Twelve males with fragile-X syndrome between the ages of three and 28 years underwent assessment of thyroid function. All 12 subjects demonstrated normal baseline values for thyroid stimulating hormone (TSH), thyroxine, thyroid binding globulin (TBG), and estimated free thyroxine (EFT). Relative to a control group reported in the literature, however, the fragile-X subjects exhibited a blunted TSH response to thyrotropin releasing hormone (TRH). This finding suggests the presence of subtle dysfunction within the hypothalamic-pituitary-thyroid axis. Elevated baseline prolactin levels were also observed among the fragile-X subjects. These results support previous reports of hypothalamic-pituitary abnormalities among fragile-X syndrome males.