A 52-year-old man with COPD on long-term inhaled corticosteroids presents with recurrent infections, proximal muscle weakness, and hypertension. Iatrogenic Cushing syndrome is suspected. Which investigation is most specific to confirm systemic corticosteroid-induced hypercortisolism in this patient?

Explanation

## Diagnosis of Iatrogenic Cushing Syndrome **Key Point:** In iatrogenic (exogenous corticosteroid-induced) Cushing syndrome, **plasma ACTH level** is the most specific investigation to confirm the diagnosis. Exogenous corticosteroids suppress the HPA axis via negative feedback, resulting in characteristically **low (suppressed) plasma ACTH**, which is the hallmark finding distinguishing iatrogenic from endogenous hypercortisolism. ### Why Plasma ACTH is Most Specific Here **High-Yield:** In iatrogenic Cushing syndrome: - Exogenous corticosteroids exert negative feedback on the hypothalamus and pituitary, suppressing CRH and ACTH secretion - A **suppressed plasma ACTH** (< 5–10 pg/mL) in the setting of clinical Cushing features and known exogenous steroid use is the **most specific confirmatory finding** - This pattern (high glucocorticoid effect + low ACTH) is pathognomonic for exogenous/iatrogenic Cushing and cannot be mimicked by endogenous causes (which would show elevated or inappropriately normal ACTH) - The 24-hour UFC may actually be **low or normal** in iatrogenic Cushing because exogenous synthetic steroids (e.g., budesonide, fluticasone) are not detected by standard cortisol immunoassays, and endogenous cortisol production is suppressed — making UFC unreliable and potentially falsely reassuring ### Why Other Options Are Less Specific | Investigation | Interpretation in Exogenous Steroid Use | Utility | |---|---|---| | **8 AM serum cortisol** | Suppressed (endogenous cortisol suppressed by exogenous steroid) | Confirms HPA suppression but not specific for systemic toxicity | | **24-hour UFC** | Often LOW or normal (exogenous steroids not measured by standard cortisol assays; endogenous cortisol suppressed) | **Unreliable — can be falsely low despite systemic toxicity** | | **Dexamethasone suppression test** | Uninterpretable (patient already on exogenous glucocorticoid) | Confounded; not useful | | **Plasma ACTH** | **Suppressed (< 5–10 pg/mL) — pathognomonic for exogenous steroid use** | **Most specific — confirms HPA axis suppression by exogenous steroid** | ### Diagnostic Pathway ``` Patient on chronic inhaled/systemic corticosteroids with Cushing features ↓ Plasma ACTH measurement ↓ Suppressed ACTH (< 5–10 pg/mL) → Confirms iatrogenic Cushing (exogenous steroid effect) Normal/elevated ACTH → Consider endogenous Cushing (pituitary/ectopic) ``` **Clinical Pearl (Harrison's Principles, 21st ed.):** In ACTH-independent Cushing syndrome — which includes iatrogenic Cushing — plasma ACTH is suppressed. This is the key distinguishing feature. The 24-hour UFC is the screening test of choice for **endogenous** Cushing syndrome, but in **iatrogenic** Cushing, UFC is unreliable because most synthetic corticosteroids (budesonide, fluticasone, prednisolone) cross-react poorly or not at all with standard cortisol immunoassays. **Warning:** Do NOT rely on 24-hour UFC to confirm iatrogenic Cushing — the test measures cortisol (endogenous), not synthetic glucocorticoids. A normal UFC in a patient on inhaled corticosteroids with Cushingoid features does NOT rule out systemic steroid toxicity. Suppressed ACTH is the specific marker. *Reference: Harrison's Principles of Internal Medicine, 21st ed., Chapter on Disorders of the Adrenal Cortex; KD Tripathi Essentials of Medical Pharmacology, 8th ed., Corticosteroids chapter.*