deiodination

Deiodination is the removal of iodine atoms from organic molecules. In biology it is a key step in thyroid hormone metabolism, affecting hormone activity and availability, while in environmental chemistry it describes the breakdown of iodinated organic compounds.

In vertebrates, deiodination is mainly enzymatic and is carried out by iodothyronine deiodinases (DIO1, DIO2, DIO3). These enzymes catalyze outer ring deiodination (ORD) and inner ring deiodination (IRD). ORD removes an iodine from the outer ring and often converts thyroxine (T4) to the more active triiodothyronine (T3). IRD removes an iodine from the inner ring and typically inactivates thyroid hormones, producing reverse T3 (rT3) from T4 and, from T3, T2.

DIO1 and DIO2 predominantly perform ORD and contribute to circulating or tissue T3 production, whereas DIO3 predominantly performs IRD and helps limit thyroid hormone action by forming rT3 and T2. Tissue distribution varies: DIO2 is common in brain, pituitary, and brown adipose tissue; DIO1 is abundant in liver and kidney; DIO3 is prominent in placenta and fetal tissues.

Regulation and clinical relevance: deiodination adjusts thyroid hormone availability in response to metabolic state, development, and disease. Altered deiodinase activity can contribute to thyroid-related disorders, developmental abnormalities, and metabolic dysregulation.

Environmental deiodination refers to microbial and chemical processes that remove iodine from iodinated xenobiotics, influencing detoxification, environmental fate, and remediation strategies. In radiochemical contexts, deiodination pathways affect the turnover of radioactive iodine.