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thyroid function test

any laboratory procedure that assesses the production of the two active thyroid hormones, thyroxine (T4) and triiodothyronine (T3), by the thyroid gland and the production of thyrotropin (thyroid-stimulating hormone, TSH), the hormone that regulates thyroid secretion, by the pituitary gland. The best and most widely used tests are measurements of serum thyrotropin and thyroxine. The secretion of thyrotropin changes substantially in response to very small changes in thyroxine and triiodothyronine production. For example, small decreases in thyroid hormone production result in relatively large increases in serum concentrations of thyrotropin, and, conversely, small increases in thyroxine and triiodothyronine production result in relatively large decreases in serum concentrations of thyrotropin. Therefore, patients with hypothyroidism (thyroid deficiency) almost invariably have not only low serum thyroid hormone but also high serum thyrotropin concentrations, and those with hyperthyroidism have high serum thyroid hormone and low serum thyrotropin concentrations. An exception is patients with pituitary disease and thyrotropin deficiency, who have low serum thyroid hormone but normal or low serum thyrotropin concentrations. Between the two thyroid hormones, measurements of serum thyroxine are preferred because serum triiodothyronine concentrations are abnormal in many patients with nonthyroid illnesses.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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