapped in

the tissues were not performed. Results were given as pg/g

and recalculated to pmol/g.

Determination of iodothyronine deiodinase activities

in liver, kidney and muscle tissues

The analyses of D1 and D2 activities were performed as

described in Lavado-Autric et al. (2013). Tissue ho-

mogenates were prepared in a buffer [1:30 wt/vol] con-

sisting of 0.32 M sucrose, 10 mm HEPES buffer and

dithiothreitol (DTT; 1 mM for D1 and 10 mM for D2), pH

7.0. The DTT was added on the day of the assay. All

samples were analysed in duplicate. Before the assays were

carried out, the [125I]-rT3 and/or [125I]-T4 were purified to

separate the labelled hormones from the contaminating

iodide, so that blanks were\1 % of the total radioactivity

used. This was done using paper electrophoresis on a

Whatman 3 MM paper strip in ammonium acetate (50 mM,

pH 6.8) at 500 V.

For analyses of D1 activity in liver (D1L activity) and

kidney (D1K activity), tissue homogenates were diluted

1:400 and 20–30 lg protein per tube was used for analysis.

[125I]-rT3 (100.000 cpm/tube) plus 400 nM rT3 were used

as the substrate, and DTT was added to have a final con-

centration of 2 mM DDT and a total volume of 100 lL.The reaction was run for 10 min at 37 �C before stopped

by adding a plasma/propylthiouracil (PTU)-mix (1:1),

placing the vials in an ice bath and adding 10 % tri-

chloroacetic acid (TCA). After centrifugation, the super-

natant was collected for the determination of the released

[125I]- during the deiodination reaction. The iodine was

separated using Dowex 50 W-2X columns. The iodine was

eluted with 10 % acetic acid and the eluates counted.

Equimolar amounts of iodine and 3,30,2-T2 should be

produced. In liver and kidney, most of the deiodinase ac-

tivity is PTU-sensitive (i.e. D1).

For the analysis of D1 and D2 activities in muscle (D1Mand D2M, respectively), [

125I]-rT3 (100.000 cpm/tube) plus

2 nM rT3 were used as substrates together with ±1 mM

PTU and 20 mM DTT at 37 �C for 1 h. Approximately

200 lg of protein per tube was used. The reactions were

stopped and the released iodine separated and counted in

the same manner as with D1 in liver and kidney. D1 ac-

tivity was found by subtracting the D2 activity (insensitive

to PTU, assayed by adding PTU) from the total 50-deiodinase activity (no PTU). When using [125I]-rT3 as

substrate for the deiodinase activity in muscle, D1 activity

ranged between 22 and 49 %, whereas D2 activity ranged

Polar Biol

123

between 51 and 78 % of the total 50- deiodinase activity.

Protein determination was done following the method of

Lowry et al. (1951) after precipitation of the homogenates

with 10 % TCA (to avoid interference from DTT). The

results are given as pmol/min/mg protein (D1 in liver and

kidney) and fmol/h/mg protein (D1 and D2 in muscle).

Age and body condition

Age determination was carried out by counting the ce-

mentum growth layer groups (GLG) of the third incisor

tooth (I3) or alternatively the first premolar tooth (P1) using

the methods described in Hensel and Sorensen (1980) and

Dietz et al. (1991). Body condition was calculated based on

the body condition index (BCI) by Cattet et al. (2002)

using body mass (BM; calculated according to Derocher

and Wiig 2002) and straight-line body length.

Data treatment and statistics

The present sub-adult female and male polar bears were

treated as one group. Univariate statistics were performed

using SPSS (PASW Statistics 18.0, SPSS Inc., Chicago,

USA, 2009). Differences in means between TH-related

parameters in tissues were tested with Student’s paired

samples t test when comparing between two tissues or one-

way repeated measures ANOVA if comparing across more

than two tissues. Relationships between biological vari-

ables and TH variables were explored using Pearson cor-

relation analysis. To meet the requirements of parametric

tests, all hormone and enzyme data were log10-transformed

to approximate normal distribution. The BCI data were

normal distributed and used untransformed in the correla-

tion analyses. Significance level for all tests was p B 0.05

(two-tailed).

Results

The concentrations of the TH hormones varied across

plasma and tissues (Table 1). The order of concentrations

for T4 was plasma � liver & kidney[muscle, whereas

the order of tissues with respect to T3 concentrations was

kidney & liver � plasma & muscle. Concentrations of

fT4 and fT3 were measured in plasma, with fT4P being

almost seven times higher than fT3P (Table 1). D1 ac-

tivities in the tissues varied in the order liver[ kid-

ney � muscle, while D2 activity was only analysed in

muscle (Table 1). D2 activity in muscle was approximately

two times higher than D1 activity in muscle. Significant

differences of TH variables in the matrices are shown in

Table 1.

There were significant relationships between TH vari-

ables within and across the tissues. The concentrations of

Table 1 Thyroid hormone concentrations and deiodinase activities in plasma (T4P, T3P, fT4P, fT3P), liver (T4L, T3L, D1L), muscle (T4M, T3M,

D1M, D2M, and kidney (T4K, T3K, D1K) in East Greenland sub-adult polar bears (Ursus maritimus) sampled in February–March 2011 (N = 7)

Plasma Liver Kidney Muscle

T4 (pmol/g) Mean ± SD 13.0 ± 3.6a 3.62 ± 1.64b 3.27 ± 0.28b 0.99 ± 0.18c

Min–Max 10.1–20.2 1.18–6.03 2.83–3.56 0.75–1.15

T3 (pmol/g) Mean ± SD 1.86 ± 0.50a 7.43 ± 1.72b 8.02 ± 1.6b 1.58 ± 0.33a

Min–Max 1.28–2.58 5.02–9.25 5.29–10.4 1.29–2.14

fT4 (fmol/g) Mean ± SD 7.22 ± 1.78* NA NA NA

Min–Max 4.20–9.23 NA NA NA

fT3 (fmol/g) Mean ± SD 1.10 ± 0.98* NA NA NA

Min–Max 0.18–2.78 NA NA NA

D1-act�) (pmol/min/mgprot) Mean ± SD NA 66.9 ± 23.7** 30.2 ± 15.5** 15.9 ± 9.56***

Min–Max NA 35–95.8 5.92–43.5 5.04–32.3

D2-act (fmol/h/mgprot) Mean ± SD NA NA NA 29.3 ± 18***

Min–Max NA NA NA 16.3–67.9

NA not analysed

Different letters (a–c) denote significant differences for T4 and T3, respectively, between tissues (p B 0.05, ANOVA repeated measures test,

log10-data, df = 6)

* Significant differences in fT4 and fT3 (p B 0.05, Students’ paired t test)

** Significant differences in D1 activity between liver and kidney (p B 0.05, Students’ paired t test)

*** Significant differences between D1 and D2 activity in muscle (p B 0.05, Students’ paired t test)�) D1M is expressed as fmols/h/mgprot

Polar Biol

123

T4 and fT4 were significantly higher than T3 and fT3 in

plasma, respectively (p[ 0.001, Student’s paired t test),

whereas T3 was significantly higher than T4 in liver, kid-

ney and muscle (p\ 0.016, Student’s paired t test). T4Pand T3P were positively correlated with each other

(r = 0.757, p = 0.049), and T4P was also positively cor-

related with T4L (r = 0.787, p = 0.036; Fig. 2a). T4K was

positively correlated with T3K (r = 0.798, p = 0.031;

Fig. 2c), while fT3P was positively correlated with both

T4K (r = 0.914, p = 0.004) and T3K (r = 0.817,

p = 0.025; Fig. 2d). T4L and D1L activity levels were also

positively correlated (r = 0.756, p = 0.049; Fig. 2b).

Furthermore, there were positive correlations between age

and T4M (r = 0.779, p = 0.039), and BCI was positively

correlated with fT3P (r = 0.779, p = 0.039; Fig. 3a, c) and

D2M activity (r = 0.774, r = 0.041; Fig. 3b).

Discussion

Circulating and tissue levels of thyroid hormones

The concentrations of T4P and T3P reported in this study

were within ranges previously reported in whole blood and

plasma from East Greenland sub-adult polar bears (Vil-

langer et al. 2011a) as well as from Svalbard and Canadian

adult bears (Cattet 2000; Skaare et al. 2001; Braathen et al.

2004). Concentrations of fT4P and fT3P were also within

the ranges reported for Svalbard polar bears in Braathen

et al. (2004). The levels of circulating THs vary along life,

and in marine mammals such as polar bears, age, sex,

season, nutritional and reproductive state are all important

factors affecting levels of THs (St. Aubin 2001; Ebling and

Barrett 2008).

Fig. 2 Positive correlations between T4 levels (pmol/g) in plasma

and liver (a), T4 levels (pmol/g) in liver and D1 activity (pmol/min/

mg protein) (b), T4 levels (pmol/g) in kidney and T3 levels (pmol/g)

in kidney (c) and T3 levels in kidney (pmol/g) and fT3 levels in

plasma (fmol/g) (d) of sub-adult polar bears (Ursus maritimus)

sampled in East Greenland February–March 2011 (N = 7). The r and

p values are presented in the plots. Significance level p B 0.05

Polar Biol

123

Few studies have analysed various tissue concentrations

of THs in large mammals. There are, however, reports of

TH levels in various tissues of the rat (Rattus norvegicus)

(van Doorn et al. 1985; Lavado-Autric et al. 2013). Tissue

levels of T4 in the present polar bears were lower than

reported for T4 in the corresponding tissues in the rat (van

Doorn et al. 1985; Lavado-Autric et al. 2013). The patterns

observed in the present polar bears, which were liv-

er & kidney[muscle and kidney & liver � muscle for

T4 and T3, respectively, were still consistent with the

patterns reported in the same tissues in the rat (van Doorn

et al. 1985; Lavado-Autric et al. 2013)..

The levels of T3 in the present polar bears were higher

than T4 in liver, kidney and muscle. In general, T4 is

higher than T3 in liver and kidney. It should be noted that

the polar bears were sampled post-mortem, 1–3 h after

death. Continued post-mortem deiodination activities could

have caused higher T3 concentrations in the tissues. It

cannot be ruled out that this could influence the results.

However, at low temperatures (0�–5�), the deiodination is

low. The port-mortem transport of the polar bears to the

sampling site was performed as quickly as logistically

possible in order to start the sampling procedure within

reasonable time.

T4M was positively correlated with age. Intracellular

levels of THs are regulated in response to tissue-specific

needs (Bianco and Kim 2006). THs play an important role

during muscle development and maturation (Dentice et al.

2013). Since the present polar bears were between 2 and

4 years of age, it is possible that the relationship is related

to expansion of muscle volume in these sub-adult bears.

However, it should be noted that there were no correlations

between any of the other TH variables and the age of the

animals in the present study. It is therefore not possible to

elucidate further on this positive relationship between T4Mand age. Future studies should therefore include a larger

sample size and a broader age range to elucidate this

question.

Deiodinase activity: influencing intracellular levels

of thyroid hormones

When assessing hepatic D1 activity across studies, it is

obvious that great variation exists depending on species,

specimen condition, environment and analytical methods.

The hepatic D1 activity reported in the present polar bears

was much lower than hepatic D1 activity reported in, e.g.

humans, rats, mice (Mus musculus) and dogs (Canis fa-

miliaris) (Schoenmakers et al. 1992; Araujo et al. 2009).

The renal D1 activity in the present polar bears was also

lower than renal D1 activity reported in rats (Araujo et al.

2009). Thus, there appear to be pronounced species dif-

ferences in the abundance and activity of these deiodina-

tion enzymes, although the active centre of the enzyme is

highly conserved among species (Bianco et al. 2002; St.

Germain et al. 2008). D2 activity in the present study was

somewhat higher than reported in human skeletal muscle in

Heemstra et al. (2009) but lower than reported in human

skeletal muscle in Maia et al. (2005). However, D2 activity

has in general been challenging to measure in skeletal

muscle, and thus, values differ substantially (Arrojo e

Drigo et al. 2013). Since some studies suggest that age

might alter deiodinase activity (Donda and Lemarchand-

Beraud 1989), it is important to emphasise that the present

polar bears were sub-adults with an age of 2–4 years and

that adult enzyme activities might be somewhat higher.

Fig. 3 Positive correlations between body condition index (BCI) and

D2 activity (fmol/h/mg protein) in muscle (a) and BCI and fT3

(fmol/g) in plasma (b) of sub-adult polar bears (Ursus maritimus)

sampled in East Greenland February–March 2011 (N = 7). The r and

p values are presented in the plots. Significance level p B 0.05

Polar Biol

123

Deiodinase activities are also rapidly altered in response

to endogenous factors, one of the most important ones

being the THs themselves (Kohrle, 1999). Thus, TH status

in the individuals and organs investigated might influence

the results. Recently, the deiodinases have been proposed

to be the major regulators of TH concentrations and con-

sequently TH action (Bianco et al. 2002; Bianco and Kim

2006; Arrojo e Drigo et al. 2013; Dentice et al. 2013). In

the present study, D1L activity was approximately two

times higher than D1K activity, whereas D1M had consid-

erably lower activity. That liver was the tissue with the

highest D1 activity in the present polar bears is in accor-

dance with literature on D1 activity in mammals (McNabb

1992; Bianco et al. 2002; Zoeller et al. 2007; Gereben et al.

2008). The positive correlation between T4L and D1L ac-

tivity in the present polar bears may reflect the ability of T4

to induce D1 activity (Kohrle 1999; Bianco et al. 2002).

The positive correlation reported between T4P and T4Lprobably demonstrates that liver is a primary receiver of

circulating T4, as it is an organ that receives a high blood

input and is a key organ for D1-mediated T3 production.

Thus, as in other mammals, the liver in polar bears is most

likely the primary site for D1 activity and deiodination of

T4–T3.

The relatively high D1K activity in the present study

demonstrates that the kidney is also an important organ for

D1-mediated conversion of T4 in polar bears, as it is in

other mammals (McNabb 1992; Bianco et al. 2002; Zoeller

et al. 2007; Gereben et al. 2008). The low D1M activity in

the polar bears is general in accordance with previous re-

ports of low D1 activity in skeletal muscle (Lavado-Autric

et al. 2013). D2 activity was only analysed in skeletal

muscle in the present polar bears, and it was approximately

two times higher than D1M activity. This coincides with the

less important role skeletal muscle and other D2-rich tis-

sues have in producing T3 for the plasma pool (Larsen

2009; Arrojo e Drigo et al. 2013). Due to its intracellular

position close to the nucleus, the endoplasmic reticulum

(ER)-bound D2 is thought to primarily contribute to the

production of T3 to the local nuclear pool (Bianco et al.

2002; Gereben et al. 2008; Arrojo e Drigo et al. 2013). In

certain tissues, e.g. the brain, D2 can be responsible for up

to 75 % of the nuclear pool of T3. In comparison, in the

liver only approximately 5 % of the nuclear T3 is derived

from local conversion of T4 (Hennemann et al. 2001).

Thus, D2-produced T3 is thought to be more important

locally in tissues where D2 is abundant (Bianco et al. 2002;

Zoeller et al. 2007; Gereben et al. 2008). In this manner,

TH levels can be regulated locally in response to, e.g.,

specific development periods (Gereben et al. 2008), local

tissue damage (Mourouzis et al. 2013) or metabolic and

thermogenic needs (Bianco and Kim 2006; Arrojo e Drigo

et al. 2013).

Relationships among plasma and tissue levels

of thyroid hormones

The study identified some significant relationships between

plasma TH concentrations and tissue concentrations of

THs. There were significant relationships between T4P and

T4L, and between fT3P and both T3K and T4K. As previ-

ously suggested, the positive relationship between T4P and

T4L is probably related to the importance of the hepatic

tissue in producing T3 to the plasma pool in mammals

(Gereben et al. 2008). The liver is shown to extract 5–10 %

of T4 available in plasma during just one single passage,

which is a much higher amount than can be attributed to by

fT4 alone (Mendel et al. 1988; Malik and Hodgson 2002).

Thus, a substantial amount of protein-bound T4 is sug-

gested to be available for uptake by the liver, and this is

probably related to the important role the liver plays in the

extrathyroidal tissue conversion of T4–T3.

It is also possible that the positive relationships between

T3K and fT3K and between T4K and fT3P reflect that kidney

has an important role in supplying T3 to the plasma pool in

polar bears. In general, it has become clear that tissue levels

of THs are regulated independently of plasma levels (Bianco

et al. 2002; Gereben et al. 2008; Arrojo e Drigo et al. 2013;

Dentice et al. 2013). This suggests that the plasma levels of

THs in polar bears may not directly give information re-

garding TH status in tissues. Thus, it might be that TH status

in the tissues is sufficient even if plasma levels are low, or

opposite, that tissues might show hypo- or hyperthyroid

characteristics while plasma levels are within the normal,

euthyroid range (Castillo 2011; McAninch and Bianco

2014). It does therefore appear to be important to investigate

processes that occur at the organ and tissue level in addition

to circulating levels in the future.

Relationships with body condition

In the present polar bears, fT3P and D2M were both

positively associated with BCI. BCI is a measure of indi-

vidual health and nutritional status (Cattet et al. 2002;

Robbins et al. 2004). Since BCI is related to skeletal

muscle mass and total body fat in polar bears, which will

be decreased upon fasting (Polischuk et al. 2002), one

might hypothesise that bears with a low BCI have experi-

enced a reduced availability of food and thus be in a fasting

state. In mammals, it is believed that fasting in general

results in a down-regulation of metabolic rate through a

suppression of the thyroid axis (Boelen et al. 2008). Re-

duced levels of fT3 and/or T3 in plasma have been asso-

ciated with food restriction in ursids and arctic foxes

previously (Azizi et al. 1979; Tomasi et al. 1998; Fuglei

et al. 2000; Nieminen et al. 2004). The reduction of T3 in

combination with food restriction has been suggested to be

Polar Biol

123

associated with an energy-saving mechanism to ensure

protein conservation (Carter et al. 1975; Cox et al. 1984).

As deiodinase activities also are reduced upon fasting

(Maia et al. 2011), the reduced levels of circulating T3 in

combination with fasting have been thought to be caused

by a decreased deiodination of T4–T3 by D1, as D1 is the

main producer of T3 to the plasma pool (Hulbert 2000).

However, whether the decreased D1 or D2 activities are

responsible for or secondary to the reduction on T3 levels

associated with food restriction is debated (Bianco et al.

2002; Araujo et al. 2009).

In the present polar bears, a positive relationship be-

tween BCI and D2M activity was present, indicating that

polar bears with low body condition also had low D2Mactivity. There were no relationships between BCI and D1

activity in any of the tissues. Skeletal muscle is one of the

primary target tissues of THs, as it is a tissue involved in

energy consumption as well as heat production (Dentice

et al. 2013). Thus, in muscle tissue, D2-produced T3 is

important for energy expenditure (Bianco and Kim 2006;

Arrojo e Drigo et al. 2013; Dentice et al. 2013). Fasting has

been shown to cause a down-regulation of D2 mRNA gene

expression in human skeletal muscle (Heemstra et al. 2009)

and adult male elephant seals (Lee et al. 2014), while D2

activity in BAT was decreased in food restricted rats

(Araujo et al. 2009). According to Bianco et al. (2002), it is

more likely that a fasting-associated reduction in T3 is a

consequence of D2 deficiency rather than a reduced D1-

mediated T3 production. This is because D2 have an ex-

tremely short half-life as compared to both D1 and D3.

Thus, production of D2 must be persistent in order to

maintain normal levels, and this might not be the case

during nutritional stress. Thus, it might be that the lower

levels of fT3P and reduced D2M activity in the polar bears

with a low BCI in the present study reflect energy con-

servation and/or a fasting state. This is in contrast to the

results found by Martinez et al. (2013), where D2 mRNA

gene expression and protein content in muscle were in-

creased despite prolonged fasting in developing elephant

seal pups. This indicates that there may be age- and de-

velopmental-specific responses linked to fasting, in addi-

tion to differences in responses across species. The

duration of the fasting period may also play a role, and

especially prolonged fasting, resulting in extreme ema-

ciation, may alter the responses as compared to responses

caused by fasting periods of shorter duration.

The available data in the present study do not allow for

any conclusions, as the relationships between BCI, fasting

and TH homoeostasis need further investigation. In addition,

this study has some limitations which should be addressed in

future studies. The health of the animals should be more

investigated in addition to estimating their condition, and

their actual fasting state should be determined by, e.g., using

urea/creatinine ratios. Future studies should also try to in-

crease the sample size and include additional age groups.

Nevertheless, as periods of fasting are expected to be pro-

longed for polar bears in the prospect of climatewarming and

loss of sea ice habitat, subsequently resulting in decreasing

polar bear body condition (Stirling andDerocher 1993, 2012;

Stirling et al. 1999), the role of THs in adaptations to long

periods of fasting and low body condition should be further

investigated in polar bears.

In addition, other stressors also might affect the TH ho-

moeostasis. Subsequent lipid mobilisation upon fasting

might increase concentrations of lipophilic contaminants

(Polischuk et al. 2002), another threat for the polar bears. In

addition, climate warming-mediated diet change is reported

to increase concentrations of certain legacy contaminants

and alter exposure scenarios (McKinney et al. 2009). Future

studies should therefore try to gain further insight into

regulation of tissue levels of THs in response to energy-

conserving adaptations and other TH-dependent mechan-

isms in mammals and to do so in relation to stressors such as

climate warming and contaminant exposures.

Conclusion

The liver and kidney are important extrathyroidal tissues

for deiodination of T3–T4 and supplying the plasma pool

with T3 in polar bears as it is for other mammals. The

lower fT3p concentrations and D2M activities might reflect

food restriction and energy conservation in polar bears with

a relatively low BCI. Future studies are necessary to shed

light on the involvement of THs and deiodinase enzymes in

relation to energy conservation and adaptation to prolonged

periods of fasting resulting from a warmer Arctic.

Acknowledgments The field work for this study was funded by the

Danish Co-operation for Environment in the Arctic (DANCEA). The

analyses of thyroid hormones and deiodinase activities in tissues were

supported by research Grants SAF2012-32491 from MINECO and

S2010/BMD-2423 from CAM, Spain. The authors thank all those

who contributed to the field work in the Scoresby Sound area in East

Greenland, including the local hunters who provided the polar bears

for this study through their aboriginal hunting quotas. The authors

would also like to thank Sigga Joensen and Rune Dietz at Aarhus

University for performing the ageing of the polar bears and Grethe

Stavik Eggen at NTNU for help with the thyroid hormone analysis in

plasma. The study was funded by the Norwegian University of Sci-

ence and Technology (NTNU).

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