[Determination of serum free thyroxine: normal range, relation to the free thyroxine ratio and total thyroxine (author's transl)].

The validity of a commercially available f-T4 radioimmunoassay was investigated in 122 patients with different thyroid status. The normal range of f-T4 was 0.60-2.20 ng/dl in euthyroid patients from an endemic goiter area. No significant difference was found between normals and a group taking oral contraceptives (mean = 1.37 ng/dl vs. 1.45 nd/dl). In patients on thyroid hormone or antithyroid drug therapy, f-T4-values showed a better correlation with the results of an oral TRH-test than total T4 or the free thyroxine ratio (ETR). Specificity of ETR was better than that of f-T4. Sensitivity of f-T4 was higher in the group with negative TRH-test, but lower in hypothyroidism as compared to ETR. Determination of f-T4 by radioimmunoassay is a simple diagnostic procedure. Since f-T4 RIA is not influenced by changes in TBG-levels its use is particularly recommended in cases where such changes are to be expected such as estrogen application or gravidity.