Clinical significance of the measurements of serum free thyroxine and free triiodothyronine concentrations

A commercially available ''DPC'' radioimmunoassay kit was used to study the serum concentrations of free triiodothyronine (FT3) and free thyroxine (FT4) in a series of 189 patients with various thyroid diseases and 120 healthy controls. The basal serum concentrations of FT3 and FT4 in normal controls ranged from 0.98 to 2.04 ng/dl and from 1.43 to 3.66 pg/ml, respectively. All untreated patients with Graves' disease had abnormally high FT3 and FT4 values, indicating the discrimination between hyperthyroid and normal subjects. A decreased ratio of FT4 to FT3 was observed in patients managed with antithyroid drugs. In diagnosing hypothyroidism, ''DPC'' FT4 kits were more sensitive than ''DPC'' TT4 and ''Amerlex'' FT4 kits. In the case of non-thyroid diseases, FT4 and FT3 values, as well as total T4 and total T3, were normal or decreased, with the exception of occasionally unknown high values. The interference of thyroxine binding globulin abnormablity was successfully eliminated by using new versions of ''DPC'' kits. (Namekawa, K.)