1. The authors recently reported that acutely ill depressed adolescents have elevated plasma T4 and fT4 compared to controls. Studies in adult depression suggest antidepressant response is associated with decreases in these elevated levels. The effect of antidepressant treatment on adolescent thyroid indices has not been examined. 2. Thyroid indices were examined in 12 adolescent patients (4 male, 8 female; age 14-19y) in the active treatment arm of a double-blind, placebo-controlled desipramine trial (200 mg/day for 6 weeks). Antidepressant responders had higher pre-treatment levels of T4 and larger decreases were observed responders vs. nonresponders. 3. These results replicate findings observed in adult depressed patients and suggest similar alterations in the hypothalamic-pituitary-thyroid (HPT) axis function in adolescent depression.