hormone
Triiodothyronine (T3)
T3 is the more biologically active thyroid hormone that acts through nuclear receptors to change gene expression.
Most circulating T3 comes from conversion of thyroxine (T4) in peripheral tissues, while the thyroid also secretes some T3.
A receptor-active thyroid hormone
T3 enters cells and binds thyroid-hormone receptors in the nucleus, changing transcription of target genes. These effects develop over hours to days and vary among tissues.
Related topics
Thyroxine (T4) is converted in tissues to Triiodothyronine (T3)
Peripheral tissues convert some T4 to the more receptor-active T3.
Worth knowing
T3 is made from T4 as well as secreted. Peripheral deiodination converts T4 to T3, making T4 a major circulating precursor.
A closer look
Feedback controls production
The hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to release thyroid-stimulating hormone (TSH). TSH stimulates thyroid-hormone production, while T3 and T4 feed back to reduce upstream signals.
References
Source-checked local draft.
Human editorial review pending.
- NIH National Library of MedicinePhysiology, Thyroid HormoneChecked 2026-09-05
- NIH National Library of MedicinePhysiology, Endocrine HormonesChecked 2026-09-05