Free T4 (Thyroxin) MICRO-ELISA Test Kit

The principal tests used in the laboratory evaluation of thyroid function are Total Thyroxin (T4), Total Triiodothyronine (T3), T-Uptake (T-Up), a calculated Free Thyroxin Index (FTI) and Thyroid Stimulating Hormone (TSH). The results of these tests are interrelated and help the clinician in making a diagnosis. Clinical hypothyroidism results from underproduction of thyroid hormones by the thyroid gland, consequently an abnormally low circulating T4 and T3 concentration in blood. Clinical hyperthyroidism results from excessive production of thyroid hormones and resulting elevation of T4 and T3 concentrations. The manifestations of thyroid dysfunction can result from disease of the thyroid gland (PRIMARY hyperthyroidism or hypothyroidism), disease of the pituitary gland (SECONDARY hyperthyroidism or hypothyroidism) or disease of the hypothalamus (TERTIARY hyperthyroidism or hypothyroidism). Thyroxin (3,5,3',5'-tetraiodo-l-thyronine, T4) and Triiodothyronine (3,5,3'- triiodo-l-thyronine, T3), are the hormones originating from the thyroid gland. T4 and T3 are responsible for regulating diverse biochemical processes 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 the bloodstream. Approximately 30% of the circulating T4 is enzymatically deiodinated at the 5' position in the peripheral tissues to yield T3. The T4 likely serves as a "prohormone" for T3, which has a much greater metabolic activity. T4 and T3 are transported through the peripheral blood stream largely bound to serum proteins. The major transport protein is Thyroxin Binding Globulin (TBG) which normally accounts for 80% of the bound hormone. The other thyroid hormone binding proteins are Thyroxin Binding Prealbumin and Albumin. Only about 0.3% of the total serum T3 and only about 0.1% of the total serum T4 are unbound and free to diffuse into tissue 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 or free T4 in the bloodstream of an euthyroid individual. Simply determining the total T4 concentration fails to take into account the variations in TBG levels that affect the unbound thyroxin (free T4) concentration. TBG levels can vary for reasons incidental to the patient's thyroid 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 the TBG molecule (unsaturated TBG) in the patient specimen. The number obtained from the multiplication of the Total T4 concentration by the Thyroid Uptake value is called a Free Thyroxin Index (FTI). The FTI correlates more closely with Free T4 (the metabolic active fraction) concentration than does the total T4 concentration alone. The FTI is therefore a better method of monitoring thyroid function and diagnosing thyroid illness than is a Total T4 determination alone. Diseases of the thyroid gland can result in clinical signs of thyroid dysfunction. Primary hypothyroidism results in underproduction of T4 by the thyroid gland and consequently an abnormally low circulating T4 concentration in the blood. Primary hyperthyroidism leads to excessive thyroid production of T4 and a resulting elevated T4 concentration. The determination of total serum T3 is used in the differential diagnosis of thyroid disease, particularly hyperthyroidism. In most hyperthyroid patients, both serum T3 and T4 are elevated. However, approximately 5- 10% of hyperthyroid patients have elevated T3 concentrations but normal serum T4, a condition known as T3-thyrotoxicosis. Such clinical conditions make it vital to establish that serum T3 is normal before excluding the diagnosis of hyperthyroidism. Serum T3 level is also an excellent indicator for the ability of the thyroid to respond to both stimulatory and suppressive tests. The thyroid gland function is regulated by the level of Thyroid Stimulating Hormone (TSH) which is produced and secreted by the pituitary gland. TSH is produced by the anterior lobe of the pituitary gland and acts on the thyroid gland to release thyroid hormones. The release of TSH from the pituitary is regulated by the hypothalamus when it secretes TRH (thyrotropin releasing hormone). In an euthyroid individual, the levels of thyroid hormones in the blood are inversely related to the levels of TSH and TRH. When the levels of thyroid hormones rise, the levels of TRH and TSH fall; and when the levels of thyroid hormones fall, the levels of TRH and TSH rise. In the event of failure of the thyroid gland, the levels of thyroid hormones fall and the negative feedback results in an elevated level of TSH in the blood. Elevated levels of TSH are thus useful in the diagnosis of primary hypothyroidism. Conversely, in the case of primary hyperthyroidism, the elevated levels of thyroid hormones will result in decreased levels of TSH. When there is a failure of the pituitary or the hypothalamus (secondary or tertiary hypothyroidism), the level of TSH is decreased in the presence of low levels of thyroid hormones. In secondary or tertiary hyperthyroidism, the level of TSH is increased in the presence of high levels of thyroid hormones.