The influence of carbamazepine on thyroid hormones and thyroxine binding globulin in hypothyroid patients substituted with thyroxine.

Carbamazepine (CBZ) decreases the serum concentration of thyroid hormones. It is proposed that CBZ increases the extra-thyroidal metabolism of thyroid hormones. In order to test this hypothesis CBZ was given to nine hypothyroid patients substituted with thyroxine (T4). A significant decrease in serum concentrations of T4, calculated free T4 (FT4), triiodothyronine (T3), and calculated free T3(FT3) was found after 3 weeks of CBZ medication. The serum concentrations of TSH and the T4:T3 ratios were unaltered, while the serum concentrations of T4-binding globulin (TBG) increased markedly in eight of the nine patients. These findings support the hypothesis of a CBZ induced increase in the extra-thyroidal metabolism of thyroid hormones.