Serum free thyroxine methods. A comparative study

Abstract

As part of a clinical trial, a new commercial RIA kit for the determination of serum free thyroxine (FT₄) utilizing a labelled T₄ derivative was used. This method was compared with our hitherto used Sephadex binding method, with two indirect methods for FT₄ estimation, the total T₄ to TBG ratio and free T₄ index, and with determinations of total T₄, total T₃, and TSH.

The samples were from patients from a cancer hospital, 50% had a malignant disease, 25% had a malignant thyroid disease, and 25% had a non-malignant thyroid disease. The patients consisted of 16 hyperthyroid, 15 hypothyroid and 184 euthyroid patients, including 61 T₄-treated and 14 T₃-treated patients.

A reasonably good agreement between the four FT₄ methods was found for hyperthyroid and hypothyroid patients, whereas disagreements between the methods were found for a number of euthyroid patients, especially in the group of T₄-treated patients, where 30%–60% of the patients had elevated FT₄-values dependent on the FT₄-method used.

From the present series of patients it is concluded that the new FT₄-RIA is of at least as much diagnostic value as the other three methods. In T₄-treated patients a FT₄ determination should be supplemented with determinations of TSH and T₃.