TSH (Thyroid-Stimulating Hormone) – Understanding Your Blood Test Result

Section Editor: Christopher McCudden Ph.D., DABCC, FADLM, FCACB
October 28, 2026

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.

What does the TSH test measure?

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.

Why a high TSH means an underactive thyroid

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.

When the thyroid is underactive, thyroid hormone falls, and the pituitary responds by sending out more TSH to push the thyroid harder. A high TSH therefore points to hypothyroidism, an underactive thyroid.
When the thyroid is overactive, thyroid hormone rises, and the pituitary responds by reducing TSH, sometimes almost to zero. A low TSH therefore points to hyperthyroidism, an overactive thyroid.

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.

How is the TSH test performed?

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.

What do TSH results mean?

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.

A normal TSH. Many laboratories use a range of approximately 0.4 to 4.0 mIU/L, and some extend the upper limit to 4.5 or 5.0 mIU/L. A normal TSH usually means the thyroid is producing the right amount of hormone.
A mildly high TSH. A TSH above the range, up to about 10 mIU/L, with a normal free T4 is called subclinical hypothyroidism. It is common, often causes no symptoms, and about 6 in 10 people with a TSH between 4 and 10 mIU/L return to normal within five years without treatment.
A clearly high TSH. A TSH above 10 mIU/L, or any high TSH with a low free T4, points more firmly to an underactive thyroid. Treatment is more often considered at these levels.
A low TSH. A TSH below the range with a normal free T4 is called subclinical hyperthyroidism. A low TSH with high free T4 or T3 suggests an overactive thyroid.
TSH in older adults. TSH tends to rise with age. In people over 65, TSH up to about 6 mIU/L can be normal, even though many laboratories don’t use age-specific ranges.
TSH in pregnancy. TSH falls in early pregnancy because of pregnancy hormones. The American Thyroid Association’s 2017 guidelines suggest an upper limit of about 4.0 mIU/L when a laboratory has no pregnancy-specific range.

What conditions and factors can affect TSH?

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:

Hashimoto thyroiditis. This autoimmune condition, also called chronic lymphocytic thyroiditis, is the most common cause of an underactive thyroid in countries with enough iodine in the diet.
Thyroid surgery or radioactive iodine treatment. Removing or treating part of the thyroid reduces how much hormone it can make.
Too low a dose of thyroid hormone medication. In people already on treatment, a high TSH usually means the dose needs adjusting or the tablets are not being absorbed well.
Recovery from a recent illness. TSH can rise briefly as the body recovers from a serious illness.
Some medications. Lithium and amiodarone, a heart rhythm medicine, are the best-known examples.

Conditions and factors that can lower TSH include:

An overactive thyroid. Graves disease and overactive thyroid nodules are the most common causes.
Inflammation of the thyroid. Some forms of thyroiditis release stored hormone for a few weeks, lowering TSH before the thyroid settles or becomes underactive.
Too high a dose of thyroid hormone medication. This is a common reason for a low TSH in people on treatment.
Early pregnancy and serious illness. Both can lower TSH temporarily.
Steroid medications and biotin. Steroids genuinely lower TSH, while biotin can make it appear low on the test.
Pituitary disease. A damaged pituitary gland may not produce enough TSH, as covered in our article on pituitary adenoma.

What other tests are ordered with TSH?

TSH is often the first thyroid test, and an abnormal result usually leads to one or more of the following.

Free T4. This measures the active form of the main thyroid hormone. It shows whether an abnormal TSH is mild or is already affecting hormone levels.
Free T3. Order this mainly when TSH is low, because some overactive thyroids raise T3 before T4.
Thyroid antibodies. Antibodies against thyroid peroxidase (TPO) suggest Hashimoto thyroiditis. TSH receptor antibodies suggest Graves disease.
A repeat TSH. A mildly abnormal TSH is usually repeated after 6 to 12 weeks before any decision is made.
A thyroid ultrasound or scan. These are ordered when a lump is felt in the neck or when doctors need to identify the cause of an overactive thyroid.

What happens after a TSH test?

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.

Questions to ask your doctor

What was my TSH, and what is the reference range at this laboratory?
Was my free T4 measured, and was it normal?
Does my result mean my thyroid is underactive or overactive?
Could a recent illness or a medication have affected my TSH?
Should I stop taking biotin before my next thyroid test?
Do I need thyroid antibody testing?
Is my result normal for my age, or for my stage of pregnancy?
Do I need treatment now, or should I repeat the test first?
If I am on thyroid medication, does my dose need to change?
When should I have my TSH checked again?