Plasma TSH Levels in Rats Exposed to Cold

Plasma TSH levels in rats exposed to cold were investigated and the following findings were obtained. Plasma TSH levels elevated soon after exposure to a moderate cold of 8°C and reached the peak 30 minutes after exposure. The elevated level of plasma TSH was sustained for only 2 hours. Thereafter, it decreased. rapidly toward pre-exposure levels. A relatively low level, but significantly higher than control value, was observed to be maintained in rats kept at 8°C for 4 weeks. During the initial stages of cold exposure at 8°C pituitary content of TSH decreased markedly from control value of 803 mU per gland to 624 mU after 6 hours and 303 mU after 24 hours. Percentages of the decrease were 22.3 and 62.3 per cent, respectively, indicating an initial burst of the stored TSH in the adenohypophysis. Exposure of rats to a mild cold of 15°C resulted also in a marked increase in the plasma TSH level after 30 minutes and 2 hours. Thereafter it decreased to the level of preexposure, with an exception of value at 16 hours after exposure. Prolonged exposure at 15°C for 4 weeks showed no more increase in the plasma TSH content. A similar but a little prolonged elevation of the plasma TSH was observed in rats exposed to a severe cold of 0°C. These results indicate that a bulk of TSH is released from the adenohypophysis in the early phase of cold exposure irrespective of its intensity. (pp. 1227--1231)