Imaging for Low ACTH and Positive Dexamethasone Suppression Test

Understanding the Clinical Picture

The Endocrine Society recommends that a patient with low ACTH levels and a positive dexamethasone suppression test undergo adrenal CT or MRI imaging to identify an adrenal lesion causing ACTH-independent Cushing's syndrome 1, 2
A combination of low ACTH levels (indicating ACTH-independent Cushing's syndrome) and a positive dexamethasone suppression test (confirming hypercortisolism) is diagnostic, with the low ACTH level definitively indicating an adrenal source of cortisol excess 1, 3

The Lancet Diabetes and Endocrinology consensus guidelines recommend measuring ACTH levels to determine if Cushing's syndrome is ACTH-dependent or ACTH-independent, and if ACTH is low, proceeding directly to adrenal CT or MRI 1, 2
No pituitary imaging is needed because low ACTH excludes pituitary adenoma (Cushing's disease) and ectopic ACTH syndrome, according to The Lancet Diabetes and Endocrinology guidelines 1, 3

The Endocrine Society guidelines explicitly state that for ACTH-independent Cushing's syndrome, adrenal imaging is the definitive next step, not pituitary MRI, to identify adrenal adenoma, adrenal carcinoma, or bilateral adrenal hyperplasia 1, 2, 3

The Lancet Diabetes and Endocrinology guidelines advise against ordering pituitary MRI in patients with low ACTH levels, as it definitively excludes Cushing's disease (pituitary ACTH-secreting adenoma) and ectopic ACTH syndrome 1, 3
Inferior petrosal sinus sampling (IPSS) is not indicated for ACTH-independent Cushing's syndrome, according to The Lancet Diabetes and Endocrinology guidelines, as it is only used to distinguish between pituitary and ectopic sources in ACTH-dependent Cushing's syndrome 1, 4

Once adrenal imaging identifies the lesion, treatment options include laparoscopic adrenalectomy for adrenal adenoma, open adrenalectomy with possible adjuvant therapy for adrenal carcinoma, and medical management or unilateral adrenalectomy for bilateral hyperplasia, as recommended by Praxis Medical Insights: Practical Summaries of Clinical Guidelines 3