Triiodothyronine (T3) is one of the hormones produced by the thyroid gland. It is involved in the regulation of metabolism, affects the functioning of almost all organs and systems of the body.

Free T3 is the active form of the hormone. It is formed mainly as a result of the metabolic transformation of thyroxine (T4) by its deiodization (iodine cleavage). This process is especially active in the anterior pituitary gland and in the peripheral tissues of the body.