A 38-year-old woman presents with a solitary thyroid nodule detected on palpation. TSH is normal. Ultrasound shows a 1.5 cm hypoechoic nodule with irregular margins and microcalcifications. What is the investigation of choice to confirm malignancy?

Explanation

## Investigation of Choice for Thyroid Nodule Characterization **Key Point:** Fine needle aspiration cytology (FNAC) under ultrasound guidance is the gold standard initial investigation for evaluating thyroid nodules with suspicious ultrasound features. ### Why FNAC is the Investigation of Choice 1. **Minimally invasive** — needle gauge 23–27 G, outpatient procedure 2. **High sensitivity and specificity** — 65–98% sensitivity, 72–100% specificity depending on nodule characteristics [cite:Robbins 10e Ch 24] 3. **Cost-effective** — inexpensive compared to other modalities 4. **Rapid results** — cytology report available within days 5. **Bethesda System classification** — standardized reporting (non-diagnostic, benign, AUS/FLUS, FN/SFN, suspicious for malignancy, malignant) ### Ultrasound Features Indicating Malignancy Risk The nodule in this case has **high-risk features**: - Hypoechoic appearance - Irregular margins - Microcalcifications (coarse calcifications suggest benignity; fine punctate calcifications suggest malignancy) **High-Yield:** Nodules >1 cm with suspicious ultrasound features warrant FNAC. Nodules <1 cm are generally followed with ultrasound unless they have very high-risk features. ### Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | **FNAC** | Initial cytologic evaluation | Non-diagnostic in 10–15% | | **Core needle biopsy** | Upgrade when FNAC is non-diagnostic or AUS/FLUS | More invasive, reserved for indeterminate FNAC | | **Thyroid scintigraphy** | Assess function ("hot" vs "cold" nodules) | Poor specificity for malignancy; cold nodules can be benign | | **CT/MRI** | Assess local invasion, lymph node involvement | Not first-line; reserved for staging after diagnosis | **Clinical Pearl:** A "cold" nodule (non-functioning) on scintigraphy has higher malignancy risk than a "hot" nodule, but this is not sensitive enough to replace FNAC. ### Management Algorithm Post-FNAC ```mermaid flowchart TD A[Thyroid nodule with suspicious US features]:::outcome --> B[FNAC under US guidance]:::action B --> C{Bethesda Category?}:::decision C -->|Benign| D[Follow-up ultrasound at 12 months]:::action C -->|AUS/FLUS| E[Repeat FNAC or Core biopsy]:::action C -->|FN/SFN| F[Repeat FNAC or Core biopsy]:::action C -->|Suspicious for malignancy| G[Thyroidectomy]:::action C -->|Malignant| G C -->|Non-diagnostic| E ``` **Mnemonic:** **FNAC-FIRST** — Fine Needle Aspiration Cytology is the First Investigation for Routine Suspicious nodules.