| Summary: | Food deprivation in mammals is associated with reduced thyroid hormone (TH) to suppress metabolism. However, in prolonged‐fasted, elephant seal pups, cellular thyroid hormone‐mediated proteins are up‐regulated with fasting duration. The functional relevance of this apparent paradox is unknown. To address our hypothesis that the elephant seal thyroid gland remains active during fasting, we infused early and late fasted pups with thyroid stimulating hormone (TSH) and measured the concentration/expression of total thyroxine, (tT4), total tri‐iodothyronine, (tT3), DI1, and THrβ1. During the early fast, concentrations of tT4 and tT3 increased, with levels peaking at 120 minutes, 158 ±9%, and 154 ±6%, respectively. In the late fast, concentrations of tT4 and tT3 also increased, with levels peaking at 120 minutes, 156 ±10%, and 160 ±9%, respectively; levels remained 215% and 242% elevated in tT4 and tT3, respectively after 24 hours in the late fast. Similarly, expression of THrβ1 increased 196 ±23% at 120 minutes in adipose tissue, with levels increasing 421 ±56 % in muscle at only 60 minutes in the late fast. Lastly, the mRNA expression of DI1 increased 415 ±49 % at 120 minutes in adipose, with levels increasing 263 ±28 % at only 60 minutes in muscle during the late fast. The data suggest that the regulation and function of the thyroid gland and of cellular thyroid hormone mediated activity of the northern elephant seal is atypical.
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