What diagnostic study is not helpful in distinguishing follicular adenoma from follicular carcinoma?

Fine needle aspiration (FNA) is a diagnostic tool commonly used to evaluate thyroid nodules, but it does not provide sufficient information to distinguish between follicular adenoma and follicular carcinoma. Both conditions can present similarly on cytological examination, as FNA primarily provides cellular samples that may not reveal essential architectural features, which are crucial for differentiation. Specifically, the presence of capsular invasion is a significant histological criterion for diagnosing follicular carcinoma, but FNA specimens generally lack the necessary tissue architecture to assess this feature accurately.

Other studies such as ultrasound, thyroid function tests, and CT scans may provide complementary information about nodule characteristics or overall thyroid function, but they are not definitive in differentiating between these two forms of thyroid neoplasm. Thus, reliance on FNA alone fails to resolve the distinction and reinforces its limited utility in this particular regard.