Because iodine atoms are constituent parts of the T4 and T3 molecules, a continual supply of iodide is required for the synthesis of these hormones. If an individual's diet is se-
verely deficient in iodide, as in some parts of the world, T4 and T3 synthesis is limited by the amount of iodide available to the thyroid gland. As a result, the concentrations of T4 and T3 in the blood fall, causing a chronic stimulation of TSH secretion, which, in turn, produces a goiter. Enlargement of the thyroid gland increases its capacity to accumulate iodide from the blood and to synthesize T4 and T3. However, the degree to which the enlarged gland can produce thyroid hormones to compensate for their deficiency in the blood depends on the severity of the deficiency of iodide in the diet. To prevent iodide deficiency and the consequent goiter formation in the human population, iodide is added to the table salt (iodized salt) sold in most developed countries.
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