Congenital adrenal hyperplasia, an autosomal recessive condition, is mainly caused by mutations in the gene 21-hydroxylase and is treated with glucocorticoids in a slightly supraphysiological dose. Adult patients seem to be characterized by insulin resistance, which may be caused by the glucocorticoids and/or the accompanying obesity. The hypothesis of this study is that pioglitazone can improve insulin sensitivity and correlated cardiovascular risk factors in this specific group of patients. This will be tested in a randomized, placebo-controlled, cross-over trial; insulin sensitivity will be quantified by euglycemic hyperinsulinemic clamp studies.

Inclusion Criteria:

• biochemical and genetically proven congenital adrenal hyperplasia
• stable corticosteroid replacement for 3 months

Exclusion Criteria:

• age < 18 years
• inability to give informed consent
• significant cardiovascular disease, defined as myocardial infarction or stroke, six months preceding the study
• significant renal disease, GFR < 30 ml/min
• significant liver disease, defined as more than 3 times upper limit of normal values of alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
• pregnancy
• mental disease