TSH, short for thyroid-stimulating hormone, is a hormone made by the pituitary gland at the base of the brain. It tells the thyroid gland in the neck how much thyroid hormone to make. TSH is usually the first test ordered to check thyroid function, either on its own or as part of a set of thyroid function tests. It is reported in milli-international units per liter (mIU/L), a unit used in both the United States and Canada.
This article explains what the TSH test measures, and why a high TSH usually means the thyroid is underactive while a low TSH usually means it is overactive. It also covers what TSH results mean, what can shift them, and what your doctor will likely do next.
The reference range for your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.
The TSH test measures thyroid-stimulating hormone in your blood. The thyroid gland makes two hormones, called T4 and T3, which set the pace of the body’s metabolism. They affect energy levels and heart rate, as well as body temperature, weight, and many other functions.
The pituitary gland senses thyroid hormone levels in the blood and adjusts its TSH output accordingly. Because the pituitary reacts to small changes in thyroid hormone, TSH often becomes abnormal before thyroid hormone levels fall outside their reference range. This is why TSH is used as the first screening test.
TSH moves in the opposite direction of thyroid function, which surprises many people reading their report. TSH is not thyroid hormone. It is the signal the pituitary sends to the thyroid, and the pituitary turns that signal up or down depending on how well the thyroid is keeping up.
This relationship holds as long as the pituitary gland itself is working normally. In the uncommon situation where the pituitary is damaged or diseased, TSH can be low or normal even though the thyroid is underactive. That’s why a free T4 test is often ordered alongside TSH.
Doctors measure TSH from a blood sample taken from a vein, usually in your arm, and no fasting is needed. TSH is naturally a little higher in the early morning than in the afternoon. The difference rarely matters, but repeat tests are most comparable when they are taken at a similar time of day.
Tell your doctor if you take biotin, a supplement often sold for hair, skin, and nails. High doses of biotin interfere with the way many laboratories measure TSH. It can produce a falsely low TSH that looks like an overactive thyroid. Stopping biotin for a few days before the test usually avoids this, but check with your doctor or laboratory first.
If you take thyroid hormone medication, follow your doctor’s instructions on when to take it on the test day. A TSH result reflects the dose you have been taking over the previous six weeks or so, not only that day’s dose.
You read your TSH result against the reference range printed on your report, usually alongside a free T4 result. The figures below are typical for non-pregnant adults.
Most abnormal TSH results reflect a change in the thyroid gland itself. Several other factors can also shift TSH, so labs usually repeat the test.
Conditions and factors that can raise TSH include:
Conditions and factors that can lower TSH include:
TSH is often the first thyroid test, and an abnormal result usually leads to one or more of the following.
If TSH is mildly high or low and you feel well, the usual next step is to repeat the TSH with a free T4 after several weeks. Many mildly abnormal results return to normal on their own, especially after a recent illness.
If your TSH stays clearly high or your free T4 is low, your doctor may discuss thyroid hormone replacement. The decision depends mostly on how high the TSH is and whether you have symptoms. Your age matters too, as does whether you are pregnant or planning a pregnancy. A persistently low TSH usually prompts further tests to find the cause and may lead to a referral to an endocrinologist.
If you are already on thyroid medication, your doctor will check TSH about six to eight weeks after any dose change, then less often once the dose is stable. Tell your doctor about a racing or irregular heartbeat, unexplained weight change, or new tiredness and feeling cold.