Thyroxine, triiodothyronine, and thyrotropin in serum during long-term diphenylhydantoin therapy

Clinical laboratory tests for assessment of thyroid function were performed in 31 patients receiving long-term diphenylhydantoin (DPH) therapy. Reduced serum total thyroxine and free thyroxine index were found. The decrease in serum total triiodothyronine and free triiodothyronine index was smaller but statistically significant. DPH had no significant effect on the serum thyrotropin (TSH) concentration and maximal TSH response to TSH-releasing hormone. The absence of reciprocal changes in TSH and serum thyroxine might be due to the near-normal triiodothyronine concentration. Such an explanation would also conform with previous findings that the pituitary contains specific receptors for triiodothyronine but not thyroxine.