EFFECT OF THYROIDECTOMY AND LOW ENVIRONMENTAL TEMPERATURE (4.4° C) UPON PLASMA AND PITUITARY THYROTROPHIN IN THE RAT

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Abstract
The content of thyrotrophin (TSH) in rat plasma and hypophyses was determined by means of the haemagglutination inhibition reaction. Mature female rats maintained at 26° C had a plasma TSH level of 0.94 mU/ml and a mean pituitary TSH level of 68.26 mU/mg. After 10 days of thyroidectomy the plasma TSH increased to 2.15 mU/ml, and to 2.81 mU/ml after 20 days. The TSH/mg of the pituitary glands was reduced to 40.4 mU after 10 days and to 35.0 mU after 20 days. After 25 days of thyroidectomy, L-Thyroxine (L-T4) at level of 2.5 μg/100 g body weight was injected for 1, 3 and 5 days. The plasma TSH level was reduced to 1.59, to 1.48 and 1.06 mU/ml, respectively. The pituitary TSH/mg was 37.5, 38.6 and 77.9 mU, respectively. When rats held at 26° C were exposed to a temperature at 4.4° C for 10 and 20 days, the plasma TSH increased to 2.15 and 2.69 mU/ml, respectively. The pituitary TSH increased to 108.04 and to 240.9 mU/mg, respectively. The combined effect of thyroidectomy and 4.4° C for 10 and 20 days, caused the plasma TSH to increase to 4.37 mU/ml after 10 days then decline to 2.4 mU/ml after 20 days. The pituitary TSH declined to 43.79 and 37.58 mU/mg, respectively. When thyroidectomized rats held at 4.4° C for 20 days were returned to 26° C for 4 days, the plasma level declined to 1.44 mU/ml and the pituitary remained low, 34.90 mU/mg.