PLASMA AND PITUITARY THYROTROPHIN (TSH) IN SEVERE AND PROLONGED HYPOTHYROIDISM, AS STUDIED IN THE RAT

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Abstract
Thyroidectomized rats, kept on a low I diet, were killed at 60, 80 and 270 days after thyroidectomy, and plasma and pituitary TSH [thyrotropin] levels measured. Pituitary TSH content was lower in the thyroidectomized rats than in the controls at 60 and 80 days, starting to increase between 60 and 80 days, and reaching higher values than those of the controls by 270 days. Plasma TSH was higher in thyroidectomized rats than in controls at all the times studied, but declined markedly between 60 days (17.53 .+-. 1.98 .mu.g/ml) and 270 days (3.63 .+-. 0.49 .mu.g/ml). This decrease in plasma TSH levels was accompanied by a decrease in plasma PBI [protein bound I]: from 0.69 .+-. 0.08 .mu.g/dl at 60 days to 0.06 .+-. 0.01 .mu.g/dl at 270 days. The daily injection of 1.75 .mu.g T4[thyroxine]/100 g body wt for 12 days in either thyroidectomized rats or normal intact rats decreased plasma TSH levels in both groups. Pituitary TSH content increased in thyroidectomized rats and decreased in controls after T4 treatment. Severe and chronic thyroid hormone deficiency is probably not accompanied by a progressive increase in circulating TSH levels, though an elevation is always found even in mild or subclinical forms or primary hypothyroidism. In the rat this cannot be accounted for by an impairment of TSH synthesis, as previously suggested; an impairment of TSH secretion is more probable.