Thyroid hormone is transported in the blood mainly bound to thyroxine-binding globulin (TBG) though other proteins can transport the hormone e.g. T4 -- binding prealbumin, albumin, high-density lipoproteins.

TBG concentration does not change significantly but free-hormone varies directly with total hormone concentration.

The active component is the "free" T3 portion of the thyroid hormone complex.

At the cellular level, T4 is de-iodinated to be transformed into T3. T3 exerts all the effects of thyroid hormone.

Thyroid hormone is mainly metabolised through hepatic conjugation and excreted in bile.