PITUITARY HORMONE PRODUCTION AND RELEASE IN THE THYROIDECTOMIZED RAT AFTER THYROXINE ADMINISTRATION

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Abstract
Female rats of the Long-Evans strain were thyroidectomized at 35–40 days of age and 56 days later were placed into three groups each receiving injections 1, 2 or 3 μg of thyroxine per day for four days. Uninjected thyroidectomized and normal rats were used as controls. Twenty-four hours after the last injection the pituitaries and plasma were bio-assayed as a measure of pituitary hormone production and secretion. The minimal effective dose in hypophysectomized recipients in terms of whole anterior pituitary, or fraction thereof, are as follows: normal control pituitary – TSH, 1/16; FSH, < 3; ICSH, 1; GH> 1/16; thyroidectomy – TSH, 1/8–1/4; FSH, > 5; ICSH, > 5; GH, 3. Thyroxine administration restored the TSH pituitary content to normal at 1 μg dose, and increased it further at the 2 and 3 μg dose. The pituitary content of GH increased from the post-thyroidectomy levels to near normal levels with increasing doses of thyroxine. The ICSH content of the pituitary was influenced by the administration of thyroxine but no effect was obvious in FSH content. GH was not detectable in plasma of thyroidectomized rats but was present in the plasma of animals receiving 2 or 3 μg of thyroxine. Thyrotrophic hormone content of plasma was decreased after administration of thyroxine. Gonadotrophin was not detected in any plasma. The above changes in hormonal content preceded the reappearance of normal pituitary cytology.