A 52-year-old man with confirmed hypercortisolism (elevated 24-hour UFC and abnormal LDDST) is being investigated for the cause of Cushing syndrome. His ACTH level is 8 pg/mL (normal 10–46 pg/mL). Which is the most appropriate next investigation to localize the source?

Explanation

## Localization of ACTH-Independent Cushing Syndrome ### Clinical Context **Key Point:** A suppressed ACTH level (<5 pg/mL) in the setting of confirmed hypercortisolism indicates **ACTH-independent (primary adrenal) Cushing syndrome**. The source is the adrenal gland itself, not the pituitary or ectopic source. ### Diagnostic Algorithm for ACTH-Independent Cushing ```mermaid flowchart TD A[Hypercortisolism confirmed]:::outcome --> B[Measure ACTH]:::action B --> C{ACTH level?}:::decision C -->|Suppressed <5 pg/mL| D[ACTH-independent Cushing]:::outcome D --> E[CT abdomen/pelvis]:::action E --> F{Adrenal mass?}:::decision F -->|Yes| G[Characterize: adenoma vs carcinoma]:::action F -->|No| H[Bilateral hyperplasia or micronodular disease]:::outcome C -->|Elevated >15 pg/mL| I[ACTH-dependent Cushing]:::outcome I --> J[HDDST or CRH test]:::action J --> K{Suppression?}:::decision K -->|Yes| L[Cushing disease - MRI pituitary]:::action K -->|No| M[Ectopic ACTH - CT chest/abdomen]:::action ``` ### Why CT Abdomen/Pelvis is Correct **High-Yield:** In ACTH-independent Cushing syndrome, the pathology is in the adrenal gland (adenoma, carcinoma, or bilateral hyperplasia). Imaging of the adrenal glands is the next logical step to identify the lesion and assess its characteristics (size, density, imaging features suggestive of malignancy). **Clinical Pearl:** - Adrenal adenomas are typically <4 cm, have low Hounsfield units (<10 HU), and show rapid washout on contrast CT - Adrenal carcinomas are usually >4 cm, heterogeneous, and may show invasion or metastases - Bilateral adrenal hyperplasia is seen in primary bilateral macronodular adrenal hyperplasia (PBMAH) or primary pigmented nodular adrenocortical disease (PPNAD) ### Comparison of Next Investigations in ACTH-Independent Cushing | Investigation | Indication | Role | |---|---|---| | **CT abdomen/pelvis** | ACTH-independent Cushing | **CORRECT** — Localizes adrenal pathology (adenoma, carcinoma, hyperplasia) | | **HDDST** | ACTH-dependent Cushing (elevated ACTH) | Differentiates Cushing disease from ectopic ACTH; not needed here | | **CRH stimulation** | ACTH-dependent Cushing (elevated ACTH) | Confirms pituitary source in Cushing disease; not needed here | | **MRI pituitary** | Cushing disease (ACTH-dependent) | Localizes pituitary adenoma; not indicated when ACTH is suppressed | **Warning:** Do NOT order HDDST, CRH test, or pituitary MRI in ACTH-independent Cushing. These tests are designed to differentiate pituitary from ectopic ACTH sources — irrelevant when ACTH is already suppressed. ### Interpretation of ACTH Levels **Mnemonic:** **ACTH-I = Adrenal Imaging** - **I**ndependent ACTH suppression → **I**maging of adrenal gland - **ACTH <5 pg/mL (suppressed)** → ACTH-independent → Adrenal source → **CT abdomen** - **ACTH 10–15 pg/mL (borderline)** → Repeat ACTH or CRH stimulation - **ACTH >15 pg/mL (elevated)** → ACTH-dependent → Pituitary/ectopic source → HDDST or CRH test, then MRI pituitary or CT chest [cite:Harrison 21e Ch 375; Endocrine Society Clinical Practice Guidelines for Cushing Syndrome (2012)]