Plasma and pituitary thyrotrophin (TSH) in severe and prolonged hypothyroidism, as studied in the rat

Thyroidectomized rats, kept on a low iodine diet, were killed at 60, 80 and 270 days after thyroidectomy and plasma and pituitary TSH levels measured. Pituitary TSH content was lowere 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 the thyroidectomized rats than in the controls at all the times studied, but declined markedly between 60 days (17.53 +/-1.98 mug/ml) and 270 days (3.63 +/-0.49 mug/ml. This decrease in plasma TSH levels was accompanied by a decrease in plasma PBI: from 0.69+/-0.08 mug/dl at 60 days to 0.06+/-0.01 mug/dl at 270 days. The daily injection of 1.75 mug T4/100 g body weight for 12 days in either thyroidectomized rats or normal intact rats resulted in a decline of plasma TSH levels in both groups. Pituitary TSH content increased in the thyroidectomized rats and decreased in the controls after T4 treatment. Present results agree with previous observations indicating that severe and chronic thyroid hormone deficiency is not accompanied by a progressive increase in circulating TSH levels, though an elevation is always fou nd even in mild or subclinical forms or primary hypothyroidism. They show that in the rat this connot be accounted for by an impairment of TSH synthesis, as previosly suggested: an impairment of TSH secretion appears more probable.