Increased insulin sensitivity in patients with aldosterone producing adenoma

OBJECTIVE Primary aldosteronism is a recognized endocrine cause of glucose intolerance. A blunted insulin response to glucose in patients with primary aldosteronism is well known, but insulin sensitivity has not been thoroughly determined. We investigated insulin sensitivity and insulin secretion in patients with aldosterone producing adenoma. DESIGN Glucose tolerance and insulin responses to glucose were evaluated using a standard 75 g oral glucose tolerance test in patients with aldosterone producing adenoma before and after surgery, and in control subjects. Parameters for insulin resistance (HOMA-R) and pancreatic β-cell function (HOMA-βF) were derived from a homeostasis model assessment. PATIENTS Fifteen patients with aldosterone producing adenoma and 41 control subjects with normal glucose tolerance matched for age and body mass index, were studied. Eight patients were re-evaluated after surgical removal of the adenoma. In 5 patients ¹²⁵I-insulin binding to erythrocytes was also measured. MEASUREMENTS Plasma glucose was measured by a glucose oxidase method. Plasma insulin, aldosterone and renin activity were measured by radioimmunoassay. RESULTS Both fasting plasma glucose and insulin were lower in the patients than in the controls. Although the areas under the curve for plasma glucose during the oral glucose tolerance test did not differ, that for plasma insulin was less in the patients with hyperaldosteronism. Insulin sensitivity was increased in the patients, as evidenced by decreased HOMA-R compared with controls, while HOMA-βF was comparable between the groups. ¹²⁵I-insulin binding to erythrocytes was higher in 5 patients than in 19 normal controls. After surgical removal of adenoma, neither fasting plasma glucose nor the glucose area under the curve during the oral glucose tolerance test changed. However, fasting plasma insulin and the insulin area under the curve increased significantly. HOMA-R increased and HOMA-βF remained unaltered. CONCLUSION Insulin sensitivity was increased in untreated patients with aldosterone producing adenoma. Enhanced insulin receptor binding may contribute to this increased insulin sensitivity.