Radioimmunoassay of free thyroid hormone concentrations in the investigation of thyroid disease.

Direct radioimmunoassay of free triiodothyronine (fT3) and free thyroxine (fT4) in serum dialysate has been performed in fifty-two normals, twenty-one hyperthyroid, and twenty-one hypothyroid patients. In health, there is an almost equimolar concentration of fT3 and fT4. In hyperthyroidism, the free thyroid hormone concentrations were clearly discriminated from the normals. In a proportion of the symptomatic hypothyroids studied, the free hormone values fell within the normal range, challenging the concept that free thyroid hormone concentrations are the sole determinants of thyroid status. In the population studied, there were statistically significant correlations between fT3 and fT4, as well as the free hormones with their respective total hormone levels. It is concluded that fT3 and fT4 estimated are unlikely to play a major role in the routine investigation of thyroid disease.