The Clinical Use of Serum Iodine Determination

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SIMPLIFICATION of the measurement of iodine in body fluids has resulted in increased use of the test and in demonstration of its importance in the study of patients suspected of having thyroid disease. Indeed, under certain circumstances, serum iodine determination may be the only laboratory test that shows with precision the abnormalities of thyroid function.* This is not surprising, since each of the three laboratory tests used in studying the thyroid — basal metabolic rate, thyroidal uptake of radioactive iodine and chemical determination of serum iodine—measures separate aspects of thyroid activity. Furthermore, each of the tests may be influenced by separate extrathyroidal factors. The purpose of this article is to outline the diagnostic uses and limitations of serum iodine determinations.

A simplified scheme (Fig. 1) shows the manner in which dietary iodine is made available for the production of thyroid hormone. In this diagram, it may be seen that the bulk of dietary iodine is first converted to iodide, mixing with the inorganic pool of the body, and then is taken up in part by the thyroid gland. The remainder is made available for renal excretion. Besides the urinary excretion of iodide, an additional small proportion of body iodine is excreted as thyroid hormone glucuronide in the feces.

In the thyroid, iodine is affixed to tyrosine from which is synthesized the thyroid hormone. The hormone is stored within the gland as thyroglobulin but is released in a free form into the blood stream. Here it circulates combined with serum . . .



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