A 42-year-old woman is found to have a thyroid nodule on routine examination. She reports a family history of medullary thyroid cancer in her mother and brother, and she has a history of pheochromocytoma diagnosed 2 years ago. Serum calcitonin is elevated at 145 pg/mL (normal <10 pg/mL). Which investigation is most appropriate to confirm medullary thyroid carcinoma (MTC) in the context of suspected MEN 2A syndrome?

Explanation

## Diagnostic Confirmation of MTC in MEN 2A ### Role of RET Mutation Testing **Key Point:** RET proto-oncogene mutation testing on peripheral blood is the gold standard for confirming MEN 2A syndrome and identifying individuals at risk for medullary thyroid carcinoma. In MEN 2A, RET mutations are present in virtually 100% of affected individuals. ### Why RET Mutation Testing Is Superior **High-Yield:** RET testing has several advantages in MEN 2A: 1. **Diagnostic:** Confirms the syndrome (not just the thyroid disease) 2. **Prognostic:** Specific RET mutations correlate with disease aggressiveness and timing of MTC onset 3. **Enables family screening:** Identifies at-risk relatives before clinical manifestation 4. **Guides prophylactic thyroidectomy:** Timing and extent depend on RET mutation type 5. **Non-invasive:** Blood test; avoids biopsy risk ### Biochemical vs. Genetic Diagnosis | Test | What It Shows | Role in MEN 2A | |---|---|---| | Serum calcitonin | Elevated in MTC | Suggests MTC but not specific to MEN 2A | | Calcitonin stimulation test | Provokes calcitonin rise | Sensitive for C-cell hyperplasia/early MTC | | RET mutation (blood) | Germline RET alteration | Confirms MEN 2A syndrome; enables family screening | | FNAC or core biopsy | Cytologic/histologic diagnosis | Confirms malignancy but does not identify syndrome | | Thyroid ultrasound | Nodule size, characteristics | Guides biopsy decision; not diagnostic of syndrome | ### Clinical Pearl **Clinical Pearl:** In MEN 2A, the combination of pheochromocytoma + elevated calcitonin + family history of MTC is virtually diagnostic of MEN 2A. RET mutation testing confirms the syndrome and allows risk stratification of relatives. Prophylactic thyroidectomy is indicated in all RET-positive individuals, with timing based on the specific mutation (aggressive mutations warrant earlier surgery, sometimes in childhood). ### Mnemonic **Mnemonic:** **RET-MEN2** = **RET** mutation testing is the gold standard for **MEN2** confirmation (both MEN 2A and MEN 2B) ### High-Yield Fact **High-Yield:** RET mutations in MEN 2A are typically in exons 10, 11, 13, or 14 (extracellular or transmembrane domains). MEN 2B mutations are usually in exon 16 (kinase domain) and are associated with earlier, more aggressive MTC and mucosal neuromas. [cite:Harrison 21e Ch 405]