What imaging is needed for a patient with a low Adrenocorticotropic hormone (ACTH) level and a positive dexamethasone suppression test?

Imaging for Low ACTH and Positive Dexamethasone Suppression Test

This patient requires adrenal CT or MRI imaging to identify an adrenal lesion causing ACTH-independent Cushing's syndrome. 1

Understanding the Clinical Picture

The combination of findings in this patient is diagnostic:

• ACTH level of 4.2 pg/mL is suppressed (low/undetectable), indicating ACTH-independent Cushing's syndrome 2
• Positive dexamethasone suppression test (cortisol failed to suppress below 1.8 μg/dL) confirms hypercortisolism 1, 2
• Low ACTH with elevated cortisol definitively indicates an adrenal source of cortisol excess, not a pituitary or ectopic source 1, 2

Diagnostic Algorithm

According to The Lancet Diabetes and Endocrinology consensus guidelines, the diagnostic pathway is straightforward:

1. Measure ACTH levels to determine if Cushing's syndrome is ACTH-dependent or ACTH-independent 1
2. If ACTH is low (as in this case), proceed directly to adrenal CT or MRI 1, 2
3. No pituitary imaging is needed because low ACTH excludes pituitary adenoma (Cushing's disease) and ectopic ACTH syndrome 1, 2

Specific Imaging Recommendation

Order adrenal CT or MRI to identify:

• Adrenal adenoma (most common cause) 2
• Adrenal carcinoma 2
• Bilateral adrenal hyperplasia 2

The Endocrine Society guidelines explicitly state that for ACTH-independent Cushing's syndrome, adrenal imaging is the definitive next step, not pituitary MRI 1, 2

Common Pitfalls to Avoid

Do not order pituitary MRI in this patient. 1, 2 The low ACTH level definitively excludes:

• Cushing's disease (pituitary ACTH-secreting adenoma) 1, 2
• Ectopic ACTH syndrome 1, 2

Do not proceed to inferior petrosal sinus sampling (IPSS). 1 IPSS is only indicated for ACTH-dependent Cushing's syndrome (normal or elevated ACTH) to distinguish between pituitary and ectopic sources 1

Treatment Implications Based on Imaging

Once adrenal imaging identifies the lesion:

• Adrenal adenoma: Laparoscopic adrenalectomy is the treatment of choice 2
• Adrenal carcinoma: Open adrenalectomy with possible adjuvant therapy 2
• Bilateral hyperplasia: Medical management or unilateral adrenalectomy may be considered 2

The key principle is that ACTH levels are the definitive factor in determining whether imaging should target the adrenal glands versus the pituitary 2, and this patient's suppressed ACTH unequivocally points to an adrenal source requiring adrenal imaging 1, 2