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

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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 131I) 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.