Nature of Thyroid Secretion in the Rat and the Manner in Which It Is Altered by Thyrotropin

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Abstract
The identity of radioiodinelabeled substances secreted into the thyroid venous effluent of the rat was determined 24 hr after 13lI administration. Only iodide, thyroxine, and a small and inconstant amount of triiodothyronine were secreted under basal conditions. Intravenous administration of 1–5 USP units of TSH caused a prompt rise in the quantity of labeled substances secreted and a pronounced qualitative shift in their composition. Diiodotyrosine and, occasionally, monoiodotyrosine were clearly detected after, but not before, TSH administration. Compared to the quantity of these substances secreted under basal conditions, TSH increased the secretion of radioiodinated components in the following order: triiodothyronine >iodide > thyroxine. The relative increase in diiodotyrosine secretion was possibly greater than that of triiodothyronine but could not be quantitated precisely since diiodotyrosine was undetectable under basal conditions. (Endocrinology76:1012, 1965)