Abnormalities of thyroid function and glucose control in subjects with Rett syndrome.

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 microgram/dl, n = 34; p < 0.001) than the adult reference range (8.5 +/- 1.75 microgram/dl, n = 200). This differences remained significant even for the 17 subjects not taking anticonvulsants (7.6 +/- 1.5 microgram/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.