Thyroxine iodine labeled

There has been much speculation in the past on the nature of the circulating thyroid hormone whether it was a simple amino acid, a peptide or polypeptide of thyroxine, or thyroglobulin itself. In 1948 Taurog and Chaikoff produced a considerable amount of evidence that the plasma hormone was indeed thyroxine. Labeled plasma iodine behaved chemically in a manner identical with that of thyroxine added to plasma it was nondialyzable it was precipitated with plasma proteins with zinc hydroxide it was extractable with butanol, and it could be fractionated with carrier thyroxine by its partition between immiscible solvents. This work received confirmation from Laidlaw (1949), and it then became generally accepted that thyroxine alone was the circulating hormone. 

West et al. [188] have used TLC to determine the thyroxine level in human serum. For 43 persons with normal thyroid function they found an average of 5.4 jxg thyroxine per 100 ml serum with a normal range from 3.2 to 7.9 jxg per 100 ml. Altogether 339 patients were examined. ScHOBN and Winkler [148] have reported the radiochromatographic determination of labelled tri- and tetraiodothyronine in the serum of a patient who had been dosed with carrier-free Na I for therapy of hyperthyroidism (Fig. 172). This separation is important because triiodothyronine is about four times as active as tetraiodothyronine. Determination of total hormone iodine in serum with the usual methods does not reveal any pathologically modified ratio of tri- to tetraiodothyronine. 

The plasma of the hormone-treated animals contained larger amounts of organically bound labeled iodine than did the plasma of the control animals, which is just what we should expect (132). After twenty hours about 80% of the plasma iodine in the animals treated with thyrotropic preparation was present as tliyroxine. This observation also suggests that the end product of iodine synthc.sis i.s thyroxine and l)eautifully demonstrates the role of thyrotropic hormone in providing thyroxine to the circulation. Hypophysectomy (c/. 35) is followed by a reduced uptake of radioiodine by the thyroid gland (115). 

Inhibitory Effect of Added Inorganic Iodine on Foruation of Labeled Thyroxine and Diidotyrosinb (131)... 

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