The Effect of Diphenylhydantoin on Thyroid Function

Abstract

The effect of diphenylhydantoin (DPH) on serum T4, a free T4 index, serum T3, serum TSH, TSH response to iv TRH and absolute iodine uptake (AIU) of the thyroid gland was studied in 26 patients receiving long-term DPH treatment and was compared to controls. In the DPH-treated group, serum T4, free T4 index and serum T3 were decreased to 75% of control values. Serum TSH and AIU were significantly increased. Twelve patients were studied before and after 1–2 weeks of treatment with DPH and a significant decrease in serum T4 and free T4 index was noted. Serum T3 decreased but not significantly. Serum TSH was significantly increased, however, the increase in TSH response to TRH and in AIU were not statistically significant. In 7 patients with hyperthyroidism 2–4 weeks' treatment with DPH was followed by a significant decrease in serum T4, serum T3 and free T44 index. It is suggested that DPH primarily acts as a stimulator of hepatic degradation of the thyroid hormones.