Role of the Thyroid in Metabolic Responses to a Cold Environment1

T HE exposure of most warm blooded animals to a cold environment gives rise to a number of apparently related metabolic phenomena. An increase in metabolic rate as measured by oxygen consumption, takes place very rapidly and in the rat at a temperature just above freezing; the rate may reach a level two or three times that observed at the critical temperature of the animal (I, 2). When exposure is prolonged for more than a few days, hyperplasia of the thyroid gland has been observed repeatedly (3-9, and part of the increased rate of metabolism has been generally ascribed to increased activity of the thyroid gland. In relation to the known action of thyroxine, however, the rapid rise in metabolic rate and almost immediate increase of muscular activity has made it appear likely that whatever role the thyroid does play is of a slower, more gradual onset which might involve other than muscular tissues. The experiments reported in this paper extend previous observations on the relationship of the thyroid and pituitary to changes in metabolism which occur in a cold environment and indicate that while iscreased thyroid activity is not indispensable for an elevation in metabolic rate, part of the increase observed is dependent on the presence of thyroid hormone and is associated with the ability of the animal to survive.