Lower Serum Free Thyroxine (T4) Levels in Painless Thyroiditis Compared with Graves' Disease Despite Similar Serum Total T4Levels

Abstract

Serum total T₄ (T₄), total T₃ (T₃), free T₄ (FT₄), free T₃ (FT₃), and T₄-binding globulin concentrations and T₃ resin uptake values were measured in 17 women with thyrotoxicosis due to painless thyroiditis (PT) and compared with the same parameters in 17 women with thyrotoxicosis due to Graves' disease (GD) with similar serum T₄ levels. The mean serum T₃ resin uptake value and T₃, FT₄, and FT₃ concentrations in the PT patients were significantly lower than those in the GD patients. The mean serum T₄-binding globulin concentration 20.2 ± 4.2 (±sd) μg/mL] in patients with PT did not differ significantly from those in patients with GD (18.0 ± 2.6 μg/ mL)and normal euthyroid women (21.9 ± 4.0 μg/mL). The serum T₃ to T₄ (nanogram per μg) ratio was higher than 20 in 14 GD patients, but lower than 20 in all patients with PT, whereas the individual serum FT₃ to FT₄ ratio values considerably overlapped in the 2 groups. In patients with PT, FT₄ correlated well with T₄ at various times during the clinical course. These findings indicate that the elevation in serum FT₄ in patients with PT is mostly due to the increase in circulating T₄ levels, whereas GD patients also have some diminution in T₄ binding. The serum T₃ to T₄ ratio, but not the FT₃ to FT₄ ratio, may be helpful for differentiation between the two diseases.