Abstract

Polychlorinated biphenyls (PCBs) and their metabolites have been related to potential deleterious changes in thyroid hormone concentrations of polar bears. Because PCBs may act synergistically with mercury (Hg) to further provoke thyroid disruption, we explored the interactions of both toxicants on the thyroid status of polar bears. Disruptions to thyroid function can occur through deficiencies of iodine or selenium (Se), conformational changes to circulating binding proteins, decreases in the transformation of thyroxine (T4) to triiodothyronine (T3) in thyroid or peripheral organs, or disruption of feedback systems. Many of these mechanisms of thyroid disruption have been suggested to be associated with exposure to toxicants in humans and wildlife. Selenium is important in antioxidant defense and is part of iododeionases in both thyroid and liver. Interactions between Hg and Se could also promote indirect effects on thyroid function.

Concentrations of thyroid hormones (total thyroxine, TT4; free thyroxine, FT4; total triiodothyronine, TT3; free triiodothyronine, FT3) and Se in the blood of 55 free-ranging Southern Beaufort Sea polar bears were examined for variations among sex and age cohorts during spring, and interactions with circulating toxicant concentrations of Hg and PCBs. Validation of radioimmunoassays was determined for use in ursids. Percent recoveries and sensitivity of assays were performed through serial dilutions of 2 pooled samples. Inter-assay and intra-assay correlations of variation ranged from 8-24 % with good parallelism (correlation mean > 0.9) between dilution of pooled samples and standard dilutions in protein buffer.

Free T3 concentrations were significantly lower in adult males (0.360 ± 0.228 pmol/l) than adult females (0.624 ± 0.241 pmol/l) and young (1.033 ± 0.413 pmol/l). Similar variations among cohorts were found for TT4, FT4 and TT3. Only free fractions of T3 significantly decreased with age. Thyroid hormones were positively correlated to concentrations of Se. Molar ratios of TT4: TT3 were greater in females than males, but TT3: FT3 and FT4: FT3 were lower in females than males. Five of the eight thyroid function indices were correlated with at least one toxicant in adult females, and two of eight thyroid function indices correlated with toxicants in adult males. There was a positive correlation between the molar ratio of TT4:TT3 with Hg, but a negative correlation between molar TT4:TT3 and elevations of both toxicants (Hg x PCBs interaction).

Numerous biological factors complicate the assessment of the impacts of toxicants on thyroid status of polar bears. For example, previous periods of fasting and food deprivation in polar bears may disrupt the normal homeostatic balance of Se. Female polar bears may be more susceptible to thyroidal effects during spring due to greater concentrations of toxicants (Hg and PCBs), lower concentrations of Se, and previous periods of food deprivation during maternal denning. Alterations in thyroid function are expected to be recoverable. Further assessment of thyroid status and regulation in polar bears are needed to evaluate the impacts of contaminants on endocrine function. Variations among seasons and cohorts, interactions with essential elements, and validation of methods must be included in these evaluations.