OBJECTIVES - While carbamazepine (CBZ) decreases thyroid hormone concentrations it rarely causes hypothyroidism. We assessed prospectively the early effect of CBZ on thyroid status in thyroxine-supplemented hypothyroid patients, when compared with patients without a thyroid disorder. METHODS - In 29 patients, thyrotropin (TSH), total thyroxine (TT4) and free thyroxine (FT4) serum levels were assayed before starting CBZ, and then weekly for 7 weeks. Nineteen patients with no thyroid disorder (group A) were compared with 10 thyroxine-supplemented hypothyroid patients, stable before CBZ treatment (group B). RESULTS - In group A, TT4 decreased significantly by ca. 15-25%, starting from the first week (Friedman, P < 0.001). FT4 decline was smaller (ca. 10-15%) and delayed till the second week. FT4/TT4 ratio increased significantly (P < 0.001), while TSH only slightly (P = 0.073), never exceeding normal range. In group B, similar TT4 and FT4 decline was followed by significantly increasing TSH (P = 0.011), while the FT4/TT4 ratio was not significantly changed. In 3 of 10 patients TSH rose over 5 mIU/l, necessitating treatment adjustment. CONCLUSIONS - In patients with no thyroid disorder, CBZ causes hormonal changes of no clinical relevance, due to adaptive response. In T4-supplemented hypothyroid patients this adaptation is lacking, CBZ may precipitate subclinical or overt hypothyroidism, and early thyroid function monitoring seems advisable.

Department of Neurology Charles University Prague Facutly of Medicine and University Hospital Hradec Kralove Czech Republic

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