In a chronic smoker which hemoptysis, he also gave a history of hypeension and obesity. Lab data showed raised ACTH levels, which are not suppressed by dexamethasone. The cause of Cushing’s syndrome in the patient is –
First, Cushing's syndrome is due to excess cortisol. The causes are either ACTH-dependent or ACTH-independent. ACTH-dependent means the pituitary or an ectopic source is producing ACTH. Ectopic ACTH is often from tumors, like small cell lung cancer. Since the patient is a chronic smoker, lung cancer is a red flag. Hemoptysis could indicate a lung tumor.
The key point here is that the ACTH levels aren't suppressed by dexamethasone. In Cushing's disease (pituitary cause), low-dose dexamethasone usually suppresses cortisol. If it's ectopic, like from a lung tumor, dexamethasone won't suppress it. The presence of hemoptysis also points towards a lung origin. Obesity and hypertension are common in Cushing's.
Looking at the options, the correct one should be ectopic ACTH from a lung tumor. The other options might include pituitary, adrenal tumors, or adrenal hyperplasia. Adrenal causes would have low ACTH, so those can be ruled out. Pituitary would have high ACTH but would respond to dexamethasone. Adrenal adenoma or hyperplasia would have low ACTH. So the only one left is ectopic ACTH, likely from small cell lung cancer given the smoking history.
**Core Concept**
Cushing's syndrome from ectopic ACTH secretion is typically caused by small cell lung cancer (SCLC), especially in smokers with hemoptysis. Dexamethasone suppression testing differentiates ACTH-dependent (Cushing's disease vs. ectopic) and ACTH-independent causes.
**Why the Correct Answer is Right**
Elevated ACTH unresponsive to dexamethasone indicates ectopic ACTH production. Small cell lung cancer is the most common ectopic source, strongly associated with smoking and hemoptysis. The tumor secretes ACTH independently of normal feedback, leading to hypercortisolism. This contrasts with Cushing's disease (pituitary adenoma), where low-dose dexamethasone suppresses cortisol.
**Why Each Wrong Option is Incorrect**
**Option A:** *Pituitary adenoma (Cushing's disease)*: ACTH is suppressed by low-dose dexamethasone, which does not occur here.
**Option B:** *Adrenal adenoma*: ACTH levels are low due to negative feedback; ACTH would not be elevated.
**Option C:** *Adrenal hyperplasia*: ACTH is suppressed due to cortisol feedback; ACTH elevation is absent.
**Clinical Pearl / High-Yield Fact**
Remember the "rule of 3 Ms" for ectopic ACTH: **M**alignancy (e.g., SCLC), **M**edullary thyroid carcinoma, and **M**eningioma. Always investigate for underlying lung cancer in smokers with ACTH-driven Cushing’s.
**Correct Answer: C. Ectopic ACTH secretion due to small cell lung cancer**