A 38-year-old woman presents with weight gain, easy bruising, proximal muscle weakness, and hypertension. 24-hour urinary free cortisol is 4× the upper limit of normal. Late-night salivary cortisol is elevated on two occasions. The next most appropriate investigation to establish the CAUSE is:

• A Plasma ACTH level ✓
• B MRI of the pituitary gland
• C Bilateral inferior petrosal sinus sampling (BIPSS)
• D High-dose dexamethasone suppression test (8 mg overnight)

Explanation

Once hypercortisolism is confirmed biochemically, the next step is to measure plasma ACTH to differentiate ACTH-dependent (Cushing's disease or ectopic) from ACTH-independent (adrenal adenoma/carcinoma) causes. ACTH < 5 pg/mL points to an adrenal source and mandates adrenal imaging. BIPSS is reserved for ACTH-dependent cases where pituitary MRI is non-diagnostic. The high-dose DST helps distinguish pituitary from ectopic but is a later step.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.