Cytologic evaluation of thyroid nodule aspirates represents a pivotal tool to assess patients with discovered thyroid lesion(s). Usually, cytologic samples are satisfactory and permit to be classified as diagnostic. In fact, false-negative and false-positive FNAC reports are very rare (i.e., <2 %) [1]. However, a non-negligible rate (up to 15–25 %) of thyroid cytology is inconclusive due to inadequate material (Thy 1, Category I) or indeterminate diagnosis (Thy 3, Category III or IV). Nodules with Thy 1 or Category I need to undergo new FNAC, and those lesions with repeated inadequate sample should be addressed to diagnostic surgery or CNB (see below) [2]. A large amount of articles published in the last decade focused on the use of several potential markers to diagnose thyroid nodules with indeterminate FNAC report. However, no laboratory, US, scintigraphic, molecular, or clinical feature can be used alone to exclude thyroid malignancy. In these cases the diagnostic surgery remains mandatory.

Until the 1990s FNAC was performed without US guidance, and the rate of inadequate samples was high; also, the overall accuracy of the technique was reported as suboptimal. In the last years US-guided FNAC has been worldwide diffused. So, the accuracy of cytologic reports has been significantly improved. Real-time US guidance improves accuracy in positioning the needle into the nodule. Moreover, complications are very rare and FNAC can be performed in ambulatory office [1, 2].

Generally, a 23–27 gauge needle attached to a syringe is used. Several fashions can be adopted for thyroid FNAC. Free-hand mode is one of those more diffuse. On the other hand, the use of a device is frequently adopted. In these methods the needle is inserted parallel to the probe or at an angle of that. The parallel approach is more comfortable for the operator because the needle may be viewed as it traverses the nodule. A perpendicular approach is largely used due to its simplicity for less experienced operators. Complications are reduced by the latter. Regardless of fashion to perform the aspiration, US is highly useful to guide the procedure.

Once obtained the cytologic sample, the specimen is traditionally prepared on slides (in a number of 4 to 6, the majority of which are fixed for Papanicolaou stain). Recently, the thin-layer preparation is diffusing; the needle is washed into a syringe of solution and the sample is prepared for cell block. In specific conditions, such as nodules with prior indeterminate FNAC report, ancillary studies (i.e., immunocytochemistry for galectin-3) may be applied to cell blocks. As an extension of cytologic examination, the measurement of calcitonin in the washout fluids of the needle achieves high interest in those patients with suspicious medullary thyroid cancer; this approach significantly improves the detection of this cancer [15, 16].

As mentioned above, US is an accurate method for identifying suspected recurrence of thyroid cancer in enlarged lymph nodes; then, neck lymph nodes suspicious for metastases from thyroid cancers can be aspirated. The procedure, generally, is easy. However, the cytologic sample may not be adequate for diagnosis. The measurement of thyroglobulin in the washout of the needle is necessary; high levels of thyroglobulin can detect a metastasis from differentiated thyroid cancer [17]. Furthermore, high calcitonin value in a cervical lymph node identifies a metastasis from medullary cancer [16].