Effect of winter sleep on pituitary-thyroid axis in American black bear

During winter sleep the black bear has decreased levels of serum total and free thyroxine (T4) and triiodothyronine (T3) and a prolonged, delayed response of serum thyrotropin (TSH) (bioassay) to thyrotropin-releasing hormone (TRH). Four weeks after the end of winter sleep, levels of serum thyroid hormones increase, and TSH response to TRH is short and brisk. Serum T4 and T3 rise after TRH administration both during and after winter sleep; however, the maximum increment in serum T3 is greater during winter sleep when the TSH rise is also prolonged and exaggerated. These observations suggest that transient hypothyroidism of possible hypothalamic origin occurs in bears during winter sleep.