Thyroid hormone changes associated with prolonged food deprivation in adult male northern elephant seals (1101.6)

Abstract

During food deprivation in mammals, thyroid hormone (TH) concentrations and deiodinase (DI) content is typically reduced to suppress metabolism. However, in metabolically active elephant seal pups the hypothalamic‐pituitary‐thyroid axis is up‐regulated during prolonged fasting. To better understand the mechanisms regulating the cellular function of TH in a fasting, yet metabolically active mammal, we measured plasma TH concentrations and mRNA expressions of DI type II (DI2) and TH receptor beta‐1 (THrβ‐1) in adult male northern elephant seals (n=8) over 8 weeks of fasting (early vs. late). We hypothesized that natural fasting in male seals decreases TH levels and DI expression. Fasting did not alter the concentrations of plasma reverse T3 and total T4. However, free T4 (0.25 ± 0.03 to 0.78 ± 0.08 ng/dL) and free T3 (0.61 ± 0.06 to 0.83 ± 0.08 pg/mL) increased between early to late fasting suggesting that there is an increased potential for TH‐mediated cellular effects. Conversely, total T3 decreased (63 ± 5 to 50 ± 4 ng/mL) suggesting that DI2 or DI3 may be contributing to the changes in TH metabolism. mRNA expressions of DI2 and THrβ‐1 decreased 87% ± 7 and 93% ± 2, respectively, between early to late fasting suggesting that there is a down‐regulation in cellular TH potential during fasting duration consistent with the effects of fasting in other mammals. When compared to the opposite effects observed in fasting pups, we conclude that elephant seals have evolved age‐dependent changes in fasting‐induced TH dynamics that may reflect differences in TH‐associated metabolic mechanisms.Grant Funding Source: Supported by NIH/NHLBI 442606‐RO‐29013