deiodinaser

Deiodinases, also known as deiodinase enzymes, are a small family of selenoproteins that regulate thyroid hormone signaling by removing iodine atoms from thyroid hormones. In humans there are three related enzymes: DIO1, DIO2, and DIO3. They differ in substrate preference, cellular location, and physiological role, allowing tissue-specific control of thyroid hormone action.

DIO1 and DIO2 primarily generate the active hormone triiodothyronine (T3) from thyroxine (T4) through outer-ring deiodination, while DIO3 mainly performs inner-ring deiodination, converting T4 to reverse T3 (rT3) and T3 to inactive metabolites such as T2. DIO2 is especially important for local T3 production in brain, pituitary, and brown adipose tissue; DIO3 protects developing tissues by inactivating thyroid hormones; DIO1 contributes to circulating T3 in liver and kidney and can perform both outer- and inner-ring reactions depending on context.

These enzymes enable tissue-specific regulation of thyroid hormone signaling, complementing circulating hormone levels. They influence development, metabolism, growth, and thermogenesis by shaping intracellular T3 availability independent of serum T3 concentrations.

Regulation of deiodinase activity is influenced by thyroid status, age, nutrition, illness, and inflammatory signals. Selenium status is important because selenocysteine is essential for their catalytic activity. Clinically, altered deiodinase expression or activity is associated with non-thyroidal illness, metabolic disorders, and developmental conditions, reflecting their pivotal role in fine-tuning thyroid hormone action. See also thyroid hormone and non-thyroidal illness.