Seasonal Variations in Thyroid Disrupting Effects of Persistent Organic Pollutants in Polar Bears (Ursus maritimus) from Svalbard
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- Institutt for biologi 
Several persistent organic pollutants (POPs) have the ability to interfere with the thyroid hormone (TH) system in arctic mammals. This endocrine system is crucial for adapting to a changing environment, and it is an important regulator of growth, thermoregulation and metabolism. Seasonal variations in prey availability forces the polar bear (Ursus maritimus) to fast in longer periods, and consequently this causes seasonal variations in their body condition. Fasting has been related to depressed TH concentrations and redistribution of POPs from the adipose tissue to target organs. However, fasting (i.e. season) is rarely considered in studies examining TH-POP relationships. Therefore, it was aimed to examine concentrations and relationships between THs and POPs in free-ranging female polar bears in different seasons (spring or autumn) and reproductive status groups (solitary, with cubs of the year, or with yearlings). A total of 112 blood samples were collected in April and September 2012 and 2013 in Svalbard, Norway. Plasma concentrations of total triiodotyronine (TT3), free triiodotyronine (FT3), total thyroxine (TT4), and free thyroxine (FT4) were analysed. Concentrations of 52 individual POPs, including polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs), per- and polyfluorinated alkyl substances (PFASs), hydroxylated PCBs (OH-PCBs), and hydroxylated PBDEs (OH-PBDEs), were also analysed in the plasma. The results showed seasonal and status differences in TH concentrations, and to a lesser degree in POP concentrations. THs were generally higher in spring compared to autumn, and higher in solitary females compared to females caring for offspring, which was explained by fasting status. Variations in POP concentrations could be related to body condition, which did not necessarily reflect the fasting status. FT3 was negatively related to ∑PCB, the sum of OCPs and PBDEs ( ∑PESTBDE ) and ∑PFAS. The ratio TT3:FT3 was positively correlated to ∑PCB and ∑PFAS, and TT4:FT4 was positively correlated to ∑PFAS. Seasonal differences in the TH-POP relationships were found for TT4:FT4-∑PFAS, TT4:TT3-∑PESTBDE, in addition to TT4:TT3-∑PFAS. In summary, the results of this study indicated that POPs may interfere with TH concentrations in polar bears, and they may be more sensitive to thyroid disruption in certain seasons. Altogether, the results emphasise the importance of accounting for environmental and biological variables, such as season and reproductive status, when examining variations of TH and POP concentrations and their relationships in arctic mammals.