We have identified subtle abnormalities of thyroid function and glucose control in patients with Rett syndrome. The mean serum total thyroxine (T₄) concentration was significantly lower in a group of subjects with Rett syndrome (6.9 ± 1.5 μg/dl, n = 34; p < 0.0001) than the adult reference range (8.5 ± 1.75 μg/dl, n = 200). This difference remained significant even for the 17 subjects not taking anticonvulsants (7.6 ± 1.5 μg/dl; p < 0.05 vs. adult reference). The difference was more marked when compared to age-adjusted normals, with 10 subjects having a serum total T₄ concentration below normal for age including 3 of 17 of the subjects not taking anticonvulsants. This decrease in serum total T₄ concentration was not due to changes in binding proteins as measured by 3,5,3’-triiodothyronine resin uptake, and was associated with a decreased concentration of thyroid-stimulating hormone (1.7 ± 1.6 mU/l, n = 23 vs. 2.5 ± 1.0 mU/l, n = 200; p < 0.01). Oral glucose tolerance tests were performed in 10 of the subjects with Rett syndrome. They had a delay in the peak glucose and insulin concentrations. Glucose levels were elevated at 1 and 2 hours (p < 0.05), and insulin levels were elevated at 1, 2, and 3 hours (p < 0.05). Two subjects fulfilled criteria for impaired glucose tolerance.