Influence of carbamazepine on serum thyroxine and triiodothyronine in patients with epilepsy.

Hypothyroidism induced by anti-epileptic drug treatment gave rise to thyroid function test studies in patients treated with carbamazepine (CBZ) only. In 42 patients on long-term CBZ treatment thyroxine (T4), free T4-index (FT4I), and triiodothyronine (T3) concentrations in serum were significantly lower than in controls, while triiodothyronine uptake (T3U) and thyrotropin (TSH) concentrations did not differ between patients and controls. In 12 patients starting on CBZ, means T4, calculated FT4 and thyroxine binding globulin (TBG) were 1-5 months later reduced compared to the initial levels. Thus, CBZ reduced thyroid hormones, TBG and FT4I. A CBZ-induced increase in conversion and metabolism of the thyroid hormones could explain this effect. The normal T3U values and decreased concentrations of TBG make a competitive CBZ binding to TBG less probable. Although the thyroid hormones levels were found lowered in the patients, all remained clinically euthyroid during the study.