If your doctor suspects you may have a thyroid condition, he or she will order testing to determine how well your thyroid gland is functioning.

A healthy thyroid gland manufactures a hormone called thyroxine (also called T4), which the body then converts to triiodothyronine (also called T3). The amount of thyroxine made by the thyroid is regulated by another hormone called thyroid stimulating hormone (TSH), which is produced by the pituitary gland. If your pituitary gland senses too-low levels of T4 in the body, it will secrete more TSH to signal the thyroid to produce more T4. If your pituitary gland senses too-high levels of T4, it will stop its own TSH production.

About the TSH Test

In order to assess your thyroid function, your physician will likely order a TSH test, particularly if you are experiencing symptoms of hyperthyroidism or hypothyroidism, or if you are planning a pregnancy. It is possible that additional thyroid tests, such as a T3 test or T4 test, may be recommended at the same time.

To perform a TSH test, a blood sample will be drawn with a needle. There is no special preparation for this test. However, if you take the vitamin biotin (B7), talk to your provider because this could impact the results of a TSH test. Because TSH values can fluctuate throughout the day, it is recommended to have the test done in the early morning.

Normal TSH values range from 0.5 to 5 microunits per milliliter (µU/mL). A too-high TSH level means your thyroid gland is not producing enough thyroxine, which indicates hypothyroidism. A too-low TSH level means your thyroid gland is producing too much thyroxine (hyperthyroidism). Your physician will fully explain your test results, as well as any further testing or treatment that may be necessary.