Serum insulin, blood sugar, and growth hormone levels were measured in response to a 50g oral glucose tolerance test in 10 patients with proven dystrophia myotonica. Three patients belonged to one family; seven patients had no known family history of the disease. One patient, a chronic invalid aged 56 years, produced a mild diabetic glucose tolerance curve and a delayed prolonged rise in serum insulin. Six of the group, including the three affected members from one family, exhibited normal glucose tolerance and fasting serum insulin values, but a markedly exaggerated rise in peripheral insulin levels maximal at 30 and 60 min. This abnormality showed no correlation with age of onset of the disease nor with severity of the muscle weakness. Growth hormone levels were normal in all of the patients studied. It is concluded that an excessive rise in circulating immunoreactive insulin in response to glucose is a common abnormality in dystrophia myotonica and reflects genetic heterogeneity in this condition. Futhermore, if the index patient in a family demostrates this abnormality, it is suggested that the 30- or 60-min blood insulin level during a glucose tolerance test is a useful methold of intra-family screen-ing for asymptomatic heterozygotes at an early stage before the development of physical defects.
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