Deiodinases in thyroid tumorigenesis.

The three deiodinase selenoenzymes are key regulators of intracellular thyroid hormone (TH) levels. The two TH-activating deiodinases (type 1 deiodinase and type 2 deiodinase (D2)) are normally expressed in follicular thyroid cells and contribute to overall TH production. During thyroid tumorigenesis, the deiodinase expression profile changes to customize intracellular TH levels to different requirements of cancer cells. Differentiated thyroid cancers overexpress the TH-inactivating type 3 deiodinase (D3), likely to reduce the TH signaling within the tumor. Strikingly, recent evidence suggests that during the late stage of thyroid tumorigenesis, D2 expression raises and this, together with a reduction in D3 expression levels, increases TH intracellular signaling in dedifferentiated thyroid cancers. These findings call into question the different functions of TH in the various stages of thyroid cancers.