Adrenal Incidentaloma: A New Cause of the Metabolic Syndrome?

A number of patients with adrenal incidentaloma are exposed to a slight degree of cortisol excess resulting from functional autonomy of the adrenal mass (usually a cortical adenoma). At present, there are only scant data on the unwanted effects of this endocrine condition referred to as subclinical Cushing's syndrome. The aim of the present study was to look for some features of the metabolic syndrome in patients with incidental adrenal adenoma. Forty-one patients (9 men and 32 women) bearing adrenal incidentaloma with typical computed tomography features of cortical adenoma were studied. For both patients and controls, exclusion criteria were age equal to 70 yr or greater, previous history of fasting hyperglycemia, or impaired glucose tolerance (IGT), severe hypertension, current use of medication or concomitant relevant illnesses, and body mass index (BMI) equal to 30 kg/m(2) or greater. Forty-one patients with euthyroid multinodular goiter accurately matched for sex, age, and BMI served for a 1:1 case-control analysis. The study design included an oral glucose tolerance test (75 g) and an endocrine workup aimed at the study of the hypothalamic-pituitary-adrenal axis. Age and BMI were fully comparable between patients (54.0 +/- 10.7 yr, 23.8 +/- 2.4 kg/m(2)) and controls (52.2 +/- 11.6 yr, 23.5 +/- 2.8 kg/m(2)). Fasting glucose and fasting insulin levels were not different between the two groups (4.96 +/- 0.61 mmol/liter vs. 4.88 +/- 0.58 mmol/liter; 67 +/- 34 pmol/liter vs. 59 +/- 32 pmol/liter), but the 2-h postchallenge glucose was significantly higher in patients than in controls (7.43 +/- 2.49 mmol/liter vs. 6.10 plus minus 1.44 mmol/liter, P = 0.01). Fifteen patients (36%) reached the World Health Organization criteria for IGT and two other patients (5%) reached those for diabetes, and 14% of the controls qualified for IGT (P = 0.01). No difference in the lipid pattern was seen between the two groups, but either systolic or diastolic blood pressure were higher in patients (135.4 +/- 15.5 mm Hg vs. 125.0 +/- 15.6 mm Hg, P = 0.003; 82.9 +/- 9.1 mm Hg vs. 75.3 +/- 6.6 mm Hg, P < 0.0001). We calculated the whole-body insulin sensitivity index derived from the oral glucose tolerance test that was significantly reduced in the patients (4.3 +/- 1.7 vs. 5.7 +/- 2.5, P = 0.01). In a multiple regression analysis, 2-h glucose was associated with BMI and midnight cortisol values (r(2) = 0.36, P = 0.002). The comparison of the patients with nonfunctioning adenoma (n = 29) with those with subclinical Cushing's syndrome (n = 12) yielded significant differences as to 2-h glucose and triglyceride levels, which were significantly higher in the second group (7.02 +/- 1.76 mmol/liter vs. 8.72 +/- 3.17 mmol/liter, P = 0.03; 1.06 +/- 0.4 mmol/liter vs. 1.73 +/- 0.96 mmol/liter, P = 0.002), but the insulin sensitivity index was conversely reduced (5.2 +/- 1.4 vs. 2.9 +/- 1.2, P < 0.0001). In conclusion, many patients with incidental adrenal adenoma display altered glucose tolerance, that may be explained by reduced insulin sensitivity, and increased blood pressure levels in comparison with carefully age- and BMI-matched controls. The slight hypercortisolism observed in some such patients may significantly contribute to this state of insulin resistance. Midnight serum cortisol appears as a sensitive marker of the metabolic effects of subclinical Cushing's syndrome.