What Is a TSH Blood Test?

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A TSH, or thyroid-stimulating hormone, blood test measures levels of thyroid-stimulating hormone in the body, according to WebMD. It is used as an effective tool to monitor thyroid functioning and check for thyroid problems, such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).

For a TSH blood test, a health professional draws a blood sample from a vein in the patient’s arm. A quick sting or pinching sensation may occur when the needle is injected. Results are typically available in two to three days, according to WebMD..

For adults, normal levels of TSH are 0.4 to 4.2 milliunits per liter, or mU/L. High values of TSH may be caused by hypothyroidism or, rarely, a pituitary gland tumor. Low values of TSH may be caused by hyperthyroidism or pregnancy. Additionally, patients with hypothyroidism who take too much thyroid medicine may develop low TSH levels and display symptoms of hyperthyroidism. Taking medications such as lithium, heparin, dopamine, levodopa and corticosteroids can affect the results of the TSH blood test as can high levels of stress and chronic illness, explains WebMD.

A TSH blood test is often ordered in conjunction with tests to measure triiodothyronine (T3) and thyroxine (T4) levels. Produced by the thyroid gland, T3 and T4 are essential for proper metabolism, according to WebMD.