Total serum thyroxine and triiodothyronine; a comparison between Graves' disease and hyperthyroxinaemia due to thyroxine replacement.

Serum concentrations of total thyroxine (T4) and total triiodothyronine (T3) were measured in a group of patients (n = 113) presenting with untreated hyperthyroidism due to Graves' disease and in subjects receiving oral T4 replacement (n = 93) in whom the total T4 concentration was supraphysiological (greater than 150 nmol/l). The mean total T4 concentration in the hyperthyroid group was 226 nmol/l, SD 59, range 151-420, and the mean total T3 concentration was 6.8 nmol/l, SD 2.73, range 3.1-17.5. For the group receiving T4 the mean total T4 concentration was 175 nmol/l, SD 25, range 150-258, and the mean total T3 concentration was 2.66 nmol/l, SD 0.45, range 1.7-4.2. In the hyperthyroid group a highly significant linear correlation was found between total T4 and total T3, T3 = 0.0354 T4 - 1.21, r = 0.761, P much less than 0.001, while in the patients taking T4 this correlation was less close, T3 = 0.0073 T4 + 1.39, r = 0.398, P much less than 0.001. The two groups are readily distinguished by expressing total T4 as a molar ratio of total T3. In the hyperthyroid group the mean T4:T3 ratio was 35.6, SD 7.8, range 19.9-56.1, compared to the patients on T4 where the mean T4:T3 ratio was 67.0, SD 11.7, range 44.3-114 (t = 22.5, P much less than 0.0001). An arbitrarily chosen value of 50 for the T4:T3 ratio affords a simple and convenient means of distinguishing the two categories: in only 3 patients with Graves' disease (2.6%) was the ratio above this, and it was below in only 5 patients (5.4%) taking T4.(ABSTRACT TRUNCATED AT 250 WORDS)