[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Triiodothyronine (T3)

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

# Triiodothyronine (T3)

The active form of thyroid hormone: it binds nuclear receptors and speeds up metabolism, the heart and brain development.

Updated 2026-11-01

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## Key facts

- It binds the thyroid receptor (TRα, TRβ) with much higher affinity than T4.
- About 80 % of circulating T3 comes from peripheral deiodination of T4.
- It raises O₂ use and heat production (more Na⁺/K⁺ pump, more UCP1), heart rate (more β1 receptors) and fat and protein breakdown.
- It is essential for nervous system and skeletal development in the fetus and infant.

**Also called**
T3, Liothyronine (drug form)

**Formula**
C15H12I3NO4

**PubChem**
[CID 5920](https://pubchem.ncbi.nlm.nih.gov/compound/5920)

## Structure

- Like T4 but with three iodines (3, 5 and 3'): it lacks the outer-ring 5' iodine.
- Reverse T3 (3, 3', 5') has the same atoms in another order and is inactive.

## Clinical notes

- Untreated congenital hypothyroidism causes intellectual disability and short stature: that is why every newborn is screened.
- In hyperthyroidism T3 often rises more than T4 (T3 thyrotoxicosis).
- In fasting and severe illness T3 falls and reverse T3 rises, an energy saving.

## 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/)

## Related

- [Thyroxine (T4)](https://durgical.com/en/biochemistry/molecules/thyroxine/)
- [Iodothyronine deiodinases (DIO1, DIO2, DIO3)](https://durgical.com/en/biochemistry/enzymes/iodothyronine-deiodinases/)
- [Thyroid peroxidase (TPO)](https://durgical.com/en/biochemistry/enzymes/thyroid-peroxidase/)
- [Nuclear receptors](https://durgical.com/en/biochemistry/topics/nuclear-receptors/)
- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)
- [UCP1 (thermogenin)](https://durgical.com/en/biochemistry/enzymes/ucp1/)

## Test yourself

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## Why is T3 more potent than T4?

Because it binds the nuclear thyroid receptor with much higher affinity.

## Where does most circulating T3 come from?

From deiodination of T4 in peripheral tissues by DIO1 and DIO2.

## What is the structural difference between T3 and reverse T3?

T3 has iodines at 3, 5 and 3'; reverse T3 at 3, 3' and 5', and it does not activate the receptor.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [Liothyronine (CID 5920)](https://pubchem.ncbi.nlm.nih.gov/compound/5920) (PubChem, NCBI, 2026) · Public domain; [T3 test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003687.htm) (MedlinePlus, US National Library of Medicine, 2026); [THRB: Thyroid hormone receptor beta (human)](https://www.uniprot.org/uniprotkb/P10828/entry) (UniProt, 2026); [3GWS: Crystal Structure of T3-Bound Thyroid Hormone Receptor](https://www.rcsb.org/structure/3GWS) (RCSB Protein Data Bank, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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