Correlation of free thyroxine index and thyroxine: thyroxine-binding globulin ratio with the free thyroxine concentration as measured by the thyroxine and thyroxine-binding globulin radioimmunoassays.

The concentration of thyroxine-binding globulin in the serum can now be measured by a simple and specific radioimmunoassay. Triiodothyronine uptake and measurement of total thyroxine have been combined to yield a free thyroxine index which has been found to correlate with the clinical state of the patients. An estimate of the free thyroxine concentration, as measured by the thyroxine and thyroxine-binding globulin radioimmunoassays, provided a good correlation with the free thyroxine index and the thyroxine: thyroxine-binding globulin ratio. However, the thyroxine: thyroxine-binding globulin ratio is inaccurate when thyroxine-binding globulin concentrations are high or low.