Differences between morning and afternoon hormonal responses to d-fenfluramine in healthy humans

The prolactin (PRL) and cortisol responses to oral d-fenfluramine (30 mg) and placebo were measured in seven healthy subjects (two women and five men) in the morning and in the afternoon. As compared to placebo, d-fenfluramine significantly increased plasma PRL levels in both the morning and the afternoon, with no significant circadian difference. On the contrary, d-fenfluramine significantly enhanced plasma cortisol levels in the afternoon, but not in the morning. These data suggest that the time of the day at which the d-fenfluramine challenge test is carried out may be an important variable in determining the glucocorticoid response to the 5-HT releasing agent in humans.