Abstract
1. 1. The turnover of all the known iodo compounds of the rat thyroid gland was followed after injections of tracer doses of 131I in rats previously brought into isotopic equilibrium with 125I. 2. 2. Both dialysable and non-dialysable iodine compounds were investigated. 3. 3. No precursor-product relationship was found between the iodo-amino acid residues (3-iodotyrosine (MIT), 3,5-diiodotyrosine (DIT), 3,5,3′-triiodothyronine (T 3) and thyroxine (T 4)) of thyroglobulin. 4. 4. The results found are compatible with the following mechanisms: MIT and DIT are synthesized by direct iodination of thyroglobulin. T 4 is produced by coupling of 2 DIT molecules, and T 3 by coupling of 1 MIT molecule and 1 DIT molecule. The mechanism of biosynthesis of the iodothyronines remains to be elucidated, but T 3 is not produced by the deiodination of T 4. 5. 5. Thyroglobulin is heterogeneous with respect to iodine turnover of each ot its four iodinated amino acids. 6. 6. The data concerning iodotyrosines turnover may be interpreted according to the scheme proposed.
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