A major reduction in T3 turnover has been demonstrated previously in clinically hypothyroid patients. We have used non-compartmental (NC) and monocompartmental (MC) analysis to study ten patients with Graves' disease who, following treatment with radioactive iodine (RAI), are now clinically euthyroid but who showed hyper-responsiveness to TRH although serum T3 and T4 concentrations are within the normal range. T3 production rate (PR), metabolic clearance rate (MRC) and fractional-turnover (K) were all significantly reduced in patients compared with seven controls (P less than 0.01). T3, MCR and PR were consistently higher, and T3 K lower, when calculated by MC, than values calculated by NC analysis. The difference in T3 production rates between patients (mean 16.6 nmol/day) and controls (mean 38.9 nmol/day) raises the question of replacement therapy in patients who are apparently euthyroid but TRH hyper-responsive.