Master Endocrinology
for MRCP Part 1

What the MRCP Part 1 Tests in Endocrinology

MRCP Part 1 Endocrinology tests the ability to diagnose and manage common endocrine disorders based on clinical presentations, biochemical patterns, and imaging. Candidates must interpret pituitary, thyroid, adrenal, and gonadal axis results, recognise hypo- and hyperfunction states, and apply diagnostic criteria (e.g., for Cushing’s, diabetes insipidus, SIADH). Knowledge of first-line treatments (e.g., carbimazole in Graves’, hydrocortisone in adrenal crisis), key drug interactions (e.g., amiodarone and thyroid function), and emergency management (e.g., DKA, Addisonian crisis) is essential. The exam emphasises pattern recognition of rare but classic syndromes (e.g., MEN, acromegaly) and the use of dynamic function tests (e.g., synacthen, OGTT for acromegaly).

High-Yield Concepts

• Diabetes Insipidus vs SIADH: Water deprivation test: in cranial DI, urine osmolality remains low (<300 mOsm/kg) but rises with desmopressin; in nephrogenic DI, no response to desmopressin. SIADH: hyponatraemia with urine osmolality >100 mOsm/kg, urine Na >40 mmol/L, low urate. First-line for SIADH: fluid restriction; consider tolvaptan if severe/resistant.
• Cushing’s Syndrome Diagnosis: Screening: 1 mg overnight dexamethasone suppression test (cortisol <50 nmol/L excludes Cushing’s), 24-hour urinary free cortisol, or late-night salivary cortisol. If positive, measure ACTH. ACTH-dependent: pituitary MRI (if adenoma >6 mm) or CRH test (cortisol rise >20% suggests pituitary). ACTH-independent: adrenal CT.
• Thyroid Storm Management: Give high-dose propylthiouracil (PTU) 200 mg q4h (preferred as blocks T4 to T3 conversion) or carbimazole. After 1 hour, give Lugol’s iodine or potassium iodide 60 mg q6h (blocks hormone release). Beta-blocker (propranolol 40-80 mg q6h) for tachycardia. Hydrocortisone 100 mg IV q8h for adrenal protection and to inhibit T4 conversion.
• Primary Hyperaldosteronism: Screen with aldosterone:renin ratio (ARR) >750 (aldosterone pmol/L / renin ng/mL/h). Confirm with saline infusion or captopril test. CT adrenals: if unilateral adenoma >1 cm, consider adrenal vein sampling to distinguish from bilateral hyperplasia. Treatment: spironolactone or eplerenone for bilateral; laparoscopic adrenalectomy for unilateral.
• Acromegaly Diagnosis: OGTT: failure to suppress GH to <0.4 mcg/L (or <1 ng/mL) after 75g glucose. Measure IGF-1 (age- and sex-adjusted). MRI pituitary to detect adenoma. First-line: transsphenoidal surgery for microadenomas; if not cured, consider somatostatin analogues (octreotide, lanreotide) or pegvisomant (GH receptor antagonist).
• Addisonian Crisis Management: Immediate IV hydrocortisone 100 mg bolus, then 100 mg q6h (or 200 mg/24h infusion). Correct hypoglycaemia with 50 mL 50% dextrose if needed. Normal saline 1 L over 1 hour, then continue based on BP and electrolytes. Do not wait for ACTH stimulation test results. Long-term: hydrocortisone 15-25 mg/day in divided doses + fludrocortisone 50-200 mcg/day.
• Multiple Endocrine Neoplasia (MEN) Types: MEN1: parathyroid hyperplasia (hypercalcaemia), pituitary tumours, pancreatic neuroendocrine tumours (gastrinoma, insulinoma). MEN2A: medullary thyroid cancer (MTC), phaeochromocytoma, hyperparathyroidism. MEN2B: MTC, phaeochromocytoma, mucosal neuromas, marfanoid habitus. Genetic testing for RET (MEN2) and MEN1 gene.
• Osteoporosis Treatment Thresholds: DXA scan: T-score ≤ -2.5 defines osteoporosis. Treat if T-score ≤ -2.5, or history of fragility fracture, or FRAX 10-year probability ≥20% major osteoporotic fracture or ≥3% hip fracture. First-line: bisphosphonates (alendronate 70 mg weekly, risedronate 35 mg weekly). Consider denosumab or teriparatide if intolerance or severe disease.

Common Traps in Endocrinology Questions

• Using the wrong dynamic test: e.g., using the short synacthen test to diagnose secondary adrenal insufficiency (it can miss recent-onset pituitary failure; insulin tolerance test is gold standard).
• Confusing primary and secondary hyperparathyroidism: in primary, calcium high with low or normal PTH; in secondary (renal failure), calcium low or normal with high PTH.
• Assuming a normal TSH alone excludes thyroid disease: in central hypothyroidism, TSH may be low or inappropriately normal with low fT4.
• Giving desmopressin to a patient with nephrogenic DI without first checking response to water deprivation; it can cause water intoxication.
• Forgetting to check calcium before starting bisphosphonates in osteoporosis: if hypocalcaemic, correct first to avoid severe hypocalcaemia.
• Misinterpreting a mildly elevated prolactin as a prolactinoma without excluding macroprolactin (polyethylene glycol precipitation) or drug causes (antipsychotics, metoclopramide).

How to Revise Endocrinology for the MRCP Part 1

Prioritise pituitary and adrenal dynamic tests (synacthen, CRH, water deprivation, OGTT for acromegaly) and their cut-offs. Memorise first-line treatments for thyroid storm, Addisonian crisis, and DKA. Focus on distinguishing SIADH from cerebral salt wasting (volume status, urate). Practice interpreting paired endocrine results (e.g., calcium/PTH, renin/aldosterone, glucose/insulin). Questions often present a clinical scenario with one abnormal lab value; you must choose the next best test or treatment. Review rare syndromes (MEN, McCune-Albright, Carney complex) and their genetic associations. Use NICE and BTA guidelines for thyroid and osteoporosis thresholds.

Practise it: MedLumen has 50 Endocrinology questions for the MRCP Part 1, each with a full explanation and references.