Controversies Regarding the Interpretation of Follicular Thyroid Nodules.

CONTEXT.—: Follicular nodules are the most common source of diagnostic difficulties in the practice of surgical pathology of the thyroid. This is due to a variety of factors, the most salient of which is the lack of well-defined criteria and evidence-based data for the diagnosis of these lesions. OBJECTIVES.—: To discuss some of the assumptions that have been accrued over the years regarding the criteria by which we evaluate such lesions. DATA SOURCES.—: The information presented herein is based on review of the literature and the author's personal experience. CONCLUSIONS.—: Thyroid nodules with a predominant follicular growth pattern span the range from benign lesions (hyperplastic nodules, adenomatoid nodules, follicular adenomas) to malignant neoplasms (follicular carcinoma, follicular variant of papillary carcinoma) with a host of intermediate or indeterminate lesions found in between. Advances in immunohistochemistry and molecular pathology have not yet provided a reliable way of separating the borderline or intermediate cases. Low-grade and intermediate or borderline follicular-patterned thyroid lesions are those most often prone to difficulties for interpretation. Newer and potential future approaches for the evaluation of these lesions are discussed.