Diagnosis of Cushing's syndrome is based on a review of your medical history, physical examination and laboratory tests, which help to determine the presence of excess levels of cortisol. Often X-ray exams of the adrenal or pituitary glands are useful for locating tumors. In addition, comparison of old and recent photographs will often highlight the changes in facial and body appearance in patients who develop Cushing's syndrome. The first step in diagnosing Cushing's disease is to confirm the presence of excessive cortisol secretion. This diagnosis is most easily made by the following tests:
Low-dose dexamethasone suppression test that involves taking a small dose of a cortisol-like drug dexamethasone (1 mg) at 11 p.m. and having blood drawn for cortisol the following morning. Normal individuals typically have very low levels of cortisol in these samples, indicating that ACTH secretion is suppressed, while cortisol is readily detected and oftentimes is increased in patients with Cushing's disease.
24-hour urine collection to measure cortisol levels.
Once the diagnosis of Cushing's syndrome is established, the source of the excess cortisol needs to be determined. The source may be from an adrenal gland tumor, ACTH-producing tumor or a pituitary ACTH-producing adenoma. Petrosal sinus sampling is a test used to distinguish the source of the ACTH secretion and should never be performed before the diagnosis of Cushing's syndrome has been established.
If laboratory tests suggest a pituitary adenoma as the cause of Cushing's syndrome, a pituitary magnetic resonance imaging (MRI) is performed to confirm the diagnosis.
Reviewed by health care specialists at UCSF Medical Center.