Fine needle aspiration (FNA) biopsy and fine needle capillary sampling are the two most widely used diagnostic techniques for assessing the cytopathology of thyroid nodules to identify those patients who have nodules that should be excised. This topic will review the most common cytopathologic findings and the clinical entities they represent. Thyroid biopsy techniques (including core needle biopsies) and their indications, utility, and complications are discussed separately. (See "Thyroid biopsy" and "Ultrasound-guided thyroid biopsy".)
The cytologic diagnosis of follicular lesion includes cells with a macrofollicular (abundant colloid, fragments of macrofollicles forming flat sheets of uniform follicular cells), microfollicular (scant colloid, small microfollicles or “rosettes” of crowded cells), or mixed (macrofollicles and microfollicles, or cells with mild nuclear atypia, or extensive oncocytic change) pattern (table 1).
Benign (macrofollicular) cytology — This pattern may also be referred to as adenomatoid, hyperplastic, or colloid adenomas. Large colloid-filled thyroid follicles are found in normal thyroid tissue, sporadic goiter, and monoclonal macrofollicular tumors.
Surgical specimens of sporadic goiter often contain many such macrofollicles (picture 1). During needle aspiration, the follicles are frequently disrupted so that the colloid is smeared across the slide, or occasionally is aggregated into droplets. The follicular cells lining the follicles may remain intact, or break apart and form sheets.
Cytology of macrofollicular nodules may show both intact macrofollicles and macrofollicles that have been broken apart forming a flat sheet (picture 2). The cells appear uniform in size and are not crowded together. Follicle size may vary, with a few microfollicles interspersed among the macrofollicles, especially if the sample was obtained from an area close to the capsule of the lesion. At high power the follicular cells are seen to be small, uniform in size, flat, and non-crowded, and smeared colloid is seen in the background (picture 3).