Thyroid hormones and deiodinase activities in plasma and tissues from East Greenland polar bears (Ursus maritimus) during winter season

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 necessary in order to understand the true impact and consequences 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 addition, type 1 deiodinase (D1) activities in liver, kidney and muscle and type 2 deiodinase (D2) activities in muscle were analysed. Concentrations of 3,5,3′,5′-tetraiodothyronine (T4) were highest in plasma, followed by liver, kidney and muscle, whereas concentrations of 3,5,3′-triiodothyronine (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 positive 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 deiodination 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.