DNA ThyroidDNA Thyroid DIO2 Gene Testing DIO2 codes for Type II iodothyronine deiodinase (D2) and forms parts of a family of three iodothyronine deiodinase enzymes which are responsible for the activation and deactivation of thyroid hormones and for the conversion of thyroxin (T4) to the active or inactive form of triiodothyronine (T3). D2 is the only enzyme able to convert T4 to T3 in the brain and is likely to play a key role in determining the ability of the