CORRELATION OF EARLY EXTRATHYROIDAL RADIOIODIDE SPACE WITH BODY WEIGHT

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Abstract
THE thyroidal iodide clearance rate is a direct measure of the iodide accumulating function of the thyroid gland. It expresses the rate of thyroidal iodide uptake divided by the mean plasma iodide concentration measured simultaneously. In this laboratory the method of Berson et al. (1) is used for routine determinations of thyroidal iodide clearance rates. The thyroidal I131 uptake is measured during the first half hour following intravenous administration of the tracer dose, with suitable corrections for changes in extrathyroidal radioactivity during this interval. The direct determination of the mean plasma radioiodide concentration would require multiple plasma assays, but in this method it is approximated as that concentration which would result if the apparent volume of distribution of I131 (exclusive of thyroidal and urinary accumulation) averaged 20 per cent of the body weight for this initial half-hour period (“mean iodide space”). This value for the mean iodide space was determined experimentally by Berson et al. (1) in a group composed principally of male patients. Thyroid disease is, however, more common in the female population by a ratio of 4 to 1 (2). Since total body water (3), plasma volume (4), and extracellular space (5) are smaller fractions of body weight in women than in men, it might be anticipated that the mean half-hour iodide space would also show a significant sex difference.

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