A 35-year-old woman with a 2 cm solitary thyroid nodule and a family history of medullary thyroid cancer presents for preoperative evaluation. Serum calcitonin is elevated at 45 pg/mL (normal <10). Which investigation is most appropriate to confirm the diagnosis before thyroidectomy?

Explanation

## Investigation of Choice for Suspected Medullary Thyroid Carcinoma ### Clinical Context The patient has a solitary thyroid nodule with elevated basal calcitonin (45 pg/mL), which is highly suggestive of medullary thyroid carcinoma (MTC). The family history of MTC raises concern for hereditary MTC or multiple endocrine neoplasia type 2 (MEN 2). ### Why Calcitonin Stimulation Test? **Key Point:** Calcitonin stimulation testing (pentagastrin or calcium gluconate infusion) is the gold standard confirmatory test for MTC when basal calcitonin is borderline or mildly elevated (10–100 pg/mL). It provokes an exaggerated calcitonin response in MTC patients. **High-Yield:** The stimulation test distinguishes: - **True MTC:** exaggerated calcitonin rise (>100 pg/mL above baseline or absolute level >200 pg/mL at 2 minutes) - **C-cell hyperplasia or benign disease:** minimal or normal response ### Diagnostic Algorithm ```mermaid flowchart TD A[Thyroid nodule + elevated basal calcitonin]:::outcome --> B{Basal calcitonin level?}:::decision B -->|>100 pg/mL| C[MTC confirmed<br/>Proceed to staging]:::action B -->|10-100 pg/mL| D[Calcitonin stimulation test]:::action D --> E{Exaggerated response?}:::decision E -->|Yes| F[MTC diagnosis confirmed]:::outcome E -->|No| G[Repeat testing or consider<br/>C-cell hyperplasia]:::outcome C --> H[Preoperative imaging<br/>Ultrasound, CT, MRI]:::action ``` ### Role of Other Tests | Investigation | Role | Timing | |---|---|---| | **Calcitonin stimulation** | Confirms MTC diagnosis | Before surgery | | **CEA level** | Prognostic marker, not diagnostic | At baseline & follow-up | | **FNAC + immunohistochemistry** | Useful but invasive; calcitonin staining supports MTC | Adjunct, not first-line | | **RET genetic testing** | Identifies hereditary MTC/MEN 2; guides family screening | After diagnosis confirmed | **Clinical Pearl:** In this patient, basal calcitonin of 45 pg/mL is in the borderline-elevated range. Stimulation testing will provide definitive confirmation before committing to total thyroidectomy and lymph node dissection. **Tip:** Remember the calcitonin cutoff: basal >100 pg/mL is diagnostic of MTC without need for stimulation; 10–100 pg/mL requires stimulation testing for confirmation.