The enzyme primarily responsible for the conversion of T4 to T3 in the periphery is

Three different deiodinases act on thyroid hormones: D1, D2, and D3. All are unique in that they contain the rare amino acid selenocysteine, with selenium in place of sulfur, which is essential for their enzymatic activity. D1 is present in high concentrations in the liver, kidneys, thyroid, and pituitary. It appears primarily to be responsible for maintaining the formation of T3 from T4 in the periphery. D2 is present in the brain, pituitary, and brown fat. D3 is also present in the brain and in reproductive tissues. It acts only on the 5 position of T4 and T3 and is probably the main source of 3 in the blood and tissues.