Thyroid function at 24-28 weeks gestation was compared between 26 mothers with hyperthyroidism (16 on antithyroid treatment and 10 in remission without treatment) and 15 normal pregnant controls. There was no difference in the total and free thyroxine (TT4 and FT4) or in the total and free triiodothyronine (TT3 and FT3) concentrations, but the erythrocyte zinc (RBC Zn) concentration was significantly lower in the study group (187.3 +/- 46.6 mumol/l RBC versus 223.6 +/- 3.7 mumol/l RBC, p less than 0.01). In the study group, both birth weight and gestation at delivery were significantly correlated with TT3 (r = -0.500, p less than 0.02 and r = -0.460, p less than 0.05, respectively) as well as with RBC Zn (r = 0.473, p less than 0.02 and r = 0.573, p less than 0.01, respectively). The results suggest that plasma concentration of TT3 and of RBC Zn at 24-28 weeks gestation may be helpful prognostic factors in the management of pregnant patients with hyperthyroidism.