Cushing’s Syndrome: Investigation Steps for MRCP Part 1
- Crack Medicine
- 7 hours ago
- 4 min read
TL;DR:
For MRCP Part 1, Cushing’s syndrome questions test a structured investigation pathway: first confirm hypercortisolism, then measure ACTH, and finally localise the source with targeted imaging. Mastering dexamethasone suppression testing and ACTH interpretation will answer most exam questions correctly.
Why this topic matters in MRCP Part 1
Cushing’s syndrome is a classic endocrine topic in MRCP Part 1, frequently appearing in questions that assess clinical reasoning rather than rare facts. The exam commonly tests whether candidates can select the correct next investigation.
Most questions follow a predictable sequence:
Confirm cortisol excess
Determine ACTH dependence
Identify the source
Understanding this sequence is more important than memorising rare causes.
For a full curriculum overview, see:
MRCP Part 1 Hub:https://www.crackmedicine.co.uk/mrcp-part-1/
The Investigation Pathway
The diagnostic approach to Cushing’s syndrome can be summarised as:
Confirm → Classify → Localise
This structured pathway is recommended by major endocrine societies.
Authoritative reference:
Endocrine Society Guidelines:https://academic.oup.com/jcem/article/93/5/1526/2598096
Step 1 — Confirm Hypercortisolism
Before imaging or further testing, cortisol excess must be confirmed.
Accepted Screening Tests
Three tests are considered first-line:
24-hour urinary free cortisol
Late-night salivary cortisol
Low-dose dexamethasone suppression test
Guideline reference:
Overnight Dexamethasone Suppression Test
This is the most commonly tested MRCP investigation.
Protocol:
1 mg dexamethasone at 23:00
Serum cortisol measured at 09:00
Interpretation:
Cortisol <50 nmol/L → Normal suppression
Cortisol >50 nmol/L → Suggests Cushing’s syndrome
Failure to suppress cortisol is the key abnormality.
Step 2 — Measure Plasma ACTH
Once hypercortisolism is confirmed, the next investigation is plasma ACTH.
ACTH determines whether the syndrome is ACTH-dependent or ACTH-independent.
ACTH Interpretation Table
ACTH Level | Interpretation | Likely Cause | Next Step |
Low ACTH | ACTH-independent | Adrenal adenoma or carcinoma | Adrenal CT |
Normal/high ACTH | ACTH-dependent | Pituitary or ectopic ACTH | Further testing |
This table represents one of the highest-yield MRCP concepts.
Authoritative reference:
NICE Clinical Knowledge Summary:https://cks.nice.org.uk/topics/cushings-syndrome/
Step 3 — High-Dose Dexamethasone Suppression Test
This test differentiates pituitary from ectopic ACTH production.
Principle
Pituitary tumours retain partial sensitivity to glucocorticoid feedback.
Ectopic ACTH production does not.
Interpretation
≥50% cortisol suppression → Pituitary Cushing disease
No suppression → Ectopic ACTH syndrome
Typical ectopic sources include:
Small cell lung carcinoma
Bronchial carcinoid
Authoritative reference:
Step 4 — Imaging Studies
Imaging is performed only after biochemical confirmation.
Premature imaging is a classic MRCP mistake.
ACTH-Independent Cushing’s
If ACTH is low:
Perform:
CT or MRI adrenal glands
Typical findings:
Adrenal adenoma
Adrenal carcinoma
Bilateral adrenal hyperplasia
Reference:
ACTH-Dependent Cushing’s
If ACTH is elevated:
Perform:
MRI pituitary
If MRI is negative:
Inferior petrosal sinus sampling (IPSS)
IPSS confirms pituitary ACTH secretion.
Reference:
High-Yield Investigation Points
The following points cover most MRCP questions:
Always confirm cortisol excess before imaging
Overnight dexamethasone suppression is commonly tested
ACTH measurement is the next step after confirmation
Low ACTH indicates adrenal disease
High ACTH suggests pituitary or ectopic causes
Pituitary Cushing partially suppresses cortisol
Ectopic ACTH does not suppress
Hypokalaemia suggests ectopic ACTH
MRI pituitary may be normal in Cushing disease
IPSS confirms pituitary origin
Five Most Tested Subtopics
1. Dexamethasone Suppression Tests
Key facts:
Overnight test confirms hypercortisolism
High-dose test differentiates causes
2. ACTH Interpretation
Memorise:
Low ACTH = adrenal source.
High ACTH = pituitary or ectopic.
3. Investigation Order
Very commonly tested.
Correct order:
Confirm → ACTH → Imaging.
4. Ectopic ACTH Syndrome
Typical exam clues:
Severe hypokalaemia
Rapid onset
Muscle weakness
Weight loss
Smoking history
5. Pseudo-Cushing States
Conditions mimicking Cushing’s:
Alcohol excess
Depression
Obesity
Severe illness
Reference:
Mini Case (Typical MRCP Question)
A 52-year-old man presents with:
Resistant hypertension
Diabetes mellitus
Proximal muscle weakness
Central obesity
Overnight dexamethasone suppression test:
Cortisol = 480 nmol/L.
What is the next investigation?
A. MRI pituitaryB. CT abdomenC. Plasma ACTHD. Urinary cortisolE. High-dose dexamethasone test
Answer
C — Plasma ACTH
Explanation
Failure to suppress cortisol confirms hypercortisolism.
The next step is to determine ACTH dependence.
Imaging should not be performed yet.
Practical Study Checklist
Before sitting the exam ensure you can:
✔ Recite the investigation sequence✔ Interpret dexamethasone suppression tests✔ Interpret ACTH levels✔ Choose correct imaging✔ Recognise ectopic ACTH clues✔ Identify pseudo-Cushing states✔ Avoid premature imaging✔ Answer investigation-order questions quickly
Practise similar questions:
MRCP Question Bank:https://www.crackmedicine.co.uk/qbank/
Mock Exams:
Video Lectures:
Common Pitfalls
Ordering CT before confirming hypercortisolism
Forgetting ACTH measurement
Confusing low-dose and high-dose dexamethasone tests
Assuming normal MRI excludes pituitary disease
Misinterpreting pseudo-Cushing states
FAQs
What is the best initial test for Cushing’s syndrome?
Accepted first-line tests include overnight dexamethasone suppression, urinary free cortisol, and late-night salivary cortisol. MRCP most often tests the overnight dexamethasone suppression test.
When should ACTH be measured?
ACTH should be measured after hypercortisolism is confirmed, as it determines whether the cause is adrenal or ACTH-dependent.
What does high-dose dexamethasone suppression indicate?
Suppression suggests pituitary Cushing disease, whereas no suppression suggests ectopic ACTH production.
When is adrenal CT required?
Adrenal imaging is indicated when ACTH is low, suggesting ACTH-independent Cushing’s syndrome.
Can pituitary MRI be normal in Cushing disease?
Yes. Small pituitary adenomas may not be visible. Inferior petrosal sinus sampling may be required to confirm the diagnosis.
Ready to start?
Build a strong foundation in endocrinology and master investigation-based questions by following a structured revision approach. Start with the MRCP Part 1 overview to cover the full syllabus systematically.
Test your understanding with exam-style questions in the Free MRCP MCQ Question Bank and simulate exam conditions using MRCP mock tests.
Sources
MRCP(UK) Examination Blueprinthttps://www.mrcpuk.org/mrcpuk-examinations/part-1/exam-blueprint
Endocrine Society Clinical Practice Guidelinehttps://academic.oup.com/jcem/article/93/5/1526/2598096
NICE Clinical Knowledge Summary – Cushing’s Syndromehttps://cks.nice.org.uk/topics/cushings-syndrome/
StatPearls – Cushing Syndromehttps://www.ncbi.nlm.nih.gov/books/NBK279088/
StatPearls – Hypercortisolismhttps://www.ncbi.nlm.nih.gov/books/NBK538241/
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