DNA DIO2 (Deiodinase 2) Gene Test to detect mutation affecting the conversion of T4 to T3
D2 deiodinase enzymes are produced in our cells to enable the conversion of T4 to the much stronger form of thyroid hormone, T3. A mutation in the DIO2 gene can cause a deficiency of T3 within the cells, but the usual thyroid hormone function tests will not show up a problem. This means that TSH, T4 and T3 blood tests will look normal.
Most of the biological effects of thyroid hormone in the body are due to the action of T3. The most common forms of thyroid hormone replacement involve giving T4 in the form of Synthroid, Levoxyl, Levothyroxine, among others. Due to polymorphism of the deiodinase 2 gene, the eripheral conversion of T4 to T3 is impaired, and the addition of T3 might be needed to compensate for reduced intracellular T3 levels which cannot be detected on blood tests.