Description

T3 (triiodothyronine) is the biologically active form of thyroid hormones. Approximately one-third to one-fifth of the total thyroid hormones secreted by the thyroid gland enter the bloodstream directly as T3. The remaining two-thirds to four-fifths are released as thyroxine (T4), a biologically inactive prohormone.

In peripheral tissues, thyroxine undergoes conversion to T3 through the action of selenium-dependent metalloenzymes known as monodeiodinases. This deiodination process is what transforms inactive T4 into active T3, making it responsible for most of the metabolic effects attributed to thyroid hormones.