Thyroid function in a hibernator, Spermophilus tridecemlineatus

Thyroid hormone secretion (assessed as thyroid 125I release rate) was monitored in Spermophilus tridecemlineatus from summer to winter (July-December) in 1973. The average release rate in July was 2.3% per day, but in August this rate decreased significantly and remained at an average 0.52% per day until December. This low rate (being similar to that of hypophysectomized rats), and the absence of any change in oxygen consumption following thyroidectomy, suggested hormone secretion was "turned off" during this period. During the period August-December, four S. tridecemlineatus showed normal thermoregulatory responses to anterior hypothalamic cooling but no thyroidal response whether restrained, unrestrained, or anesthetized. Thyroid 125I uptake (and presumably hormone manufacture) continued during the period August-December and 24-h uptake averaged 27.0% in December. Possible reasons for "turning off" thyroid hormone secretion are discussed, and it is concluded this is to allow the membrane fatty acids to revert to a less-saturated condition in preparation for low body temperatures during hibernation.