THYROTROPHIC HORMONE IN THE PLASMA AND ANTERIOR PITUITARY OF THE THYROIDECTOMIZED RAT

Abstract

SUMMARY The changes in plasma concentration and anterior pituitary content of thyrotrophic hormone (TSH) after thyroidectomy in the rat were followed by means of an in vitro assay method. A triphasic change in plasma TSH was demonstrated which consisted of a threefold rise by the 2nd day, a fall to the normal range by the 5th day, and a gradual rise from the 10th day, which continued for at least 6 weeks. The initial rise in plasma TSH was accompanied by a fall in the pituitary TSH store after 5 days to 7 % of the normal value; the store then remained low for the next 6 weeks. Thyroxine (as measured by protein-bound 131 I) disappeared from the circulation after thyroidectomy at a rate compatible with the observed rate of rise of plasma TSH; 20 % remained after 2 days. The replacement of thyroid hormone by exogenous thyroxine in thyroidectomized rats resulted in 4 days in a decline in plasma TSH concentration to a subnormal value. During the next 3 days the blood level of TSH rose to 400 % of the control level. Thyroxine administration caused a large increase in the pituitary store of TSH. The results indicate that circulating thyroxine influences primarily the rate of release of TSH from the anterior pituitary, and that a slower effect on the synthesis of TSH may be indirect and secondary to changes in the TSH content of the pituitary gland.