Long noncoding RNA (lncRNA) Cancer Susceptibility 2 (CASC2) has been proved to contribute to the development of cancers. However, the mechanism behind the action of CASC2 in thyroid cancer is not quite clear. We demonstrated that CASC2 was downregulated in thyroid cancer. We noted that CASC2 overexpression restrained the growth, migration, and invasion of thyroid cancer cells, whereas CASC2 depletion caused opposite trends. Bioinformatics analysis predicted that hypoxia inducible factor 1 subunit alpha inhibitor (FIH-1) was potentially targeted by miR-18a-5p, which was confirmed by luciferase reporter assay. Upregulation of FIH-1 abrogated the promotive effect of miR-18a-5p on the growth and invasion of thyroid cancer cells. In addition, CASC2 serves as a competing endogenous RNA (ceRNA) and a ''sponge'' for miR-18a-5p, thereby regulating the expression of FIH-1. These data elucidated the CASC2/miR-18a-5p ceRNA network in thyroid cancer pathogenesis.