Carbamazepine and false positive dexamethasone suppression tests for Cushing's syndrome

Introduction

The clinical features of Cushing's syndrome may be subtle to begin with, and they resemble those of patients with metabolic syndrome. The key to diagnosing Cushing's syndrome is a high index of suspicion and the use of screening tests. The 1 mg overnight dexamethasone suppression test is commonly used. Subsequent biochemical evaluation often uses higher doses of dexamethasone. The overnight dexamethasone suppression test is also increasingly used in the functional evaluation of incidental adrenal masses.

We describe two patients, both of whom were suspected of having Cushing's syndrome based on abnormal results from overnight dexamethasone suppression tests. Subsequent evaluation excluded Cushing's syndrome in both patients. The false positive suppression tests were because of accelerated metabolism of dexamethasone caused by carbamazepine. Awareness of this potential pitfall in the diagnosis of Cushing's syndrome is important to avoid unnecessary investigations and anxiety.