Diagnosis of Cushing’s Syndrome
Diagnosis of Cushing’s syndrome involves a detailed medical history, physical examination, and a series of tests to measure cortisol levels and identify the underlying cause. Because Cushing’s syndrome presents with symptoms that overlap with many other conditions—such as obesity, diabetes, and polycystic ovary syndrome—diagnosis can be complex and often requires input from an endocrinologist.
Initial Assessment
A doctor will start by reviewing:
History of corticosteroid medication use
Onset and progression of symptoms
Any history of hormone-related illnesses
Family history of endocrine disorders
A physical exam may reveal characteristic signs such as central obesity, purple stretch marks, high blood pressure, and skin thinning.
First-Line Cortisol Testing | Diagnosis of Cushing’s Syndrome
The next step is to assess cortisol levels using one or more of the following tests:
24-hour urinary free cortisol test: Measures cortisol in urine collected over a full day. High levels suggest excessive cortisol production.
Late-night salivary cortisol test: Cortisol should be low at night; elevated levels suggest Cushing’s.
Low-dose dexamethasone suppression test (LDDST): Involves taking dexamethasone at night, which should suppress cortisol production by morning. A failure to suppress cortisol indicates abnormal regulation.
These tests are often repeated to confirm the diagnosis.
Determining the Cause
If excess cortisol is confirmed, further tests determine whether it is ACTH-dependent (due to pituitary or ectopic sources) or ACTH-independent (from adrenal tumours):
Blood ACTH levels: Low ACTH suggests an adrenal source; high ACTH suggests a pituitary or ectopic source.
High-dose dexamethasone suppression test: Helps distinguish between Cushing’s disease (pituitary cause) and ectopic ACTH production.
Imaging Studies
To locate the source of hormone overproduction:
MRI of the pituitary gland to identify adenomas
CT scan or MRI of the adrenal glands for tumours
CT scan of the chest or abdomen if ectopic ACTH production is suspected
Inferior Petrosal Sinus Sampling (IPSS)
In challenging cases, this specialised procedure compares ACTH levels in veins near the pituitary and peripheral blood to determine if the source is pituitary or elsewhere.
In summary, diagnosis of Cushing’s syndrome is methodical and involves hormonal testing and imaging. Accurate identification of the cause ensures appropriate treatment and better outcomes.
[Next: Treatment of Cushing’s Syndrome →]
