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.

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.

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.

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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.

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) (PubChem, NCBI, 2026) · Public domain; T3 test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); THRB: Thyroid hormone receptor beta (human) (UniProt, 2026); 3GWS: Crystal Structure of T3-Bound Thyroid Hormone Receptor (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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