The lack of Na(+)/I(-) symporter (NIS) gene expression in some thyroid cancer patients has been a major hurdle that limits the efficacy of standard radioactive iodide therapy. The molecular mechanism that contributes to low NIS expression is not well understood. Activated NIS gene expression is stimulated by thyroid-stimulating hormone-mediated cAMP/protein kinase A signaling through a NIS upstream enhancer (NUE). The cAMP pathway is also stimulated by forskolin. In the current work, we studied the mechanism of transcriptional activation of NIS in normal thyroid cells and thyroid cancer cells. We identified the cAMP response element modulator (CREM) activator as a new component of the transcription complex that is important for NIS gene expression. The CREM complex is seen in the normal thyroid cells and BRAF (V600E) thyroid cancer cells (BHP 17-10) but is missing in rearranged in transformation/papillary thyroid carcinoma-1 rearrangement thyroid cancer cells (BHP 2-7). This complex is believed to be responsible for the loss of NUE activity and reduced NIS expression in the BHP 2-7 cell line. In BHP 2-7 cells, forskolin stimulated the thyroid-specific transcription factor Pax 8, but CREM activator mRNA did not increase, and this produced a small increase in NUE activity. Ectopic expression of CREM activator enhanced activity of the NUE, indicating that CREM is an essential regulator of NIS gene expression.