Nature of Intrathyroidal Iodinated Substances Released Into the Medium During in Vitro Incubation of Prelabeled Whole Rat Thyroid Lobes

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Abstract
Ap-proximately 40% of the initial thyroidal radioactivity was released into the medium during a 3-hr. in vitro incubation of prelabeled whole rat thyroid lobes. The composition of iodinated substances found in the thyroid and in the medium after incubation was essentially identical with that found in the thyroid before incubation. Almost all radioactivity appearing in the medium was in peptide-bound form. No free iodinated amino acids were detected. The qualitative and quantitative nature of this loss of radioactivity into the medium was not altered by a large variety of experimental manipulations. It is concluded that such loss of iodinated material into the medium is caused by disintegration of the interior thyroid follicles and bears no relation to thyroid secretion in vivo.