Many thyroid tumors can be readily diagnosed using cytologic and histopathologic criteria, which allow the pathologist to distinguish benign from malignant lesions and guarantee accurate classification of the majority of malignant tumors. However, in several situations, the pathologist is confronted with thyroid lesions in which the distinction between benign and malignant can be quite subtle, or the exact classification of malignancy is not evident on routine examination. The accuracy of diagnosis has clinical consequences and implies different modalities of treatment. On one hand, there is the need to avoid excessive treatment and psychological discomfort to the patient. On the other hand, patients with potentially aggressive disease need to be guaranteed effective management at the initial stages of disease when it is still curable. For this reason, the approach to these challenging situations should include the application of ancillary techniques, such as immunohistochemistry, immunophenotyping, and molecular profiling, that can improve the standard morphologic assessment both in surgical specimens [1] and in cytology samples obtained by fine needle aspiration (FNA) [2].

Genetic studies have identified a process of cumulative molecular events involved in thyroid tumor initiation and progression, resulting in genomic instability and the capacity for independent cellular growth, invasion, and metastasis [3]. It seems unrealistic to expect a single tool, in the form of a magic biomarker, to be able to effectively resolve the diagnostic dilemmas in thyroid pathology. Each marker differentially expressed in tumorous and nontumorous tissues represents a snapshot of the molecular events succeeding in the tissue environment. The amount of information a single marker offers is often insufficient to understand tumor biology or to render accurate diagnosis. The use of combined immunohistochemical markers as a panel seems to be an alternative to aid some of the diagnostic challenges in surgical pathology and cytopathology of thyroid specimens [1, 4, 5], and modern genomic and proteomic technological approaches are being developed to introduce molecular signatures capable of separating benign from malignant thyroid tumors, and in the last group, to distinguish tumors with indolent and aggressive behavior [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

The availability and application of these ancillary tools vary from laboratory to laboratory. In this section, we will review briefly the technologies available. The application of individual tests will be discussed in the sections that follow, where the significance of their results will clarify differential diagnosis.