The thyroid gland: recurring problems in histologic and cytologic evaluation.

Nodules of the thyroid gland are frequently encountered, occurring in up to 7% of the population, and although most of these nodules are benign, carcinomas of the thyroid gland are the most common malignancy of the endocrine system. Although the different types of thyroid carcinoma are few, a wide variety of recurring problems exists in both their histologic and cytologic evaluation. Here, I will review a selected group of problematic areas, including unusual histologic variants of follicular adenoma, criteria for diagnosing minimally invasive follicular carcinoma, the use of fine-needle aspiration as a screening test for follicular neoplasia, challenging variants of papillary carcinoma, and features of poorly differentiated carcinoma.