Thyroxine-binding globulin (TBG) is a globulin that binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3 but is present in the lowest concentration. Despite its low concentration, TBG carries the majority of T4 in the blood plasma.

The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in the tissues, T4 is released from the proteins and is now free to convert into the more active form called T3. For this reason, many doctors believe that measuring free T4 is a more sensitive test for thyroid hormone production.

The reverse T3 test has to be done at the same time you do the free T3, and you then measure the ratio between the two by dividing the RT3 into the Free T3. The body produces the benign RT3 naturally to rid itself of excess of T4, but in some cases, such as high or low cortisol, RT3 is produced in excess, which can clog cell receptors from receiving T3. FT3 is usually twenty or more times higher than RT3. NOTE: Reverse T3 is analyzed using a specific technique called liquid chromatography/tandem mass spectrometry (LC/MS-MS). There's a special place in the U.S. where these precise machines needed for LC/MS panels run only on two specific days a week. So, sometimes the clock ticks a bit while we wait for samples to return from their initial collection points. Results may take 7 business days.

The thyroid stimulating hormone (TSH) is produced and secreted by the pituitary gland and stimulates the thyroid to produce T3 and T4. When TSH is elevated, low thyroid function may be indicated. If TSH is low it is indicative of high thyroid function. A study found that TSH was suppressed in all subjects after food, so fasting may be suggested although it is not reqired by Quest. Free T4 and Free T4 values did not change significantly after eating.

This panel includes comprehensive testing of the thyroid stimulating hormone (TSH) in addition to free thyroxine (T4) and free triiodothyronine (T3).

T3 or triiodothyronine is the active form of the thyroid hormone. T3 acts as a metabolic throttle for fueling all of the daily cellular functions. Free T3 is the unbound portion of T3 that is biologically active.

The determination of TPO antibody levels is the most sensitive test for detecting autoimmune thyroid disease (eg, Hashimoto's thyroiditis, idiopathic myxedema, and Graves disease) and detectable concentrations of anti-TPO antibodies are observed in most patients with these disorders. The highest TPO antibody levels are observed in patients suffering from Hashimoto thyroiditis. In this disease, the prevalence of TPO antibodies is about 90% of cases, confirming the autoimmune origin of the disease. These autoantibodies also frequently occur (60%–80%) in the course of Graves disease.

This test measures thyroglobulin antibodies commonly present in patients with Hashimoto's thyroiditis. Antibodies against the protein thyroglobulin can result in the destruction of thyroid cells. This destruction can lead to hypothyroidism. NOTE: Dietary supplements containing biotin may interfere in assays and may skew results to be either falsely high or falsely low. For patients receiving the recommended daily doses of biotin, draw samples at least 8 hours following the last biotin supplementation. For patients on mega doses of biotin supplements, draw samples at least 72 hours following the last biotin supplementation.