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
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
Related conditions
Related
- Triiodothyronine (T3)
- Thyroid peroxidase (TPO)
- Iodothyronine deiodinases (DIO1, DIO2, DIO3)
- Tyrosine
- Iodine
- 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.
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) (PubChem, NCBI, 2026) · Public domain; Free T4 test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Thyroid Diseases | Hypothyroidism | Hyperthyroidism | MedlinePlus (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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