Metabolism of125I in the Thyroid Gland Studied with Electron Microscopic Autoradiography

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Abstract
Mice were injected with Na125I, and autoradiographs of the thyroid were prepared for examination under the electron microscope. Iodination of thyroglobulin was not demonstrable within the cell. Under conditions of reduced turnover of thyroglobulin, the autoradiographic reaction occurred at the colloid cell boundary overlying the microvilli of the cell. In normal or stimulated thyroids the autoradiographic reaction was evenly distributed over the colloid at the earliest time intervals after injection (15 min). At later time intervals radioactivity could be localized over intracellular droplets; this was particularly apparent after TSH stimulation. It appeared that these droplets were formed at the apical cell membrane, and they were found throughout the cell. Some of those close to the capillary pole appeared shrunken, others in this location showed partial disintegration of their membranes. From this pattern it is postulated that iodination of thyroglobulin occurs in the colloid in association with the microvilli. The intracellular droplets participate in the resorption of thyroglobulin and the release of thyroid hormone into the circulation. Since this is also associated with the release of iodotyrosines from thyroglobulin and their deiodination, it is suggested that the latter processes might take place in these droplets. (Endocrinology75: 787, 1964)