Serum free thyroxine and triiodothyronine concentrations in subacute thyroiditis.

Serum absolute free T4 (AFT4) and T3 (AFT3) concentrations were studied in 10 patients with subacute thyroiditis whose total serum T4 levels were elevated, and values were compared with those obtained in 10 patients with untreated Graves' disease, whose total serum T4 concentrations were elevated to the same level as in the former group. The mean (+/- SD) basal metabolic rate in patients with subacute thyroiditis was 16 +/- 14.8%, significantly (P less than 0.001) lower than that in patients with Graves' disease (48.7 +/- 15.7%). Serum T4-binding globulin concentrations measured by RIA were normal and did not differ from those in Graves' disease patients. The percentage of free T4 and AFT4 were both elevated in subacute thyroiditis to a degree comparable to that seen in Graves' disease. The serum total T3, serum free T3 fraction and AFT3 were also elevated, but both total T3 and AFT3 were significantly (P less than 0.05 and P less than 0.05, respectively) lower than in Graves' disease patients. Therefore, the ratios of T3 to T4 and AFT3 to AFT4 were significantly (P less than 0.01 and P less than 0.01, respectively) lower than in Graves' disease patients. These results indicate that the significantly lower basal metabolic rate and milder manifestations of thyrotoxic symptoms in subacute thyroiditis than in Graves' disease might be explained by the low ratios of T3 to T4 and AFT3 to AFT4 as well as the short duration of chemical hyperthyroidism.