**5. Medullary thyroid cancer and liquid biopsy**

Medullary thyroid cancer (MTC) accounts for only 5% of thyroid cancers; it is responsible for approximately 13% of all thyroid cancer-related deaths [4]. MTC is a malignant thyroid tumor showing evidence of C-cell differentiation [14]. One of the recently studied miRNAs in medullary thyroid cancer is miR-21, which is downregulated especially in the aggressive forms [27]. miR-129-5p also is significantly downregulated in MTC compared to normal tissue leading to increased cellular invasion and migration [28] (**Table 3**).


**Table 3.** Expression of microRNA in MTC.
