Free T4 (Thyroxin) MICRO-ELISA Test Kit - 96 tests

Free T4

The principal tests used in the laboratory evaluation of thyroid function areTotal Thyroxin (T4), Total Triiodothyronine (T3), T-Uptake (T-Up), acalculated Free Thyroxin Index (FTI) and Thyroid Stimulating Hormone(TSH). The results of these tests are interrelated and help the clinician inmaking a diagnosis. Clinical hypothyroidism results from underproductionof thyroid hormones by the thyroid gland, consequently an abnormally lowcirculating T4 and T3 concentration in blood. Clinical hyperthyroidismresults from excessive production of thyroid hormones and resultingelevation of T4 and T3 concentrations.The manifestations of thyroid dysfunction can result from disease of thethyroid gland (PRIMARY hyperthyroidism or hypothyroidism), disease ofthe pituitary gland (SECONDARY hyperthyroidism or hypothyroidism) ordisease of the hypothalamus (TERTIARY hyperthyroidism orhypothyroidism).Thyroxin (3, 5, 3', 5'-tetraiodo-l-thyronine, T4) and Triiodothyronine (3, 5, 3'-triiodo-l-thyronine, T3), are the hormones originating from the thyroidgland. T4 and T3 are responsible for regulating diverse biochemicalprocesses throughout the body that are essential for protein synthesis, normal development, metabolic and neural activity.T4 is synthesized within the thyroid gland and secreted directly into thebloodstream. Approximately 30% of the circulating T4 is enzymaticallydeiodinated at the 5' position in the peripheral tissues to yield T3. The T4likely serves as a "prohormone" for T3, which has a much greatermetabolic activity.T4 and T3 are transported through the peripheral blood stream largelybound to serum proteins. The major transport protein is Thyroxin BindingGlobulin (TBG) which normally accounts for 80% of the bound hormone.The other thyroid hormone binding proteins are Thyroxin BindingPrealbumin and Albumin. Only about 0.3% of the total serum T3 and onlyabout 0.1% of the total serum T4 are unbound and free to diffuse intotissue to exert their biological effects. When the level of TBG increases, the level of total T4 will increase to maintain the same level of unbound orfree T4 in the bloodstream of an euthyroid individual.Simply determining the total T4 concentration fails to take into account thevariations in TBG levels that affect the unbound thyroxin (free T4)concentration. TBG levels can vary for reasons incidental to the patient'sthyroid status such as the presence of certain drugs, steroid hormones, pregnancy, and various non-thyroidal diseases. The Thyroid Uptake (TUp)test is an indirect measurement of empty binding sites for T4 on theTBG molecule (unsaturated TBG) in the patient specimen. The numberobtained from the multiplication of the Total T4 concentration by theThyroid Uptake value is called a Free Thyroxin Index (FTI). The FTIcorrelates more closely with Free T4 (the metabolic active fraction)concentration than does the total T4 concentration alone. The FTI istherefore a better method of monitoring thyroid function and diagnosingthyroid illness than is a Total T4 determination alone.Diseases of the thyroid gland can result in clinical signs of thyroiddysfunction. Primary hypothyroidism results in underproduction of T4 bythe thyroid gland and consequently an abnormally low circulating T4 concentration in the blood. Primary hyperthyroidism leads to excessivethyroid production of T4 and a resulting elevated T4 concentration.The determination of total serum T3 is used in the differential diagnosis ofthyroid disease, particularly hyperthyroidism. In most hyperthyroidpatients, both serum T3 and T4 are elevated. However, approximately 5-10% of hyperthyroid patients have elevated T3 concentrations but normalserum T4, a condition known as T3-thyrotoxicosis. Such clinical conditionsmake it vital to establish that serum T3 is normal before excluding thediagnosis of hyperthyroidism. Serum T3 level is also an excellent indicatorfor the ability of the thyroid to respond to both stimulatory and suppressivetests.The thyroid gland function is regulated by the level of Thyroid StimulatingHormone (TSH) which is produced and secreted by the pituitary gland.TSH is produced by the anterior lobe of the pituitary gland and acts on thethyroid gland to release thyroid hormones. The release of TSH from thepituitary is regulated by the hypothalamus when it secretes TRH(thyrotropin releasing hormone).In an euthyroid individual, the levels of thyroid hormones in the blood areinversely related to the levels of TSH and TRH. When the levels of thyroidhormones rise, the levels of TRH and TSH fall; and when the levels ofthyroid hormones fall, the levels of TRH and TSH rise. In the event offailure of the thyroid gland, the levels of thyroid hormones fall and thenegative feedback results in an elevated level of TSH in the blood.Elevated levels of TSH are thus useful in the diagnosis of primaryhypothyroidism. Conversely, in the case of primary hyperthyroidism, theelevated levels of thyroid hormones will result in decreased levels of TSH.When there is a failure of the pituitary or the hypothalamus (secondary ortertiary hypothyroidism), the level of TSH is decreased in the presence oflow levels of thyroid hormones. In secondary or tertiary hyperthyroidism, the level of TSH is increased in the presence of high levels of thyroidhormones.