Effect of the exposure to chronic-intermittent cold on the thyrotropin and thyroid hormones in the rat.

Abstract Rats exposed to acute cold (4 °C for 2 h), chronic cold (4 °C), and chronic-intermittent cold (4 °C for 2 h daily) were killed after 1, 2, 3, 4, and 10 days of cold exposure. The control group was maintained at 25 °C. In each animal, the plasma concentration of thyrotropine (THS), triiodothyronine (T 3 ), and thyroxine (T 4 ) was determined by radioimmunoassay. At the initial time of exposure, elevations in TSH, T 3 , and T 4 were observed in the rats in each experimental group. However, on the 10th day, in rats exposed to chronic-intermittent cold, TSH, T 3 , and T 4 decreased to values lower than the control values. In animals exposed to acute cold as well as to chronic cold no differences were found, with respect to the controls, in TSH and T 4 . In rats exposed to acute cold for 10 days, the T 3 value was lower than the control value; however, in animals exposed to chronic cold, T 3 was same as that in the controls. The results indicate that, in the rat, exposure to chronic-intermittent cold produces an inhibition in the secretion of TSH and thyroid hormones.