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deiodinase, iodothyronine, type I
Symbol(s): DIO1 TXDI1
Locus: 1 p32 -p33
EC number
EntrezGene 1733
OMIM 147892
RefSeq NM_000792
UniProt P49895
deiodinase, iodothyronine, type II
Symbol(s): DIO2
Locus: 14 q24.2 -24.3
EC number [1]
EntrezGene 1734
OMIM 601413
RefSeq NM_000793
UniProt Q92813
deiodinase, iodothyronine, type III
Symbol(s): DIO3 TXDI3
Locus: 14 q32
EC number [2]
EntrezGene 1735
OMIM 601038
RefSeq NM_001362
UniProt P55073
deiodinase, iodothyronine, type III opposite strand
Symbol(s): DIO3OS C14orf134
Locus: 14 q32.33
EC number [3]
EntrezGene 64150
OMIM 608523
RefSeq NM_022345
UniProt Q9HAR6

Deiodinase (EC is an enzyme important in the action of thyroid hormones.

Deiodinases are unusual in that the enzyme contains selenium, in the form of an otherwise rare amino acid selenocysteine.

Activation and inactivationEdit

In the tissues, deiodinases can either activate or inactivate thyroid hormones:

  • Activation occurs by conversion of the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3) through the removal of an iodine atom on the outer ring.
  • Inactivation of thyroid hormones occurs by removal of an iodine atom on the inner ring, which converts thyroxine to the inactive reverse triiodothyronine (rT3), or which converts the active triiodothyronine to the inactive diiodothyronine (T2). The major part of thyroxine deiodination occurs within the target cells.


In most vertebrates, there are three types of enzymes that can deiodinate thyroid hormones:

Type Location Function
type I (DI) is commonly found in the liver, kidney, muscle tissue and thyroid gland DI can deiodinate both rings
type II deiodinase (DII) mostly in the brain but also in the testis and thyroid DII can only deiodinate the outer ring of the prohormone thyroxine (or the metabolically inactive reverse triiodothyronine) and is the major activating enzyme
type III deiodinase (DIII) found in the fetal tissue and the placenta DIII can only deiodinate the inner ring of thyroxine or triiodothyronine and is the major inactivating enzyme


File:Iodothyronine deiodinase.png

External linksEdit


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