Pituitary-thyroid feedback regulation in patients with Graves' disease during antithyroid drug therapy

In an attempt to study the mode of normalization of thyroid function in patients with Graves' disease, a study was made on 140 patients with Graves' disease who were eumetabolic after appropriate therapy with antithyroid drugs for more than 9 months. T3 administration failed to suppress thyroidal radioiodine uptake and serum T4 in patients with TRH-unresponsive TSH secretion. In addition, exogenous TSH failed to elevate serum levels of T4 and T3. In patients with TRH-responsive pituitaries, T3 administration uniformly made serum TSH undetectable but produced various effects (unsuppressible, partially suppressible, and suppressible) on radioiodine uptake and serum T4. The magnitude of suppression of radioiodine uptake paralleled that of serum T4. In patients with unsuppressible or partially suppressible thyroids, exogenous and endogenous TSH were less effective in elevating serum T4 and T3. In patients with suppressible thyroids, T3 administration depressed radioiodine uptake and serum T4; the magnitudes of depression were comparable to those found in normal subjects. Exogenous and endogenous TSH elevated serum T4 and T3 in patients with suppressible thyroids. Here again, the magnitudes of elevation were comparable to those found in the normal subjects. The serum T3 to T4 ratio was high before treatment, but decreased significantly during antithyroid drug therapy. The magnitude of decrease was roughly proportional to the degree of T3 suppressibility.