A thyroid hormone, Triiodothyronine also known as T3, effects almost every physiological process in the body including growth and development, metabolism, body temperature and heart rate. Production of T3 and its prohormone Thyroxine T4 is activated by Thyroid Stimulating Hormone (TSH) released from anterior pituitary gland. It is a loop feedback process so that if T3, T4 hormone blood levels increase, TSH levels decrease. If the levels drop, TSH will increase to stimulate the production of these target hormones to bring them back to the normal blood levels.
T3 is the true hormone effecting the target tissues and it is approximately four times potent than that of T4. The amount of T4 is higher as 80% where T3 is 20%. T3 is produced from T4 by a transformation on the molecule. The half life of T3 is shorter than that of T4. They circulate in blood by binding to a protein (Tyroxine binding globülin and albumin))The Free T3 and Free T4 is the unbound active form of the hormones.The unbound active portion is only 0.3% of the total.
Interpretation:Generally FT3 or FT4 portion measurement is not necessary, since Total T3 will be sufficient. However, FT3 levels may be required to clinically evaluate euthyroid patients who have altered distribution of binding proteins (eg pregnancy and dysalbuminemia). Elevated values are associated with thyrotoxicosis or excess thyroid hormone replacement.
Sample: Arm vein blood. Nonfasting
Working day: Everyday
Result Time: Same day 6:00 PM
