TURNOVER OF THYROID HORMONE IN COLDEXPOSED RATS DETERMINED BY RADIOACTIVE IODINE STUDIES1

Abstract

SMALL mammals subjected to low environmental temperatures for short periods (3-21 days) exhibit alterations in the thyroid gland. Although such changes as increased gland weight (1,2), cellular hypertrophy (3, 4, 5), and increased radioiodine (I131) content 2 hours after isotope injection (5, 6, 7) have been interpreted as indicating increased functional activity of the gland, they are relatively indirect measures of hormonal secretion rates. However, more direct parameters of hormonal output, such as biologic decay (8) and hormone utilization or requirement (9) are consonant with these interpretations, and suggest that secretion of thyroid hormone increases following exposure to cold. The increased metabolic rate of small mammals placed at low temperatures (1, 10, 11) appears related to these changes in the functional capacity of the thyroid gland. After animals have been exposed to 5° C for 60 days or more their metabolic rate remains markedly higher than that of unexposed animals, whereas studies of the thyroid gland including cell height (3) and I131 content (6, 7) suggest that thyroid function decreases to pre-exposure levels.’ This discrepancy may result from production of a more active hormone, a change in sensitivity to the hormone, or a maintenance of secretion not detected by these methods.