Performance characteristics and diagnostic value (vs other tests) of two radioimmunoassay kits for estimating free triiodothyronine in serum.

We compared the clinical value of "Amerlex" (Amersham International) and "Pre-release Coat-A-Count" (Diagnostic Products Corp.) radioimmunoassay kits for free triiodothyronine with that of triiodothyronine, triiodothyronine/thyroxin-binding globulin ratio, and free triiodothyronine index in patients with thyroid disease, pregnant women, oral contraceptive users, thyroxin-binding globulin-deficient subjects, and patients receiving phenytoin. Assay of free triiodothyronine in hyperthyroidism provided diagnostic information similar to that from the indirect indices, whereas the free triiodothyronine index was the most sensitive index of hypothyroidism. With respect to diagnostic value, free triiodothyronine assay was comparable with the free triiodothyronine index and superior to triiodothyronine assay and the triiodothyronine/thyroxin-binding globulin ratio in correcting for alterations in thyroid-hormone binding capacity. In neither kit for free triiodothyronine was the equilibrium between bound and free hormone significantly disturbed during assay. Overall, as a diagnostic index of thyroid status, free triiodothyronine is as good as the free triiodothyronine index and better than either triiodothyronine or the triiodothyronine/thyroxin-binding globulin ratio.