Triiodothyronine (T3) is a thyroid hormone that is produced and secreted by the thyroid gland, an essential component of the endocrine system that sits at the base of the neck.


What is triiodothyronine?

Triiodothyronine, which is also known as T3, is an active form of thyroxine (T4), a thyroid hormone that is responsible for a number of key physiological functions.

Triiodothyronine, like thyroxine plays a role in regulating the metabolism and digestion, ensuring that the heart and brain are functioning correctly as well as the maintenance of strong, healthy bones. Triiodothyronine, as a biologically active form of thyroxine (T4), also regulates thyroid hormone (TH), which is required for normal development as well as regulating metabolism.

T3 is secreted from the thyroid gland in a much smaller volume than T4. About 20% of total functioning triiodothyronine is secreted by the thyroid gland. A further 80% of the total volume of triiodothyronine is produced during the conversion process of thyroxine to triiodothyronine.

Both triiodothyronine and thyroxine are produced when thyroid stimulating hormone is activated via messages from the hypothalamus and both triiodothyronine and thyroxine feedback to the pituitary gland.

What is free triiodothyronine?

Free triiodothyronine, also known as FT3, refers to triiodothyronine, that is not bound to protein. As we know, only 20% of total triiodothyronine is released from the thyroid gland, the remaining volume comes from converted thyroxine.

In sum: Free triiodothyronine refers to the proportion of triiodothyronine that is not bound to protein.

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Written by Hannah Kingston | Medically Reviewed by Gwen Murphy, PHD, MPH