Abstract
Summary
In order to study the mechanism of inhibitory action of excess iodide on thyroid hormone secretion, effects of excess iodide on colloid droplet formation, susceptibility of thyroglobulin to exogenous protease and thyroidal hydrolytic enzyme activity were studied under several experimental conditions. Excess iodide was first converted to some form of organic iodine and then inhibited colloid droplet formation, an initial step of thyroid hormone secretion. Since this effect of excess iodide was very similar to those produced by lysosomal membrane stabilizers, a possible role of excess iodide on lysosomal membrane was suggested. Administration of excess iodide did not stabilize the thyroglobulin against an exogenous protease, pronase. Excess iodide did not affect thyroidal hydrolytic enzyme activity as evidenced by a normal autoproteolytic activity. It is suggested that excess iodide depressed the thyroid hormone secretion primarily by interfering with the engulfment of thyroglobulin at the apical cell surface and eventual formation of intrathyroidal colloid droplets, without affecting the nature of thyroglobulin or proteolytic enzyme activity.
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