Lab test6 min read
TSH test
A blood test that measures thyroid-stimulating hormone (TSH), made by the pituitary gland. It is usually the first step in finding out whether the thyroid is overactive or underactive.
The pituitary gland in the brain makes TSH to tell the thyroid how much hormone to produce. When thyroid hormone is low, TSH rises; when it is high, TSH falls. So a high TSH usually points to an underactive thyroid and a low TSH to an overactive one — but it does not show the cause.
According to NIDDK, TSH is usually checked first, and an abnormal result needs at least one more test: T4 — many clinicians prefer free T4 because changes in binding proteins don't affect it — or T3 when hyperthyroidism is suspected despite a normal T4, and thyroid antibodies to identify Graves' or Hashimoto's disease. MedlinePlus notes that TSH can be high or low even with a healthy thyroid: some serious illnesses unrelated to the thyroid lower it temporarily, it tends to be higher after 80, and it is often a little low in the first three months of pregnancy.
What is it?
According to MedlinePlus, TSH (also called thyrotropin) is a hormone made by the pituitary gland in the brain that tells the thyroid how much thyroid hormone to make. This test measures its level in the blood.
A level that is too high or too low may be a sign of a thyroid problem, but the test cannot show what is causing it.
Why is it done?
It shows how well the thyroid is working — whether it makes too much hormone (hyperthyroidism) or too little (hypothyroidism) — and is used to check that people taking thyroid hormone medicine are on the right treatment, and to monitor levels after treatment for hyperthyroidism.
Symptoms of too much thyroid hormone that may lead to the test include weight loss despite eating normally or more, a fast or irregular heartbeat, nervousness, trouble sleeping, shaky hands or muscle weakness, sweating or heat sensitivity, frequent bowel movements and an enlarged thyroid (goitre).
Symptoms of too little thyroid hormone include tiredness, weight gain, sensitivity to cold, joint and muscle pain, dry skin, dry or thinning hair, irregular periods, depression and constipation. It develops slowly, so symptoms may go unnoticed for months or years.
People over 60 may show different signs, such as loss of appetite or withdrawing from others, which can be mistaken for depression or dementia. The test may also be done with others to evaluate lumps in the thyroid.
Preparation
You usually don't need to fast unless other tests that require it are done at the same time. Tell your doctor about all your medicines, as some may need to be stopped beforehand — but don't stop any medicine unless they tell you to.
How is it done?
A health professional takes a blood sample from a vein in your arm with a small needle. It usually takes less than five minutes, and you may feel a brief sting.
Understanding the result
High TSH: the pituitary releases more TSH when thyroid hormone is low, so it may point to an underactive thyroid.
Low TSH: the pituitary makes little TSH when thyroid hormone is high, so it may point to an overactive thyroid.
An abnormal result usually leads to more tests — T4, T3 and thyroid antibodies, which help identify Graves' disease (the most common cause of hyperthyroidism) and Hashimoto's disease (the most common cause of hypothyroidism). Occasionally, an abnormal TSH reflects a problem with the pituitary gland itself.
The source gives no numeric ranges: rely on the reference range in your laboratory report and on your doctor's interpretation.
Risks and side effects
There is very little risk: you may have slight pain or bruising where the needle went in, which usually fades quickly.
Limits of the test
TSH can be high or low even with a healthy thyroid: some serious illnesses unrelated to the thyroid lower it temporarily, and it tends to be higher in people over 80 without thyroid disease.
In pregnancy it is often a little low in the first three months. Anyone with a history of thyroid disease should talk to their doctor when pregnant or planning a pregnancy.
After the test
Newborns are routinely screened with a TSH test for congenital hypothyroidism. To find the cause of hyperthyroidism, a radioactive iodine uptake test or thyroid scan may be done, and ultrasound is used for nodules.
Common questions
Do I need to fast before a TSH test?
Usually not, unless other tests that need fasting are done at the same time. Ask the laboratory or your doctor.1
Does a high TSH mean an underactive thyroid?
It often points to it, but the test cannot show the cause and can change for other reasons, so T4, T3 and antibody tests are usually done too.12
Does pregnancy affect TSH?
According to MedlinePlus, it is often a little low in the first three months of pregnancy.1
Questions for your doctor
- Do I need T4, T3 or antibody tests as well as TSH?
- Could my medicines affect the result?
- What does my result mean alongside my symptoms?
- When should I repeat the test?
References
- 1MedlinePlus / NLM. TSH (Thyroid-stimulating hormone) Test. medlineplus.gov/lab-tests/tsh-thyroid-stimulating-hormone-test/Health agencies & guidelines · Accessed 2026-10-11
- 2NIDDK / NIH. Thyroid Tests. www.niddk.nih.gov/health-information/diagnostic-tests/thyroidHealth agencies & guidelines · Accessed 2026-10-11
Review status: Edited content · Last updated:
Change log
- — Page created.
Educational content only — no diagnosis, and no substitute for a clinician.