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.

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.