During genetic counseling of a family with MEN 2A syndrome, the physician discusses the pathophysiology and clinical manifestations. Which of the following statements about MEN 2A is INCORRECT?

Explanation

## MEN 2A Syndrome: Clinical Features and Management **Key Point:** MEN 2A is characterized by medullary thyroid carcinoma (MTC), pheochromocytoma, and primary hyperparathyroidism. However, **primary hyperparathyroidism is NOT the most common endocrine manifestation** — medullary thyroid carcinoma is. ### MEN 2A: The Classic Triad | Component | Frequency | Key Features | |-----------|-----------|-------------| | **Medullary thyroid carcinoma** | ~100% (nearly universal) | Most common and earliest manifestation; RET-dependent | | **Pheochromocytoma** | ~50% | Bilateral in ~50% of cases; rarely malignant (~5%) | | **Primary hyperparathyroidism** | ~20–30% | Multiglandular hyperplasia; LEAST common of the three | **High-Yield:** MTC is the **most common and most penetrant** feature of MEN 2A, occurring in nearly 100% of RET mutation carriers. Primary hyperparathyroidism, by contrast, occurs in only 20–30% of MEN 2A patients, making it the **least common** of the triad. ### RET Mutations and Penetrance **Key Point:** RET proto-oncogene mutations cause both MEN 2A and MEN 2B with **very high penetrance** for medullary thyroid carcinoma (>95% by age 70 in most families). Penetrance is NOT 100% in all families, but it is exceptionally high. ### Pheochromocytoma in MEN 2A - Bilateral in ~50% of MEN 2A cases (vs. ~10% in sporadic pheochromocytoma) - Rarely malignant (~5% malignancy rate) - Screening with plasma metanephrines or 24-hour urine metanephrines is recommended - Alpha-blockade required before any surgical intervention ### Prophylactic Thyroidectomy Strategy **Clinical Pearl:** Prophylactic thyroidectomy is the **standard of care** for RET mutation carriers in MEN 2A: 1. **Timing depends on RET codon involved:** - High-risk codons (634, 918, 883): thyroidectomy before age 5–10 years - Intermediate-risk codons: thyroidectomy before age 10–15 years - Lower-risk codons: individualized approach 2. **Rationale:** Prevents development of medullary thyroid carcinoma, which is the leading cause of death in MEN 2A 3. **Outcome:** Prophylactic thyroidectomy has dramatically improved survival in MEN 2 families **Mnemonic for MEN 2A — "MTC + Pheo + PTH":** **M**edullary **T**hyroid **C**arcinoma (most common), **Pheo**chromocytoma (50%), **PTH** hyperparathyroidism (20–30%, least common). ```mermaid flowchart TD A[RET Mutation Identified]:::outcome --> B{Codon Risk Assessment}:::decision B -->|High-risk<br/>codons 634,918,883| C[Thyroidectomy before age 5-10]:::action B -->|Intermediate-risk| D[Thyroidectomy before age 10-15]:::action B -->|Lower-risk| E[Individualized approach<br/>with close monitoring]:::action C --> F[Prevent MTC development]:::outcome D --> F E --> G[Regular calcitonin screening]:::action G --> F ```