Multiple Endocrine Neoplasia (MEN 1, 2A, 2B)
A Step 1–focused walkthrough of MEN 1, 2A, and 2B — contrasting the menin tumor-suppressor (2-hit) versus RET oncogene (1-hit) mechanisms, then the organ-by-organ tumor patterns, dynamic hormone testing, classic vignettes (refractory ulcers + hypercalcemia; pheo-before-thyroidectomy), and management including risk-directed prophylactic thyroidectomy. Corrected the MEN1 pancreatic-NET framing (gastrinoma = most common functional tumor; metastatic pancreatic NETs = leading cause of death) and the MEN2B prophylactic-thyroidectomy timing (first 6 months of life per ATA highest-risk).
Overview & pathophysiology
Multiple endocrine neoplasia (MEN) syndromes are autosomal dominant cancer-predisposition disorders causing tumors or hyperplasia in ≥2 endocrine organs. The board-tested split is mechanistic:
- MEN1 — loss-of-function of the MEN1 tumor-suppressor gene (chromosome 11q13, protein menin). Like all tumor suppressors it follows the two-hit model (inherited germline mutation + somatic loss of the second allele).
- MEN2A & 2B — gain-of-function of the RET proto-oncogene (chromosome 10), a receptor tyrosine kinase. A single activating mutation drives tumorigenesis (oncogene → one hit).
This tumor-suppressor-vs-oncogene contrast is itself a classic vignette hook. Because the affected glands secrete hormones, patients surface either through hormone excess (hypercalcemia, refractory ulcers, hypoglycemia, catecholamine spells) or through family screening / genetic testing.
MEN1 (Wermer) — Parathyroid, Pancreas, Pituitary:
- Parathyroid hyperplasia → primary hyperparathyroidism — most common and usually the earliest manifestation (~90%): hypercalcemia, characteristically multigland.
- Pancreatic / duodenal neuroendocrine tumors — gastrinoma is the most common functional entero-pancreatic NET → Zollinger-Ellison syndrome (recurrent/refractory or distal-duodenal/jejunal ulcers + secretory diarrhea). Malignant/metastatic pancreatic NETs are a leading cause of MEN1 death. Others: insulinoma (Whipple triad), VIPoma, glucagonoma. (Non-functioning pancreatic NETs are actually the most common tumor when counting screen-detected lesions.)
- Pituitary adenoma — prolactinoma most common (amenorrhea, galactorrhea, ↓libido).
- Also: foregut carcinoid (thymic/bronchial/gastric), subcutaneous lipomas, facial angiofibromas, collagenomas.
MEN2A (Sipple) — RET codon 634:
- Medullary thyroid carcinoma (MTC) — ~100%; arises from parafollicular C cells → calcitonin.
- Pheochromocytoma — ~50%; often bilateral (adrenal medulla).
- Primary hyperparathyroidism (parathyroid hyperplasia).
- Associations: cutaneous lichen amyloidosis, Hirschsprung disease.
MEN2B — RET codon M918T:
- MTC — most aggressive, earliest onset.
- Pheochromocytoma.
- Mucosal neuromas (lips/tongue/GI) + intestinal ganglioneuromatosis.
- Marfanoid habitus.
- NO parathyroid disease — the key discriminator from MEN2A.
All MEN are autosomal dominant. Count the P's:
- MEN1 = 3 P's — Parathyroid, Pancreas, Pituitary.
- MEN2A = 2 P's + MTC — Parathyroid, Pheochromocytoma, Medullary thyroid carcinoma.
- MEN2B = 1 P + MTC + neuromas — Pheochromocytoma, Medullary thyroid carcinoma, Mucosal neuromas / Marfanoid (no parathyroid).
Gene cue: MEN1 = "menin" tumor suppressor; MEN2 = RET oncogene.
MEN2B extras: *B for Bumps (mucosal neuromas) and Body habitus (marfanoid).*
MEN1 vs 2A vs 2B
| Feature | MEN1 (Wermer) | MEN2A (Sipple) | MEN2B |
|---|---|---|---|
| Gene / mechanism | MEN1 (menin), 11q13; tumor suppressor, 2 hits | RET codon 634; oncogene, 1 hit | RET codon M918T; oncogene, 1 hit |
| Thyroid (C cells) | — | MTC (↑calcitonin, ↑CEA) | MTC — aggressive, earliest |
| Adrenal medulla | — | Pheochromocytoma (~50%) | Pheochromocytoma |
| Parathyroid | Hyperplasia → ↑Ca, ↑/inappropriately-normal PTH | Hyperparathyroidism | Absent |
| Pancreas / GI | Gastrinoma, insulinoma, VIPoma | — | Ganglioneuromatosis |
| Pituitary | Prolactinoma (most common) | — | — |
| Other | Lipoma, angiofibroma, carcinoid | Lichen amyloidosis, Hirschsprung | Mucosal neuromas, marfanoid |
Vignette: A 32-year-old man has duodenal ulcers that recur despite high-dose PPIs, chronic diarrhea, and a calcium-oxalate kidney stone. Labs: Ca²⁺ 11.8 mg/dL, PTH inappropriately high-normal (not suppressed despite hypercalcemia), phosphate low. His sister had a prolactinoma.
Diagnosis: MEN1 — gastrinoma (Zollinger-Ellison) + primary hyperparathyroidism (± pituitary).
Next best step:
- Confirm ZES: fasting serum gastrin (off PPI); if equivocal, secretin stimulation test → paradoxical rise in gastrin (normal antral G cells are suppressed by secretin; gastrinoma cells are not).
- Confirm hyperparathyroidism: ionized Ca²⁺ + intact PTH.
- Germline MEN1 testing and screen first-degree relatives.
Note: hypercalcemia stimulates gastrin release — correcting the hyperparathyroidism improves ZES control.
Vignette: A 25-year-old with a firm thyroid nodule and a known RET mutation is scheduled for thyroidectomy. She reports episodic pounding headaches, palpitations, and sweating; BP 170/105. Serum calcitonin markedly elevated.
Diagnosis: MEN2 with MTC and a likely pheochromocytoma.
Next best step — do NOT operate yet:
- Screen for pheochromocytoma first: plasma free metanephrines (or 24-h urine metanephrines); localize with CT/MRI.
- If pheo present, resect the pheochromocytoma before the thyroid, after α-blockade (phenoxybenzamine) then β-blockade with volume/salt expansion. Never give β-blockade before α-blockade (unopposed α-vasoconstriction → crisis).
*Operating on an unrecognized pheo risks a fatal intraoperative hypertensive crisis. Pheo is always excluded and treated before thyroid or parathyroid surgery in MEN2.*
Hormone axes & dynamic tests (high-yield):
- Primary hyperparathyroidism: ↑Ca²⁺, ↑ or inappropriately-normal PTH, ↓phosphate, ↑urine Ca.
- Gastrinoma (ZES): ↑fasting gastrin + ↑gastric acid; secretin stimulation → paradoxical ↑gastrin.
- Insulinoma: supervised 72-hour fast → hypoglycemia with ↑insulin, ↑C-peptide, ↑proinsulin, negative sulfonylurea screen (C-peptide distinguishes endogenous insulin from exogenous insulin, which has low C-peptide).
- Pheochromocytoma: plasma free / 24-h urine metanephrines (most sensitive); imaging CT/MRI, MIBG for extra-adrenal/metastatic disease.
- MTC: ↑calcitonin, ↑CEA; C-cell tumor does not concentrate iodine → radioactive iodine is useless.
- RET carriers → prophylactic total thyroidectomy (MTC prevention/cure):
- MEN2B (ATA highest risk, M918T): within the first 6 months of life.
- MEN2A: codon-risk-directed; high-risk (e.g., codon 634) generally at or before age 5 (guided by calcitonin).
- Always exclude and treat pheochromocytoma before any surgery (α- then β-blockade; resect pheo first).
- MTC: total thyroidectomy + central-neck dissection; follow calcitonin/CEA; advanced disease → RET-targeted TKIs (e.g., selpercatinib; also vandetanib/cabozantinib).
- Primary hyperparathyroidism: parathyroidectomy (subtotal, or total with autotransplant, in MEN1 multigland disease).
- MEN1: no prophylactic surgery — lifelong biochemical + imaging surveillance (Ca/PTH, gastrin, prolactin, IGF-1, fasting glucose/insulin); PPIs for ZES, cabergoline for prolactinoma.
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