SERUM TRI-IODOTHYRONINE UPTAKE USING COATED CHARCOAL IN THE ASSESSMENT OF THYROID FUNCTION

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Abstract
SUMMARY: A modification of the tri-iodothyronine T3 uptake test using coated charcoal has been developed. When the method was compared with the resin sponge test, the charcoal method was found to be more rapid, accurate and economical. For patients who were not pregnant or receiving drugs known to affect the degree of saturation of thyroxine-binding globulin, the T3 charcoal uptake method made a good distinction between the hyperthyroid and euthyroid patients but the distinction between the hypothyroid and euthyroid patients was less satisfactory. Separation between the three groups of patients was improved by using the product of the T3 uptake and protein-bound iodine measurement (the free thyroxine index). The [125I]T3 charcoal results are reported for 199 blood donors as normal controls. A sex difference was apparent but there was no significant variation in the age groups studied (18–60 yr.). Thirty-seven pregnant women were studied. Calculation of the free thyroxine index did not give results within the non-pregnant euthyroid range.