Stimulation ofin VitroIodothyronine Synthesis in Thyroid Lobes from Rats Given a Low Iodine Diet, Propylthiouracil or TSHin Vivo

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Abstract
Feeding intact rats an iodine-deficient diet for prolonged periods or propylthiouracil for 10 or more days markedly stimulated in vitro labeled iodothyronine synthesis by their thyroid lobes. In vivo administration of 5 U TSH (thyroid-stimulating) twice daily for 2 days also improved in vitro I131-iodothyronine synthesis in both intact and hypophysectomized rats, but to a lesser extent than the above dietary regimens. The stimulation of in vitro I131-iodothyronine synthesis is probably due to a combination of prolonged TSH stimulation of the thyroid gland in vivo combined with iodine depletion of the gland. In vitro triiodothy-ronine synthesis is apparently more dependent on TSH than is that of thyroxine. Earle''s solution seems to be a more satisfactory medium for iodothyronine synthesis than is Krebs-Ringer phosphate.