Free thyroxine and triiodothyronine

Value of the free triiodothyronine index in the diagnosis of hyperthyroidism

Normalized T4 (T4N), total T3, T3 resin sponge uptake (T3U) and the response of TSH to TRH were determined in 264 patients sent for diagnosis of thyroid function. Many of these patients were diagnostic problem cases. Those with disease or medication known to invalidate the TRH test, but not those with abnormal thyroid hormone binding capacity in serum, were excluded. A free T3 index (FT3 index) was calculated as the product of total T3 and T3U. The FT3 index tended to decline with increasing age, the mean value being significantly higher in the youngest patients (12-20-years-old) than in the older age groups. In the diagnosis of hyperthyroidism, the FT3 index was superior not only to T4N but also to total T3, particularly in the presence of elevated hormone binding capacity (relatively low T3U). It is anticipated that the FT3 index will be most useful as a routine test for hyperthyroidism, especially in several (though not all) instances where the TRH test is invalidated.

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.