A solid phase radioimmunoassay for the measurement of free thyroxine. A new screening test for thyroid function?

A solid phase radioimmunoassay for the measurement of free thyroxine, utilizing immunoextraction of free T4, was evaluated for its clinical utility and compared to the free thyroxine indices obtained by the measurement of thyroxine-binding globulin (TBG) or assessment of TBG by talc T3 uptake (TT3U) and total T4 (FTI-1 = total T4/TBG and FT1-2 = Total T4 × TT3U). Free T4, FTI-1, and FTI-2 were analyzed in a total of 202 specimens: 62 normal healthy subjects, 40 hypothyroid, 11 hyperthyroid, 63 euthyroid, 16 euthyroid women on oral contraceptives, seven euthyroid pregnant women, two euthyroid patients with TBG deficiency, and one euthyroid patient with an excessive increase in TBG. Also, free T4 and FTI-2 were measured in nine patients with nonthyroidal illnesses who had decreased triiodothyronine and total T4 levels. Free T4 assay was simple and rapid but showed poor precision. Also, it was less sensitive for detecting hypothyroidism (sensitivity 75%) as compared to FTI-1 and FTI-2 (sensitivity, 95% and 90%, respectively) and, therefore, is not recommended for screening purposes. This test, however, seems to be helpful in confirming the diagnosis of hyperthyroidism in occasional patients with high normal FTI levels as well as for ruling out hypothyroidism in patients with nonthyroidal illnesses.