Deiodinase (monodeiodinase) is a peroxidase enzyme that is involved in the activation or deactivation of thyroid hormones.
Iodotyrosine deiodinase employs a flavin mononucleotide cofactor and belongs to the NADH oxidase/flavin reductase superfamily.
[2] In starvation or severe somatic stress, deiodinase type 1 is inhibited which lowers circulating levels of T3 (due to it being the main source of peripherally converted T3 from T4 in the plasma), causing a decrease in the metabolic rate.
This effect of caloric restriction makes sense for someone who is starving because it tends to conserve body stores of fuel.
Selenium in iodothyronine deiodinase, as selenocysteine, plays a crucial role in determining the free circulating levels of T3.