Best Confirmatory Investigation for Thyroid Nodule >1.3 cm with Normal Thyroid Function

Rationale for FNAC as First-Line Investigation

The American College of Oncology recommends fine-needle aspiration cytology (FNAC) as the best confirmatory investigation for a thyroid nodule larger than 1.3 cm with normal thyroid function tests, as it provides the most sensitive and specific method for differentiating between benign and malignant nodules 1, 2, 3
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, 2, 4
Thyroid cancer is relatively rare (approximately 5% of all thyroid nodules), making FNAC crucial for appropriate risk stratification in a population with a high prevalence of thyroid nodules (4-50% depending on diagnostic procedures and patient age) 3
Thyroid function tests (TSH, T3, T4) are of limited value in diagnosing thyroid cancer, as most thyroid cancers present with normal thyroid function 3, 4

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 3, 5
Ultrasound features associated with increased malignancy risk include hypoechogenicity, microcalcifications, absence of peripheral halo, irregular borders, solid aspect, and abnormal blood flow 3, 4

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 3, 5

If FNAC yields inadequate samples, the procedure should be repeated, as recommended by the American College of Oncology 1, 2
In cases of follicular neoplasia with normal TSH and "cold" appearance on thyroid scan (if performed), surgery should be considered, according to the American College of Oncology guidelines 1, 6
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, 3

For nodules with indeterminate FNAC results, molecular testing for mutations (BRAF, RAS, RET/PTC, PAX8/PPARc) may provide additional diagnostic information, as recommended by the American College of Oncology 4