Effects of thyroxine as compared with thyroxine plus triiodothyronine in patients with hypothyroidism

Background Patients with hypothyroidism are usually treated with thyroxine (levothyroxine) only, although both thyroxine and triiodothyronine are secr eted by the normal thyroid gland. Whether thyroid secretion of triiodothyro nine is physiologically important is unknown. Methods We compared the effects of thyroxine alone with those of thyroxine plus triiodothyronine (liothyronine) in 33 patients with hypothyroidism. Ea ch patient was studied for two five-week periods. During one period, the pa tient received his or her usual dose of thyroxine. During the other, the pa tient received a regimen in which 50 mu g of the usual dose of thyroxine wa s replaced by 12.5 mu g of triiodothyronine. The order in which each patien t received the two treatments was randomized. Biochemical, physiologic, and psychological tests were performed at the end of each treatment period. Results The patients had tower serum free and total thyroxine concentration s and higher serum total triiodothyronine concentrations after treatment wi th thyroxine plus triiodothyronine than after thyroxine alone, whereas the serum thyrotropin concentrations were similar after both treatments. Among 17 scores on tests of cognitive performance and assessments of mood, 6 were better or closer to normal after treatment with thyroxine plus triiodothyr onine. Similarly, among 15 visual-analogue scales used to indicate mood and physical status, the results for 10 were significantly better after treatm ent with thyroxine plus triiodothyronine. The pulse rate and serum sex horm one-binding globulin concentrations were slightly higher after treatment wi th thyroxine plus triiodothyronine, but blood pressure, serum lipid concent rations, and the results of neurophysiologic tests were similar after the t wo treatments. Conclusions In patients with hypothyroidism, partial substitution of triiod othyronine for thyroxine may improve mood and neuropsychological function; this finding suggests a specific effect of the triiodothyronine normally se creted by the thyroid gland. (N Engl J Med 1999;340:424-9.) (C)1999. Massac husetts Medical Society.