Multiple Endocrine Neoplasia (MEN) Syndromes

MEN syndromes are autosomal dominant disorders in which two or more endocrine glands develop hyperplasia, adenomas, or carcinomas in a single patient (and across the family). They are a favourite NEET PG topic because they tie together genetics (MEN1 vs RET), surgery (prophylactic thyroidectomy), and clinical endocrinology in one neat package.

Definition & classification

A MEN syndrome is defined as the occurrence of tumours of two or more endocrine glands in the same individual. All are inherited in an autosomal dominant fashion with high penetrance but variable expressivity. The germline mutation is present in every cell, so tumours are typically multicentric and bilateral and present at a younger age than their sporadic counterparts.

The three classic syndromes:

Feature	MEN 1	MEN 2A	MEN 2B
Eponym	Wermer syndrome	Sipple syndrome	Wagenmann–Froboese / MEN 3
Gene	MEN1 (menin)	RET	RET
Chromosome	11q13	10q11.2	10q11.2
Gene type	Tumour suppressor	Proto-oncogene	Proto-oncogene
Medullary thyroid Ca	Absent	Present (~95%)	Present (~100%, earliest)
Phaeochromocytoma	Absent	~50%	~50%
Parathyroid	Hyperplasia (90%)	Present (~20–30%)	Absent
Pituitary	Adenoma	Absent	Absent
Pancreas/duodenum	Present (gastrinoma, insulinoma)	Absent	Absent
Marfanoid + mucosal neuromas	Absent	Absent	Present

High-yield: The single quickest discriminator — MEN1 = tumour-suppressor gene (needs two hits, Knudson) while MEN2 = RET proto-oncogene (single activating mutation, gain of function). This explains why RET carriers get prophylactic surgery but MEN1 carriers do not.

The "3 Ps" framework

• MEN 1 → 3 Ps: Pituitary, Parathyroid, Pancreas (pancreatic/duodenal neuroendocrine tumours).
• MEN 2A → 2 Ps + 1 M: Parathyroid, Phaeochromocytoma, Medullary thyroid carcinoma.
• MEN 2B → 1 P + 2 Ms + M: Phaeochromocytoma, Medullary thyroid carcinoma, Mucosal neuromas, Marfanoid habitus (no parathyroid disease).

Mnemonic: "MEN 1, parathyroid is number one" — the commonest and usually first manifestation of MEN 1 is primary hyperparathyroidism (~90–95% lifetime). In MEN 2A the first/most consistent lesion is medullary thyroid carcinoma (MTC).

Etiology & pathophysiology

MEN 1 — the menin story

The MEN1 gene at 11q13 encodes menin, a nuclear scaffolding protein that interacts with transcription factors (JunD) and chromatin-modifying complexes (MLL histone methyltransferase). Menin is a tumour suppressor; loss of function removes a brake on proliferation in parathyroid, pituitary, and enteropancreatic neuroendocrine cells. Following Knudson's two-hit hypothesis, the patient inherits one mutated allele (germline first hit) and a somatic mutation/loss of heterozygosity inactivates the second allele within a target cell — leading to multifocal, multiglandular tumours.

MEN 2 — the RET story

The RET proto-oncogene at 10q11.2 encodes a receptor tyrosine kinase expressed in cells of neural crest origin (thyroid C cells, adrenal medulla, parathyroid, enteric autonomic ganglia). An activating (gain-of-function) germline mutation causes constitutive, ligand-independent kinase activity, driving C-cell hyperplasia → MTC, and adrenal medullary hyperplasia → phaeochromocytoma.

Genotype–phenotype correlation is strong and drives the age of prophylactic thyroidectomy:

MEN 2 variant	Classic RET codon	C-cell aggressiveness (ATA risk)
MEN 2B	M918T (exon 16)	Highest
MEN 2A (classic)	Codon 634 (exon 11, e.g. C634R)	High / moderate–high
Familial MTC (FMTC)	609, 611, 618, 620, 768, 804	Moderate

High-yield: RET M918T = MEN 2B = the most aggressive MTC, presents in infancy. Codon 634 = MEN 2A, the codon most associated with phaeochromocytoma and hyperparathyroidism. The ATA stratifies into "highest" (M918T), "high" (634, A883F), and "moderate" (all others) risk to time surgery.

High-yield: RET mutations also cause Hirschsprung disease (loss-of-function in this case) — note the paradox: gain-of-function RET → MEN 2; loss-of-function RET → Hirschsprung. Both can rarely coexist (codons 609/618/620).

Clinical features

MEN 1 manifestations

1. Primary hyperparathyroidism (90–95%, usually first): Due to four-gland hyperplasia (not a solitary adenoma as in sporadic disease). Presents with hypercalcaemia — "stones, bones, abdominal groans, psychic moans" — renal calculi, bone pain, constipation, fatigue, depression. Onset typically by the 20s–30s, decades earlier than sporadic.
2. Enteropancreatic neuroendocrine tumours (30–80%):
   • Gastrinoma is the commonest functional and the leading cause of MEN1-related death → Zollinger–Ellison syndrome (recurrent/refractory peptic ulcers, diarrhoea, high fasting gastrin). In MEN 1 gastrinomas are typically multiple and located in the duodenum (gastrinoma triangle).
   • Insulinoma — fasting hypoglycaemia (Whipple's triad).
   • Non-functioning PNETs, glucagonoma, VIPoma (less common).
3. Pituitary adenoma (15–40%): Prolactinoma is the commonest (amenorrhoea–galactorrhoea, hypogonadism); also GH-secreting (acromegaly).
4. Associated: facial angiofibromas, collagenomas, lipomas, foregut carcinoids (thymic, bronchial, gastric ECL-cell).

High-yield: In MEN 1, hyperparathyroidism is the most common manifestation but gastrinoma/malignant PNET is the most common cause of death. Thymic carcinoid carries a poor prognosis (commoner in male smokers).

MEN 2A (Sipple)

• MTC in ~95% — neck nodule, may secrete calcitonin (flushing, diarrhoea in advanced disease) and CEA.
• Phaeochromocytoma in ~50% — often bilateral, episodic headache, palpitations, sweating, hypertension; rarely malignant.
• Primary hyperparathyroidism in ~20–30% — milder than MEN 1.
• Variants: MEN 2A with cutaneous lichen amyloidosis (interscapular pruritic patch) and MEN 2A with Hirschsprung disease.

MEN 2B

• MTC in essentially 100%, earliest and most aggressive (may metastasise in infancy).
• Phaeochromocytoma ~50%.
• Mucosal neuromas — lips, tongue, eyelids ("bumpy lip" appearance), conjunctiva.
• Intestinal ganglioneuromatosis — constipation, megacolon.
• Marfanoid habitus, pes cavus, slipped femoral epiphysis, but no lens dislocation and no aortic disease (distinguishes from true Marfan).
• No hyperparathyroidism.

High-yield: A child with marfanoid habitus + bumpy lips (mucosal neuromas) + a thyroid nodule is MEN 2B until proven otherwise — test for RET M918T and check plasma metanephrines before any surgery.

Diagnosis & investigation of choice

Biochemical / hormonal work-up

• MTC: Basal and stimulated serum calcitonin (tumour marker) and CEA; calcitonin doubling time prognosticates. Calcitonin >100 pg/mL strongly suggests nodal/distant spread.
• Phaeochromocytoma: Plasma free metanephrines / 24-h urinary fractionated metanephrines (investigation of choice — highest sensitivity), then CT/MRI adrenals; MIBG scan for extra-adrenal/metastatic disease.
• Hyperparathyroidism: Raised calcium with inappropriately normal/high PTH; localisation with sestamibi (MIBI) scan ± neck ultrasound (though four-gland disease limits the value of localisation in MEN 1).
• Gastrinoma (ZES): Fasting serum gastrin (markedly raised) with gastric pH < 2; secretin stimulation test (paradoxical rise in gastrin >120 pg/mL confirms). Localise with endoscopic ultrasound and Ga-68 DOTANOC/DOTATATE PET (somatostatin-receptor imaging).
• Insulinoma: 72-hour fast — low glucose, high insulin, high C-peptide, raised proinsulin, negative sulfonylurea screen.
• Prolactinoma: Serum prolactin + pituitary MRI.

Genetic testing — the cornerstone

High-yield: Confirmation and screening are by germline mutation analysis — MEN1 sequencing for MEN 1, and RET sequencing for MEN 2. RET testing in a newborn/at-risk child of a known carrier dictates the timing of prophylactic total thyroidectomy — this is the most exam-tested management decision in the whole topic.

Stepwise approach to a suspected MEN 2 / MTC patient

Clinical suspicion (MTC or family history) → RET germline mutation analysis → exclude phaeochromocytoma first (plasma metanephrines + imaging) → check serum calcium/PTH → treat/resect phaeochromocytoma BEFORE thyroid or parathyroid surgery → total thyroidectomy + central neck dissection for MTC → manage parathyroid disease.

High-yield: Always rule out and treat phaeochromocytoma FIRST before any other surgery in MEN 2. Operating on an undiagnosed phaeo precipitates a fatal hypertensive crisis. Alpha-blockade (phenoxybenzamine) precedes beta-blockade.

Management & drug / procedure of choice

MEN 1

• Hyperparathyroidism: Subtotal parathyroidectomy (3½ glands) OR total parathyroidectomy with autotransplantation + transcervical thymectomy (removes supernumerary/ectopic glands and prevents thymic carcinoid). Recurrence is higher than sporadic disease because all glands are abnormal.
• Gastrinoma / ZES: High-dose proton-pump inhibitors control acid hypersecretion (medical mainstay); surgery for localised disease is debated as tumours are multiple and duodenal.
• Insulinoma: Enucleation/resection; diazoxide for medical control.
• Prolactinoma: Cabergoline / bromocriptine (dopamine agonist) — medical therapy is first line, surgery reserved for resistant/large tumours.

MEN 2 — prophylactic thyroidectomy (the headline)

Because MTC is virtually inevitable and lethal once metastatic, prophylactic total thyroidectomy is offered to RET carriers, timed by ATA risk category:

ATA risk	RET mutation	Recommended timing of total thyroidectomy
Highest	M918T (MEN 2B)	Within the first 6 months of life (often first month)
High	Codon 634, A883F	At/before age 5 years (or earlier if calcitonin rises)
Moderate	All other RET (FMTC)	When calcitonin becomes elevated, or by childhood/adolescence based on calcitonin

• Established MTC: Total thyroidectomy + central compartment (level VI) lymph node dissection; lateral neck dissection if nodes involved. MTC arises from C cells → does NOT take up radioiodine and is not TSH-dependent, so radioactive iodine and TSH suppression are ineffective. Advanced/metastatic disease: tyrosine kinase inhibitors vandetanib or cabozantinib; selpercatinib/pralsetinib for RET-altered tumours.
• Phaeochromocytoma: Adrenalectomy (laparoscopic, cortical-sparing if bilateral) after adequate alpha-blockade with phenoxybenzamine, then beta-blockade and volume repletion.
• Hyperparathyroidism (MEN 2A): Resect only enlarged glands or subtotal parathyroidectomy.

High-yield: MTC does not respond to radioiodine or TSH suppression (C cells are not follicular) — total thyroidectomy is the only curative option, and surgery must be early/prophylactic in RET carriers.

Complications

• MEN 1: Metastatic gastrinoma/PNET (leading cause of death), recurrent peptic ulceration, nephrolithiasis and renal impairment from chronic hypercalcaemia, malignant thymic carcinoid, hypopituitarism.
• MEN 2: Metastatic MTC (lung, liver, bone), hypertensive crisis from unrecognised phaeochromocytoma during surgery/anaesthesia/labour, hypocalcaemia and recurrent laryngeal nerve injury after thyroidectomy, recurrent MTC with rising calcitonin/CEA.
• General: Psychological burden of genetic testing; need for lifelong biochemical surveillance of all family members.

Key differentials

• Sporadic vs syndromic disease: A solitary parathyroid adenoma, single phaeochromocytoma, or sporadic MTC — suspect MEN if young age, bilaterality/multifocality, or positive family history.
• MEN 4: Caused by CDKN1B (p27/cyclin-dependent kinase inhibitor) mutations — a MEN1-like phenotype (parathyroid + pituitary) without an MEN1 mutation.
• von Hippel–Lindau (VHL): Phaeochromocytoma + retinal/CNS haemangioblastomas + renal cell carcinoma + pancreatic cysts/PNETs — overlaps with MEN for phaeo and PNET.
• Neurofibromatosis type 1 (NF1): Phaeochromocytoma + café-au-lait spots + neurofibromas.
• Carney complex: Cardiac myxomas + spotty skin pigmentation + endocrine overactivity (PPNAD, GH adenoma) — PRKAR1A gene.
• Familial isolated hyperparathyroidism / HPT-jaw tumour syndrome (CDC73/HRPT2): Parathyroid tumours with ossifying jaw fibromas.

Syndrome	Defining extra-endocrine clue	Gene
MEN 2B	Mucosal neuromas + marfanoid	RET (M918T)
VHL	Retinal/cerebellar haemangioblastoma, RCC	VHL
NF1	Café-au-lait, neurofibromas, Lisch nodules	NF1
Carney complex	Cardiac myxoma, lentigines	PRKAR1A

Recently asked / exam angle

• "Pituitary + parathyroid + pancreas tumours" → MEN 1 (Wermer); commonest manifestation = hyperparathyroidism; commonest cause of death = gastrinoma/PNET.
• "Marfanoid + mucosal neuromas + thyroid cancer" → MEN 2B; gene = RET M918T; do thyroidectomy before 6 months/1 year.
• "Earliest/most aggressive MTC" → MEN 2B.
• "Codon most linked to phaeochromocytoma in MEN 2A" → 634.
• "MEN syndrome without parathyroid disease" → MEN 2B.
• "Investigation of choice for phaeochromocytoma" → plasma free / urinary fractionated metanephrines.
• "Which is a tumour suppressor — MEN1 or RET?" → MEN1 (menin); RET is a proto-oncogene.
• "RET also causes which non-MEN disease?" → Hirschsprung disease (loss of function).
• "MEN1-like phenotype, negative MEN1 gene" → MEN 4 (CDKN1B/p27).
• "Before thyroidectomy in MEN 2, what must you exclude?" → phaeochromocytoma.
• "Tumour marker for MTC" → calcitonin (and CEA).
• "Why is radioiodine useless in MTC?" → MTC arises from parafollicular C cells, not iodine-trapping follicular cells.

Rapid revision

1. MEN syndromes are autosomal dominant; tumours are multifocal, bilateral, early-onset.
2. MEN 1 = Wermer = 3 Ps (Pituitary, Parathyroid, Pancreas); gene MEN1/menin, 11q13, tumour suppressor.
3. Commonest MEN 1 lesion = primary hyperparathyroidism (four-gland hyperplasia); commonest MEN 1 death = gastrinoma/PNET.
4. MEN 2A = Sipple: MTC + phaeochromocytoma + hyperparathyroidism; MEN 2B: MTC + phaeo + mucosal neuromas + marfanoid, no parathyroid disease.
5. MEN 2 gene = RET proto-oncogene, 10q11.2, gain of function; M918T = 2B (worst), codon 634 = 2A.
6. Prophylactic total thyroidectomy timing: MEN 2B < 6 months/1 yr; codon 634 by age 5; moderate-risk guided by calcitonin.
7. Always exclude and treat phaeochromocytoma FIRST; alpha-blockade (phenoxybenzamine) before beta-blockade.
8. MTC does not respond to radioiodine or TSH suppression; tumour markers = calcitonin + CEA; advanced disease → vandetanib/cabozantinib/selpercatinib.
9. Phaeochromocytoma IOC = plasma/urinary metanephrines; ZES IOC = fasting gastrin + secretin stimulation.
10. MEN 1 hyperparathyroidism surgery = subtotal (3½-gland) or total parathyroidectomy + autotransplant + transcervical thymectomy.
11. RET loss-of-function → Hirschsprung; gain-of-function → MEN 2 (the RET paradox).
12. MEN 4 = CDKN1B (p27), a MEN1-like phenotype with a negative MEN1 gene test.