Best Confirmatory Investigation for Thyroid Nodule >1.3 cm with Normal Thyroid Function
Fine-needle aspiration cytology (FNAC) is the best confirmatory investigation for a thyroid nodule larger than 1.3 cm with normal TSH, T3, and T4 levels. 1
Rationale for FNAC as First-Line Investigation
- FNAC should be performed in any thyroid nodule >1 cm regardless of thyroid function test results, as it provides the most sensitive and specific method for differentiating between benign and malignant nodules 1
- While thyroid nodules are common (4-50% of the population depending on diagnostic procedures and patient age), thyroid cancer is relatively rare (approximately 5% of all thyroid nodules), making FNAC crucial for appropriate risk stratification 1
- Thyroid function tests (TSH, T3, T4) are of limited value in diagnosing thyroid cancer, as most thyroid cancers present with normal thyroid function 1
- FNAC has high sensitivity for the differential diagnosis of benign and malignant nodules, though there are limitations with inadequate samples and follicular neoplasia 1
Role of Ultrasound in the Diagnostic Algorithm
- Ultrasound should be used as a first-line imaging procedure to characterize the nodule and guide FNAC, but is not sufficient alone for definitive diagnosis 1, 2
- Ultrasound features associated with increased malignancy risk include hypoechogenicity, microcalcifications, absence of peripheral halo, irregular borders, solid aspect, and abnormal blood flow 1
- The combination of multiple high-risk ultrasound features substantially increases the overall risk of malignancy 2
- However, ultrasound size alone is a poor predictor of malignancy 3, 4
Limitations of Other Imaging Modalities
- MRI and CT are not recommended as first-line investigations for thyroid nodules as they provide less specific information about nodule characteristics compared to ultrasound followed by FNAC 1, 2
- Thyroid scintigraphy (thyroid scan) is now rarely used in the initial work-up of thyroid nodules except when TSH is suppressed, suggesting an autonomously functioning nodule 5
Management of FNAC Results
- If FNAC yields inadequate samples, the procedure should be repeated 1
- In cases of follicular neoplasia with normal TSH and "cold" appearance on thyroid scan (if performed), surgery should be considered 1
- Measurement of serum calcitonin should be considered as part of the diagnostic evaluation, as it is a reliable tool for diagnosing medullary thyroid cancer (5-7% of all thyroid cancers) 1
Important Clinical Considerations
- The risk of malignancy may be higher in patients with multiple nodules or smaller nodule size 6
- There is often discordance between nodule sizes measured by ultrasound and gross pathology examination, which may affect management decisions 7
- Nodule size alone should not be used for therapeutic decision making in lieu of FNAC 4
- For nodules with indeterminate FNAC results, molecular testing for mutations (BRAF, RAS, RET/PTC, PAX8/PPARc) may provide additional diagnostic information 1
In conclusion, for a patient with a thyroid nodule larger than 1.3 cm and normal thyroid function tests, FNAC is the best confirmatory investigation to determine the risk of malignancy and guide further management.