COMPARISON OF THE EFFECT OF HYPOPHYSECTOMY AND THYROXINE ADMINISTRATION ON THYROID FUNCTION IN THE RAT1

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Abstract
Comparison was made of the changes in various thyroid parameters at different intervals following thyroxine administration and hypophysectomy in 332 adult male rats. There was a distinct temporal difference among the responses of the different parameters to the acute fall in thyrotropin levels induced by thyroxine administration or hypophysectomy. The most sensitive parameter was thyroidal release of radioiodine, the next I131 uptake and the least sensitive, thyroid weight. Hypophysectomy and thyroxine injection produced nearly identical effects on thjToidal I131 uptake and release. After thyroxine administration a rebound phenomenon was observed in both I131 uptake and in thyroid weight, but it was not observed in I131 release. Potassium iodide, in amounts equivalent to the doses of thyroxine, decreased thyroidal I131 for a short period but had little effect on thyroid weight. Sham hypophysectomy resulted in decreased I131 loss from the thyroid, the degree of thyroidal suppression depending upon the extent of surgically induced pituitary trauma.