Site of Iodination in the Rat Thyroid Gland Deduced from Electron Microscopic Autoradiographs

The site of iodination of protein in the thyroid gland (whether intracellular or intraluminal) was ascertained by autoradiographic studies using iodide-^l25I. In tissue fixed within about 40 sec after intravenous injection of radioiodide the silver grains of autoradiographs were concentrated over the follicular lumen generally as a ring of grains close to the apical border of the follicular cells. The zone of grains was sharply limited toward the cells. No concentration of silver grains was detected associated with any intracellular organelle. The autoradiographic ring which had a minimum width of about 2 μm was continuous along the apical plasma membrane of the follicle cells but there was a drastic reduction in grain density along the plasma membrane of the distal portion of pseudopods. Tissue was fixed so soon after radioiodide injection that it appeared likely that a negligible fraction of radioiodoprotein, if formed in the cell, could have been transferred to the lumen.

The observations strongly indicate that the iodination of thyroglobulin occurs in the follicle lumen, probably at the apical surface of the follicle cells. Since in the TSH-treated animals the distribution of the labeling along the apical plasma membrane agrees well with the reported histochemical distribution of thyroperoxidase in this membrane, it is further concluded that iodination may well be catalyzed by peroxidase in the apical plasma membrane. (Endocrinology97: 1432, 1975