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

# Thyroxine (T4)

The hormone the thyroid makes most; it is mainly a prohormone that tissues turn into active T3.

Updated 2026-11-01

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## Key facts

- It makes up most of what the thyroid secretes; the rest is T3.
- Over 99 % circulates bound to proteins (thyroxine-binding globulin, transthyretin, albumin); its half-life is about a week.
- Deiodinases DIO1 and DIO2 turn it into T3; DIO3 turns it into inactive reverse T3.
- It enters cells through transporters such as MCT8.

**Also called**
T4, Tetraiodothyronine, Levothyroxine (drug form)

**Formula**
C15H11I4NO4

**PubChem**
[CID 5819](https://pubchem.ncbi.nlm.nih.gov/compound/5819)

## Structure

- Two tyrosine rings joined by an ether, with four iodine atoms (3, 5, 3' and 5').
- It forms when two diiodotyrosine residues couple inside thyroglobulin.

## Clinical notes

- Levothyroxine (synthetic T4) is the standard treatment for hypothyroidism.
- Free T4 is measured together with TSH to assess thyroid function.
- Lack of MCT8 (Allan–Herndon–Dudley syndrome) stops the hormone from reaching neurons.

## 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

- [Triiodothyronine (T3)](https://durgical.com/en/biochemistry/molecules/triiodothyronine/)
- [Thyroid peroxidase (TPO)](https://durgical.com/en/biochemistry/enzymes/thyroid-peroxidase/)
- [Iodothyronine deiodinases (DIO1, DIO2, DIO3)](https://durgical.com/en/biochemistry/enzymes/iodothyronine-deiodinases/)
- [Tyrosine](https://durgical.com/en/biochemistry/molecules/tyrosine/)
- [Iodine](https://durgical.com/en/biochemistry/topics/iodine/)
- [Hormone classes and mechanisms](https://durgical.com/en/biochemistry/topics/hormone-classes-and-mechanisms/)

## Test yourself

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

## How many iodine atoms does thyroxine have?

Four, at positions 3, 5, 3' and 5'.

## Why is T4 considered a prohormone?

Because it binds the nuclear receptor weakly; tissues turn it into T3, the active form.

## Which proteins carry T4 in blood?

Thyroxine-binding globulin, transthyretin and albumin.

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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: [Thyroxine (CID 5819)](https://pubchem.ncbi.nlm.nih.gov/compound/5819) (PubChem, NCBI, 2026) · Public domain; [Free T4 test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003517.htm) (MedlinePlus, US National Library of Medicine, 2026); [Thyroid Diseases | Hypothyroidism | Hyperthyroidism | MedlinePlus](https://medlineplus.gov/thyroiddiseases.html) (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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