Assessment of Thyroid Function by the Combined Use of the Serum Protein-Bound Iodine and Resin Uptake of131I-Triiodothyronine

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Abstract
A comparison has been made between the diagnostic accuracy of the PBI, the resin uptake of I131-T3 and their mathematical product (PBI X resin uptake), the "free thyroxine" index, an arbitrary figure, assumed to be proportional to the concentration of free thyroxine in the blood. The 3 parameters have been determined in a total of 154 patients (108 clinically euthyroid, of whom 23 were pregnant and 4 had the nephrotic syndrome; 30 thyrotoxic and 16 hypothyroid). PBI values outside the normal range were found in 17.4% of the pregnant women and in 93.3% of thyrotoxic and 100% of hypothyroid patients. Resin uptake values outside the normal range were found in 91.3% of the pregnant women, and in 96.7% of thyrotoxic and 87.5% of hypothyroid patients. All thyrotoxic and hypothyroid patients fell outside the nor-mal range for the "free thyroxine" index, whereas all the pregnant women gave values inside the normal range. Although the PBI and resin uptake test are highly satisfactory in the diagnosis of thyroid dysfunction, they may both be invalid as indicators of thyroid status, when variations in the thyroxine-binding proteins of plasma or inter-ference with their action occurs. If taken in conjunction with each other, however, these 2 indices may disclose such instances because of the characteristic deviations produced therein. In these individuals the use of the "free thyroxine" index may indicate the true thyroid status, and may also be a more accurate diagnostic aid than the PBI or resin uptake test alone in the assessment of thyrotoxicosis or hypothyroidism.