A 35-year-old woman presents with a solitary thyroid nodule detected on clinical examination. Thyroid function tests are normal. Ultrasound shows a 1.5 cm hypoechoic nodule with irregular margins and microcalcifications. Which is the investigation of choice to establish a definitive diagnosis?

Question

Accepted Answer

A. Fine needle aspiration cytology (FNAC) under ultrasound guidance. ## Investigation of Choice for Thyroid Nodule Evaluation

**Key Point:** Fine needle aspiration cytology (FNAC) under ultrasound guidance is the gold standard initial investigation for thyroid nodules with suspicious features.

### Why FNAC is the Investigation of Choice

1. **High sensitivity and specificity** — FNAC has >95% accuracy when performed under ultrasound guidance and interpreted using the Bethesda System for Reporting Thyroid Cytopathology.
2. **Minimal invasiveness** — needle gauge 23–27 G, outpatient procedure with minimal morbidity.
3. **Cost-effective** — inexpensive, reproducible, and widely available.
4. **Guides management** — results directly inform whether surgery is indicated (malignancy/suspicious for malignancy) or conservative follow-up (benign, atypia of undetermined significance).

### Bethesda Classification for Thyroid Cytology

| Category | Risk of Malignancy | Recommended Action |
|---|---|---|
| Non-diagnostic/Unsatisfactory | 1–4% | Repeat FNAC or ultrasound follow-up |
| Benign | 0–3% | Clinical and ultrasound follow-up |
| Atypia of Undetermined Significance (AUS) | 10–40% | Repeat FNAC or molecular testing |
| Follicular Neoplasm | 25–40% | Thyroidectomy or molecular testing |
| Suspicious for Malignancy | 50–75% | Thyroidectomy |
| Malignant | 97–99% | Thyroidectomy |

**Clinical Pearl:** The combination of ultrasound findings (hypoechoic, irregular margins, microcalcifications) and FNAC results provides the highest diagnostic accuracy for distinguishing benign from malignant thyroid lesions.

**High-Yield:** FNAC is preferred over core needle biopsy as the first-line tissue diagnosis because it has equivalent diagnostic accuracy, lower cost, fewer complications, and is faster to perform and report.

### Why Other Investigations Are Not First-Line

- **Core needle biopsy:** Reserved for non-diagnostic FNAC or when FNAC results are equivocal (e.g., AUS category). Provides better histologic architecture but is more invasive and expensive.
- **Thyroid scintigraphy:** Assesses functional status (hot vs. cold nodules) but has poor sensitivity for malignancy; cold nodules have only ~15% malignancy risk. Not used for diagnostic confirmation.
- **CT neck:** Useful for staging and assessing extrathyroidal extension in known malignancy, not for initial diagnosis of a nodule.

[cite:Robbins 10e Ch 24]

![Thyroid Neoplasms diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/4114.webp)

Answer

B. Computed tomography of the neck

Answer

C. Core needle biopsy

Answer

D. Thyroid scintigraphy with Tc-99m pertechnetate

A 35-year-old woman presents with a solitary thyroid nodule detected on clinical examination. Thyroid function tests are normal. Ultrasound shows a 1.5 cm hypoechoic nodule with irregular margins and microcalcifications. Which is the investigation of choice to establish a definitive diagnosis?

Explanation

Investigation of Choice for Thyroid Nodule Evaluation

Why FNAC is the Investigation of Choice

Bethesda Classification for Thyroid Cytology

Why Other Investigations Are Not First-Line

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Category	Risk of Malignancy	Recommended Action
Non-diagnostic/Unsatisfactory	1–4%	Repeat FNAC or ultrasound follow-up
Benign	0–3%	Clinical and ultrasound follow-up
Atypia of Undetermined Significance (AUS)	10–40%	Repeat FNAC or molecular testing
Follicular Neoplasm	25–40%	Thyroidectomy or molecular testing
Suspicious for Malignancy	50–75%	Thyroidectomy
Malignant	97–99%	Thyroidectomy