Exaggeration of defective platelet function by methyldopa in hypothyroidism.

Platelet function was analyzed in two hypothyroid patients who were taking methyldopa. Results from these two patients are compared to those obtained in seven others with hypothyroidism on no medications. Bleeding times in the two drug patients were 33 and 26 minutes compared to normal values (4.0 to 8.0 minutes) in the other hypothyroid individuals. In addition, platelet aggregation responses to epinephrine, collagen, and ristocetin were abnormal in these two patients; these were normal in the others with hypothyroidism. Repeat studies were done in the first drug patient when she was euthyroid and still receiving methyldopa. Her bleeding time decreased to 11 minutes, platelet retention increased from 31% to 75%, and abnormal aggregation responses corrected to normal. Factor VIII coagulant activity, VIII antigenic activity, VIII ristocetin cofactor activity, and two-dimensional immunoelectrophoresis of VIII antigen were normal. These findings suggest that methyldopa can potentiate the mild platelet function defect seen in patients with hypothyroidism.