[Insulin resistance in patients with Graves' disease and reduced glucose tolerance. The normalization of fasting insulin secretion in parallel with the restoration of thyroid function].

In order to investigate the nature of impaired glucose tolerance (IGT) in 3 young patients with Graves' disease we have studied their insulin secretion fasting and in response to oral glucose by means of measurement of serum C-peptide. Fasting levels of serum C-peptide of these patients were beyond the range of 15 age-matched normal subjects; the C-peptide/glucose ratio was also significantly higher (p less than 0.001) in the patients than in the controls. Following glucose ingestion serum levels of C-peptide resulted high in the range of normals, with a mean C-peptide/glucose ratio greater than in the controls, but without reaching of statistical significance. To investigate whether the anomaly in fasting insulin secretion of these patients had any correlation with their hyperthyroidism, afterwards we surveyed fasting concentrations of serum C-peptide in parallel with progressive variations of serum free thyroxine and triiodothyronine (FT4 and FT3) and thyrotrophin (TSH) during antithyrotoxic treatment with methimazole. The data of 23 tests on serum FT3 and FT4 levels, carried out during 16-18 months, resulted in significant correlation with contemporaneous measurements of fasting serum C-peptide (p less than 0.001). No significant correlation was found between serum TSH and fasting C-peptide levels. The results suggest that IGT of the patients in this study is not dependent on lacking insulin secretion, but on mild insulin resistance. Such glucose metabolic anomaly appears to be in clear correlation with the degree of hyperthyroidism, even if its pathogenesis remains to be further investigated.