Hypothyroidism is associated with both reduced central 5-HT function and an increased incidence of depression. This study tested the hypothesis that the reduced 5-HT function returns to normal with thyroxine replacement therapy. Seven hypothyroid patients were tested before and after adequate thyroxine replacement. Cortisol and prolactin responses to d-fenfluramine, a centrally acting 5-HT-releasing agent, were used as an index of central (hypothalamic) 5-HT responsiveness. 5-HT-mediated cortisol responses were significantly higher after thyroxine replacement. Basal prolactin levels were reduced, but 5-HT-mediated prolactin responses were not significantly higher after treatment, perhaps due to the pre-treatment responses being elevated by the direct stimulatory effects of hypothyroidism itself on pituitary prolactin secretion. Depressive symptomatology improved with thyroxine. TSH levels were positively related to depressive symptomatology, and inversely to cortisol responses. Depressive symptomatology was inversely related to cortisol responses. These findings thus provide further support that central 5-HT neurotransmission is affected by hypothyroidism. They also suggest that the reduction in 5-HT responsiveness is reversible with thyroxine replacement therapy.