THE MECHANISM OF THE SECRETION OF THYROID HORMONE*

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Abstract
Autographs of thyroid gland from radioiodine-treated rats fed on either a low or a high iodine diet showed that the iodide taken from the blood by the thyroid cell is transformed into thyroglobulin in the apex of the cell. This thyroglobulin is then released into the colloid of the thyroid follicle. The presence of free thyroxin was demonstrated in the thyroid tissue by the help of the isotope dilution technique, indicating that the thyroglobulin is hydrolyzed to yield thyroxin, presumably by the proteolytic enzymes contained in the colloid. The presence of free thyroxin has also been demonstrated in blood plasma. The possibility that this substance originates from the free thyroxin fraction of the thyroid gland is supported by the presence of a gradient in concn. between the thyroid and blood. It is suggested that L-thyroxin is in fact the form in which the thyroid hormone is released by the thyroid and circulates in the body.