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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Thyroid peroxidase (TPO)

The thyroid's key enzyme: it oxidises iodide, attaches it to thyroglobulin and joins iodinated tyrosines to form T4 and T3.

Updated 2026-11-01

## Key facts

- A haem protein of the thyrocyte apical membrane; it works at the interface with the colloid.
- It uses hydrogen peroxide made by the dual oxidase DUOX2.
- It does three things: oxidises iodide, iodinates thyroglobulin tyrosines (MIT and DIT) and couples DIT + DIT → T4 and MIT + DIT → T3.
- It is the main autoantigen of Hashimoto thyroiditis (anti-TPO antibodies).
- There is no human ligand-bound structure in the PDB, so this page shows no 3D model.

**Also called**
Iodide peroxidase

**EC number**
[1.11.1.8](https://www.enzyme-database.org/query.php?ec=1.11.1.8)

**Genes**
TPO

## Regulation

- TSH raises its expression and H₂O₂ production (through cAMP).
- An acute iodide excess holds organification back (Wolff–Chaikoff effect), a temporary brake.

## Inhibitors and toxins

- Methimazole and carbimazole (thionamides)
- Propylthiouracil (also inhibits type 1 deiodinase)

## Clinical notes

- Thionamides block TPO and are used in the hyperthyroidism of Graves disease.
- Anti-TPO antibodies are positive in most Hashimoto thyroiditis and in many cases of Graves disease.
- TPO mutations are a common cause of dyshormonogenesis: congenital hypothyroidism with goitre.

## Pathways

- [Thyroid hormone synthesis](https://durgical.com/en/biochemistry/pathways/thyroid-hormone-synthesis/)

## Related conditions

- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Goitre](https://durgical.com/en/conditions/goitre/)

## Related

- [Thyroxine (T4)](https://durgical.com/en/biochemistry/molecules/thyroxine/)
- [Triiodothyronine (T3)](https://durgical.com/en/biochemistry/molecules/triiodothyronine/)
- [Iodothyronine deiodinases (DIO1, DIO2, DIO3)](https://durgical.com/en/biochemistry/enzymes/iodothyronine-deiodinases/)
- [Iodine](https://durgical.com/en/biochemistry/topics/iodine/)
- [Tyrosine](https://durgical.com/en/biochemistry/molecules/tyrosine/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which three reactions does thyroid peroxidase catalyse?

It oxidises iodide, iodinates thyroglobulin tyrosines and couples the iodotyrosines to form T4 and T3.

## Where does TPO get its hydrogen peroxide?

From the dual oxidase DUOX2, on the thyrocyte apical membrane.

## Which antithyroid drugs inhibit TPO?

The thionamides: methimazole, carbimazole and propylthiouracil.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [TPO: Thyroid peroxidase (human)](https://www.uniprot.org/uniprotkb/P07202/entry) (UniProt, 2026); [TPO gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/tpo/) (MedlinePlus, US National Library of Medicine, 2026); [Congenital hypothyroidism: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/congenital-hypothyroidism/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/thyroid-peroxidase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/thyroid-peroxidase/