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

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Abstract
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 T3 uptake (T3U) and thyrotropin (TSH) concentrations did not differ between patients and controls. In 12 patients starting on CBZ, mean T4, calculated FT4 and T4 binding globulin (TBG) were 1-5 mo. later reduced compared to the initial levels. 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 hormone levels found were lower in the patients, all remained clinically euthyroid during the study.