Risks of Impaired Glucose Tolerance in Turner Syndrome
© 2005 Prime Health Consultants, Inc.
Choi and colleagues performed oral glucose tolerance tests (OGTT) in 103 patients with Turner syndrome who had normal fasting and postprandial glucose levels. Plasma glucose, insulin, C-peptide, and proinsulin were measured every 30 minutes and insulin resistance was evaluated using a homeostatic model assessment (HOMA) and a quantitative insulin sensitivity check index (QUICKI). The OGTT was a standard 1.75 g/kg with a maximum of 75 g of glucose. Impaired glucose tolerance (IGT) was defined according to ADA criteria of a fasting glucose below 126 mg/dl and a 2-hour level between 140 - 200 mg/dl. Some of the patients were receiving recombinant growth hormone.
Eighteen of the subjects were noted to have IGT and 2 were diagnosed as having diabetes mellitus. BMI and positive family history of diabetes were not different in the IGT group compared to the normal glucose tolerance (NGT) group. There was no relationship between positive history of growth hormone administration and duration of therapy in either group as well. During the OGTT plasma glucose levels were higher in the IGT group than in the NGT group, particularly at 60, 90, and 120 minutes while plasma insulin levels were higher at 0 and 120 minutes. HOMA was similar in both groups, but QUICKI was lower in the IGT group than in the NGT group. Fasting plasma triglyceride level was higher in the IGT group while total cholesterol, HDL, LDL and free fatty acid levels were similar in both groups. Fasting and 2-hour plasma insulin, proinsulin, and C-peptide, as well as triglycerides and free fatty acid levels showed significant correlations with 2-hour plasma glucose level. However, only fasting insulin and plasma triglyceride levels strongly predicted the 2-hour glucose level.
The authors suggested that testing with an oral glucose tolerance test is superior to the fasting and postprandial glucose test for the early detection of abnormalities of carbohydrate metabolism in girls with Turner syndrome.
Editor’s Comment: This is a very comprehensive study of carbohydrate metabolism in patients with Turner syndrome. It is not entirely surprising that over 10% had impaired glucose tolerance and that a variety of hormone levels were elevated in the IGT group - including insulin, C-peptide, and proinsulin. What is not clear is why the authors claim that the oral glucose tolerance test is superior to the fasting and postprandial glucose test for the early detection of abnormalities of carbohydrate metabolism. Clearly their data show that the fasting plasma insulin and triglycerides most strongly and significantly predicted the 2-hour glucose level and, in fact, it is the 2-hour glucose level that predicted the diagnosis of impaired glucose tolerance.
It is interesting that there was no difference between the NGT and IGT groups with regard to growth hormone administration or duration of growth hormone, nor lipids other than triglycerides. Pediatric endocrinologists and geneticists have been aware of the potential for growth hormone to be associated with impairment in glucose tolerance. Since such metabolic abnormalities are known to occur in girls without Turner syndrome who have never received growth hormone, these findings are particularly reassuring. Since BMIs were not different between the NGT and IGT groups, this suggests that caloric restriction and increased physical activity may not have the same effects in girls with Turner syndrome as have been shown in large populations of allegedly normal chromosomal individuals with IGT.
William L. Clarke, MD