Comparison of serum free thyroxine indices and "corrected" thyroxine tests.

A dual competitive protein-binding assay for serum thyroxine (T4) is described. The assay is called the "corrected" T4 index and can be performed simultaneously with the determination of total T4. This method correlated significantly with the free T4 index and the effective T4 ratio. The "corrected" T4 index was compared for precision and diagnostic accuracy with the effective T4 ratio and two different free T4 indices. Improved modifications of the thyroxine and triiodothyronine Sephadex uptake tests, used for calculation of the free T4 indices, are also reported. In spite of better precision, the "corrected" T4 index and effective T4 ratio gave twice as many incorect classifications of both hyper- and hypothyroidism as the free T4 indices. Previous reports on the effective T4 ratio and other basically similar tests might give an overly optimistic view of what can be expected in routine clinical practice. The free T4 index, which is a good estimate of the free T4 concentration, may still be the thyroid function test of choice when serum T4 and clinical impression are not in conformity.