THE EFFECT OF VARYING TEMPERATURES ON THYROID ACTIVITY AND THE SURVIVAL OF RATS EXPOSED TO COLD AND TREATED WITH l -THYROXINE OR CORTICOSTERONE

SUMMARY Rates of thyroid secretion (TSR) were determined in female Sprague-Dawley-Rolfsmeyer and male Holtzman rats at varying temperatures. When exposed to cold, TSR reached the highest value within the first 20 days after exposure, with a gradual increase up to 90 days. When rats reared in the cold were exposed to warm temperatures, TSR declined slightly at first, but 35–40 days were required to produce a significant decrease. Treatment before and after exposure to cold with l-thyroxine (T4), corticosterone and T4 plus corticosterone significantly increased survival at 4·5°. Corticosterone plus T4 was most effective, T4 alone was next best and corticosterone alone had the poorest effect.