Free Thyroxine (FT4), Serum/Plasma


In the thyroid gland, the trapped iodide is oxidised by thyroid peroxidase (TPO) and incorporated into thyroglobulin yielding monoiodinated and diiodinated residues which are enzymatically coupled to form triiodothyronine (T3) and thyroxin (T4). T4 concentration is around 40 fold higher than T3. Only 0.03% of T4 and 0.3% of T3 are found in the free form while the rest are found bound to carrier proteins. It is the free Thyroid hormone (TH) that generates a biological response. In peripheral tissue, the majority of T4 is enzymatically converted to T3. In addition to their role on the metabolism of macronutrients and overall energy and oxygen consumption, THs are critical for normal bone growth and development, they lower systemic vascular resistance, increase blood volume, and have inotropic and chronotropic effects on cardiac function, play important roles in the development and function of brown and white adipose tissue, have multiple effects on liver function including stimulation of enzymes regulating lipogenesis and lipolysis as well as oxidative processes, regulate the synthesis and secretion of several pituitary hormones and have major effects on the developing brain in utero and during the neonatal period.


FT4 along with TSH are used in the diagnosis of Hyperthyroidism (increased production of THs) which can result from several disorders including Grave’s disease (an autoimmune disease), Toxic Adenoma, Toxic Multinodular Goitre, Plummer’s disease, and Thyroditis. FT4 and TSH are also used in the diagnosis of Hypothyroidism (decreased production of THs) which can result from Hashimoto’s Thyroditis (an autoimmune disease), Iodine insufficiency, secondary to depressed secretion of TSH, Thyroid surgery, certain medications and congenital thyroid defects or absence. Hypothyroidism can also develop during pregnancy or shortly after delivery. FT4 is also used in monitoring thyroid disorders therapy. Only 0.03% of T4 and 0.3% of T3 are found in the free form while the rest are found bound to carrier proteins. See Also Thyroid Stimulating Hormone (TSH) , and Thyroglobulin, Serum

Sample Type, Quantity & Conditions

1 ml Serum 1 ml Li-Heparin or K2, K3-EDTA Plasma Stability: 7 Days at 2-8 °C 30 Days at -20 °C

Special Precautions

Freeze only once.

Normal Range

Newborn: 0.86 - 2.49 ng/dL 6 Days - 3 Months: 0.89-2.20 ng/dL 4 Months - 12 Months: 0.92-1.99 ng/dL 1 Year - 6 Years: 0.96-1.77 ng/dL 7 Years - 11 Years: 0.97-1.67ng/dL 11 - 20 Years: 0.98-1.63 ng/dL Male 20 - 39 Years 1.04 - 1.65 ng/dL Male 40 - ˃69 0.97 - 1.65 ng/dL Female 20 - 39 0.97 - 1.58 ng/dL Female 40- ˃69 0.89 - 1.52 ng/dL

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