Interference of anticonvulsants with the metabolism of hormones may lead to side effects. Anticonvulsants influence the hypothalamic-pituitary-thyroid axis, in that lowering of thyrotropin [thyroid-stimulating hormone (TSH)] and thyroxine levels is observed. In the present study, flunarizine, a calcium ion blocker, administered as add-on medication, further lowered serum thyroxine but not TSH. The underlying biochemical principle may be an interference with the mediation of signals by calcium-calmodulin at the level of the thyroid follicle.