Studies on the Effect of Hypothermia II. The Active Role of the Thyroid Gland in Hypothermic States in the Rabbit

In rabbits subjected to lowered body temperature (28 ~ to 10 ~ C. rectal temperature) for various periods, an acute profound physical and histological change has been noted consistently in the thyroid gland. Believing that these changes have an important bearing on the ability of the animals, and perhaps of human beings, to withstand periods of reduced body temperature, we are describing them in some detail. Extensive studies have been performed by numerous investigators (1, 3, 4, 5, 7, 8, 13, 23) on the effect of prolonged exposure to low temperature upon the thyroid gland of various animals, but the results do not always agree. Thyroidectomized animals have been said to suffer more severely in a cold environment than do normal ones (2, 11, 21, 22), yet the contrary is reported also (10). Dietrich and Schweigk (6) recorded an immediate increase of the blood flow through the thyroids of chilled dogs. In a series of extensive experiments, Cramer and Ludford (3) demonstrated that exposure to a cold environment for a short period was a powerful stimulus to the functional activity of the adrenals, but that low environmental temperatures of longer duration were necessary to induce hyperactivity of the thyroid. Ludford and Cramer (15) noted congestion of the thyroid capillaries and increased secretory activity of the follicular cells in epilated rats after 24 hours in an ice chest. Ring (20) observed that in rats exposed to cold for a short period of time the elevation of the basal metabolism was associated with a rise in body temperature. If they lived for 3 weeks or more in an environment of 8-5 ~ C., there was an average elevation in metabolism of 16 to 21 per cent, and the increase in the basal metabolic rate was brought about largely, if not entirely, by the thyroid gland since a smaller