The Action of T3 and T4 Within The Body

Within the blood plasma, over 99% of the T3 and T4 are bound to proteins e.g. thyroid binding globulin and albumin.

This is metabolically inactive and only the small unbound component is able to have any clinical effects. Free (unbound T3, T4) hormone level measurements are preferred as they are not influenced by concentrations of binding proteins.

At the cells, T4 is a prohormone whereas T3 is active. Both are taken into the cells where the T4 can be converted in to T3. The T3 can bind onto receptors on the nucleus influencing the activities of the mitochondria and gene transcription [2].