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Primary Hyperaldosteronism (Conn’s) MRCP Part 1

TL;DR

Primary Hyperaldosteronism (Conn’s) is a high-yield endocrine cause of secondary hypertension frequently tested in MRCP Part 1. It typically presents with hypertension, suppressed renin, and elevated aldosterone, sometimes with hypokalaemia. Diagnosis relies on the aldosterone–renin ratio followed by confirmatory testing. Early recognition and correct subtype classification are essential for both exams and clinical practice.


Why this matters

Primary Hyperaldosteronism (Conn’s) is one of the most examinable causes of secondary hypertension in MRCP Part 1. It combines renal physiology, endocrine regulation, and clinical reasoning—making it ideal for single-best-answer questions and interpretation of lab data.

In exams, candidates are expected to:

  • Recognise biochemical patterns

  • Select the correct diagnostic sequence

  • Differentiate subtypes

  • Choose appropriate management

For a broader exam framework, review the MRCP Part 1 overview.


Core sections

1. Definition and Pathophysiology

Primary hyperaldosteronism is characterised by autonomous aldosterone secretion independent of the renin–angiotensin system.

Physiological effects:

  • Increased sodium reabsorption → hypertension

  • Increased potassium excretion → hypokalaemia

  • Increased hydrogen ion loss → metabolic alkalosis

Aldosterone acts primarily on the distal convoluted tubule and collecting duct, increasing ENaC activity.

2. Causes (Most Tested)

Cause

Key Feature

Bilateral adrenal hyperplasia

Most common overall

Aldosterone-producing adenoma (Conn’s syndrome)

Most common unilateral cause

Adrenal carcinoma

Rare, aggressive

Familial hyperaldosteronism

Genetic forms (Type I–III)

3. Clinical Features

Classic triad:

  • Hypertension (often resistant)

  • Hypokalaemia

  • Metabolic alkalosis

However, examiners frequently test atypical presentations:

  • Normokalaemic hypertension

  • Incidental adrenal mass

  • Mild or asymptomatic disease

Additional features:

  • Muscle weakness

  • Polyuria and polydipsia

  • Paraesthesia

Exam tip: Do not exclude the diagnosis if potassium is normal.

4. Screening Test (Key Concept)

Aldosterone–Renin Ratio (ARR)

Typical findings:

  • Elevated plasma aldosterone

  • Suppressed plasma renin

  • High ARR

Pre-test considerations:

  • Stop ACE inhibitors, ARBs, diuretics where possible

  • Correct hypokalaemia

  • Ensure adequate salt intake

ARR is the single most tested investigation step in MRCP.

5. Confirmatory Testing

After a positive ARR, confirm autonomous secretion:

  • Saline infusion test

  • Oral sodium loading test

  • Fludrocortisone suppression test

Failure of aldosterone suppression confirms the diagnosis.

6. Subtype Differentiation

Critical for management decisions:

  • CT adrenal imaging (initial step)

  • Adrenal vein sampling (AVS) = gold standard

AVS distinguishes:

  • Unilateral adenoma → surgery

  • Bilateral hyperplasia → medical therapy

7. Management

Unilateral disease (adenoma):

  • Laparoscopic adrenalectomy

Bilateral adrenal hyperplasia:

  • Medical therapy:

    • Spironolactone (first-line)

    • Eplerenone (alternative with fewer side effects)

8. High-Yield Summary (Exam Checklist)

  1. Most common cause: bilateral hyperplasia

  2. Most common surgically correctable cause: adenoma

  3. Hypokalaemia may be absent

  4. ARR = best screening test

  5. Always confirm before imaging

  6. AVS is gold standard for localisation

  7. Spironolactone is first-line treatment

  8. Resistant hypertension is a key clue

  9. Metabolic alkalosis supports diagnosis

  10. Renin is suppressed


Practical examples / mini-cases

MCQ:A 45-year-old woman presents with resistant hypertension. Blood tests show potassium 3.1 mmol/L and metabolic alkalosis. Plasma renin is low and aldosterone is elevated.

What is the most appropriate next step?

A. CT scan of adrenal glandsB. Start spironolactone immediatelyC. Measure aldosterone–renin ratioD. Renal artery Doppler

Answer: C. Measure aldosterone–renin ratio

Explanation: The aldosterone–renin ratio is the initial screening test. Imaging should only follow biochemical confirmation.

Practise similar questions in the Free MRCP MCQs or simulate exam conditions with a Start a mock test.


MRCP Part 1 study setup with notes and medical textbooks for endocrine revision

Common pitfalls (5 bullets)

  • Assuming hypokalaemia is mandatory

  • Ordering CT before biochemical confirmation

  • Not adjusting medications before ARR

  • Confusing with secondary hyperaldosteronism (renin elevated)

  • Skipping adrenal vein sampling before surgery


FAQs

1. What is the best initial test for Primary Hyperaldosteronism (Conn’s)?

The aldosterone–renin ratio (ARR) is the preferred screening test. A high ratio indicates autonomous aldosterone production.

2. Can patients have normal potassium levels?

Yes. Many patients are normokalaemic, especially early in the disease—this is a common exam trap.

3. Why is adrenal vein sampling important?

It differentiates unilateral from bilateral disease and guides management, especially before surgery.

4. What is the treatment of choice?

Unilateral adenoma is treated with adrenalectomy, while bilateral hyperplasia is managed medically with mineralocorticoid receptor antagonists.

5. How is it different from secondary hyperaldosteronism?

Primary: low renin, high aldosterone. Secondary: both renin and aldosterone are elevated (e.g. renal artery stenosis).


Ready to start?

To strengthen your endocrine revision, practise targeted questions using the Free MRCP MCQs and assess your readiness with a Start a mock test.

For structured preparation, explore the MRCP Part 1 overview and integrate this topic with adrenal and hypertension modules in your study plan.


Sources

 
 
 

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