Free T3 (FT3)

Triiodothyronine, the thyroid hormone also known as T3, affects almost every physiological process in the body, including growth and development, metabolism, body temperature and heart rate. The production of T3 and its prohormone Thyroxine T4 is activated by Thyroid Stimulating Hormone (TSH) released from the anterior pituitary gland. If T3, T4 hormone levels in the blood rise, TSH levels fall, this is a loop feedback process. If levels fall, TSH will increase to stimulate the production of these target hormones, bringing these values back to normal blood levels. T3 is the main hormone affecting the target tissues and is about four times more effective than T4. The amount of T3 is 20 per cent, while the amount of T4 is higher, 80 per cent. T3 is produced from T4 by deiodination on the molecule. The half-life of T3 is shorter than the half-life of T4. They circulate in the blood by binding to a protein (Thyroxine binding globulin and albumin). Free T3 and Free T4 are the unbound active form of the hormones. The unbound active portion is only 0.3% of the total.
Interpretation:Measurement of FT3 or the FT4 moiety is usually not necessary as total T3 will be sufficient. However, FT3 levels may be required to clinically evaluate euthyroid patients with altered distribution of binding proteins (e.g., pregnancy and dysalbuminemia). Elevated values are associated with thyrotoxicosis or excessive thyroid hormone replacement.
Sample: Venous blood from the arm. Fasting status is not required.
Working day: Every day
Result Time: Same day at 18:00