Evidence of hyperthyroidism in apparently euthyroid patients treated with levothyroxine

A concentration of serum thyroxine (T4) above the accepted normal range has recently been recognized to result from commonly prescribed replacement dosages of levothyroxine sodium. To determine if the high levels of serum T4 have undesirable metabolic effects, despite the fact that the subjects are accepted as euthyroid, we studied 28 patients receiving long-term levothyroxine therapy; 19 patients were considered to be receiving replacement dosages and nine suppressive dosages of levothyroxine. To assess the effect of levothyroxine on target tissue, we measured the thyrotropin response to protirelin and systolic time intervals obtained by simultaneous electrocardiography and echocardiography. Thyrotropin response to protirelin was suppressed in patients with elevated serum T4 levels and normal serum triiodothyronine levels. These patients also had shortened systolic time intervals typical of hyperthyroidism. Our data indicate that commonly given replacement dosages of levothyroxine may induce undesirable metabolic consequences and that these patients perhaps ought to be seen as having "subclinical hyperthyroidism." The prescribed dosage of levothyroxine as therapy for hypothyroidism is still frequently excessive.