• Thyroid function was examined in 50 affectively ill patients before and four weeks after carbamazepine treatment. Carbamazepine significantly and substantially decreased peripheral thyroid hormone levels while increases in thyrotropin levels, although significant, were of much smaller magnitude. Furthermore, the decreases in levels of thyroxine (T4) and free T, were significantly greater in carbamazepine responders than in nonresponders. These findings are discussed in light of current theories of the role of the thyroid axis in affective illness.