hormone
Thyroxine (T4)
T4 is the main thyroid hormone released into circulation and a precursor that tissues convert to T3.
Educational draft · Human scientific and editorial review pending. About review status
T4 contains four iodine atoms and has a longer circulating half-life than T3; most activity comes after conversion or receptor interaction in tissues.
How it works
A circulating precursor
The thyroid secretes mostly T4, which travels in blood bound to transport proteins. Peripheral tissues remove an iodine atom to make T3 or produce reverse T3. These conversion routes help explain why T4 and T3 are separate entities.
Related topics
Iodine is incorporated into Thyroxine (T4)
The thyroid incorporates iodine into thyroxine (T4), one of the thyroid hormones.
Evidence: Established · Source checked 2026-09-05 · Sources: NIH Office of Dietary Supplements: Iodine: Fact Sheet for Health Professionals
Thyroxine (T4) is converted in tissues to Triiodothyronine (T3)
Peripheral tissues convert some T4 to the more receptor-active T3.
Evidence: Established · Source checked 2026-09-05 · Sources: NIH National Library of Medicine: Physiology, Thyroid Hormone
Worth knowing
T4 is not simply inactive. T4 has biological actions and serves as the major circulating reservoir for T3 production; “inactive precursor” is an oversimplification.
It contains four iodine atoms. The four iodine atoms in thyroxine distinguish T4 structurally from triiodothyronine, which contains three.
A closer look
Iodine is a structural input
Iodine is incorporated into thyroid hormones during synthesis. Iodine supply and thyroid-hormone production are related but distinct processes.
References
Source-checked local draft.
Human editorial review pending.
- NIH National Library of Medicine (opens in new tab)Physiology, Thyroid HormoneChecked 2026-09-05
- NIH National Library of Medicine (opens in new tab)Physiology, Endocrine HormonesChecked 2026-09-05
- NIH Office of Dietary Supplements (opens in new tab)Iodine: Fact Sheet for Health ProfessionalsChecked 2026-09-05