TSH (Thyroid Stimulating Hormone)
A hormone from the pituitary gland that tells your thyroid how much hormone to make. It is the test most often ordered first to check your thyroid.
AI-verified against cited sources. Awaiting expert reviewAlso called TSH, hTSHLOINC 3016-3
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What it is
TSH is made by the pituitary, a small gland at the base of the brain. It works like a thermostat for your thyroid. When thyroid hormone in your blood runs low, the pituitary releases more TSH to nudge the thyroid. When thyroid hormone runs high, it releases less. Because of this feedback loop, TSH is usually the first number clinicians look at to see how well the thyroid is working. A TSH result is read together with free T4, the main thyroid hormone itself.
Why it matters
Thyroid hormones help set how fast your body uses energy. They affect heart rate, body temperature, weight, mood, digestion and more. A raised TSH is linked to an underactive thyroid, and a low TSH is linked to an overactive one, though many other things can move it. TSH is also the main number clinicians follow to see whether thyroid hormone treatment is at the right level for a person.
If it’s low
A TSH below your lab's range means the pituitary is sending less signal because there is enough thyroid hormone around, or more than enough. It is often linked to an overactive thyroid. It is also seen in early pregnancy, during serious illness, and in people taking thyroid hormone. A mildly low TSH with normal thyroid hormones is common and is often repeated before anything is decided. In rare pituitary conditions TSH can be low or normal even when thyroid hormone is also low, which is why free T4 is usually checked too.
Common causes
- Early pregnancy, when pregnancy hormones can lower TSH for a while
- A recent illness, or medicines such as glucocorticoids and dopamine
- Taking thyroid hormone, including medicine at a higher dose than needed or supplements that contain thyroid hormone
- High-dose biotin, which can make TSH read falsely low on some tests
- Graves' disease or thyroid nodules that make extra hormone
- Thyroiditis, an inflammation that releases stored hormone for a while
- Rarely, a pituitary problem
What can help
- Ask whether free T4 and T3 should be checked alongside it
- Tell your clinician about every supplement you take, including biotin and anything sold for thyroid support
- Ask whether to repeat the test, since a mildly low value often returns to normal
- If you are pregnant, ask which trimester-specific range applies
If it’s high
A TSH above your lab's range means the pituitary is asking for more thyroid hormone than the thyroid is supplying. The most common lasting cause is an underactive thyroid, often from Hashimoto's thyroiditis. Mildly raised values are common, are often found without any symptoms, and sometimes return to normal on a repeat test. Guidelines usually suggest repeating TSH with a free T4 after a couple of months before deciding what it means.
Common causes
- A temporary rise while recovering from an illness
- Normal ageing, because TSH tends to drift higher with age
- Hashimoto's thyroiditis, an autoimmune condition and the most common cause of a lasting rise
- Thyroid hormone medicine at a dose that is too low, doses missed, or doses taken together with calcium or iron supplements, which can reduce absorption
- Some medicines, including lithium and amiodarone
- Iodine intake that is too low or too high
- Past thyroid surgery or radioactive iodine treatment
- A lab interference, or rarely a pituitary problem
What can help
- Ask whether to repeat TSH with a free T4, and when
- Ask whether TPO antibodies would help explain the result
- If you take thyroid medicine, ask your clinician how and when to take it, including timing with food, calcium and iron
- Tell your clinician about every supplement you take, including biotin
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on age. Choose an age to see the one that applies.
See TSH by age and sex, with the U.S. survey numbers where they exist.
ETA 2013 categories for a raised TSH (they apply when free T4 is in range; the guideline gives 0.4 to 4.0 mIU/L as the adult reference range, 4.0 to 10.0 as mildly raised and above 10.0 as more severely raised)
- Below the guideline reference rangebelow 0.4 mIU/L
- Within the guideline reference range0.4 to under 4 mIU/L
- Mildly raised4 to under 10.01 mIU/L
- More severely raised10.01 and above mIU/L
If this applies to you
TSH means a little more for some people. Open the one that fits you.
Thyroid condition
TSH is the main number used to check whether thyroid hormone treatment is at the right level. The ATA suggests rechecking it 4 to 6 weeks after any dose change. Once results are steady, NICE suggests checking about once a year.
Age 65 or older
TSH tends to drift a little higher with age. The ETA suggests considering age-specific TSH ranges for older people, so a result slightly above the usual adult range can be common at older ages.
PCOS
The guideline's checklist includes ruling out other causes of similar symptoms. TSH, the main thyroid test, is one of the tests it lists for this.
Trying to conceive
Guidelines differ here. ASRM lists TSH as a possible routine test, with free T4 and thyroid antibodies if it's outside the range. NICE offers thyroid tests only when there are symptoms of thyroid disease.
Perimenopause or menopause
NICE notes that thyroid symptoms can be mistaken for menopause. For people who take thyroid hormone, the FDA label for estradiol tablets says they may need a higher dose while taking estradiol, and advises monitoring thyroid tests.
Pregnant or postpartum
TSH is lowest near the end of the first trimester, because hCG, a pregnancy hormone, acts on the thyroid's TSH receptor. The ATA's 2026 guideline prefers the lab's own trimester-specific ranges and, when none are available, says a range of 0.1 to 4.0 mIU/L can be used in the first and second trimesters.
Taking lithium
Lithium can slow the thyroid and lead to an underactive thyroid. The FDA label suggests thyroid tests at 3 months and every 6 to 12 months after that, and NICE suggests every 6 months.
What can affect your result
- Age: TSH tends to rise with age, and the usual upper limit is higher in older adults
- Time of day: higher overnight, lower in the afternoon
- Pregnancy: ranges are lower in pregnancy and differ by trimester
- Illness and recovery from illness
- Medicines such as lithium, amiodarone, glucocorticoids and dopamine
- Biotin supplements
- Ethnicity: US survey data show lower average TSH in Black Americans than in white Americans
- Sex has only a small effect, no more than about 0.1 mIU/L in one very large study
- Lab method: assays differ slightly between labs
- In a healthy person TSH changes little from test to test, so a result well away from your usual is worth a repeat
- High-dose biotin (vitamin B7) can distort results on immunoassays that use biotin chemistry, and has caused results that mimic an overactive thyroid (low TSH with high free T4). Labcorp asks patients to stop biotin at least 72 hours before the draw; check with your clinician or lab first.
- Recovery from a serious illness can leave TSH temporarily raised, and acute illness can push it either way.
- Medicines including lithium and amiodarone can raise TSH, and glucocorticoids and dopamine can lower it.
- Pregnancy lowers the usual TSH range, especially in the first trimester.
- Rarely, antibodies in the blood (heterophile antibodies) or large TSH complexes (macro-TSH) can give a result that does not match how you feel or your other thyroid tests.
- Assays differ slightly between labs, so compare trends using the same lab and method where you can.
TSH follows a daily rhythm, with higher levels overnight and lower levels in the afternoon. MedlinePlus suggests an early-morning draw, and drawing at about the same time each visit makes results easier to compare. TSH reaches a steady level about 6 weeks after thyroid hormone treatment is started, and the ATA (2014) suggests rechecking it 4 to 6 weeks after any dose change. TSH can lag behind changes in thyroid hormone levels.
When to talk to a clinician
Talk with a clinician about any TSH outside your lab's range, especially if you have symptoms such as unusual tiredness, feeling cold, constipation or weight gain (more linked to a raised TSH) or a racing heart, tremor, heat intolerance, weight loss or poor sleep (more linked to a low TSH). The ETA guideline treats a TSH above 10 mIU/L as more severe than a mild rise, so a result that high is worth a prompt conversation. If you are pregnant or planning a pregnancy, tell your prenatal clinician about any TSH outside your lab's range. If you have chest pain, a very fast or irregular heartbeat, fainting or confusion, call 911 or your local emergency number.
Questions you could ask
- How often should my thyroid be tested?
- Does my TSH matter for any of my other conditions or medicines?
- If I am pregnant or planning to be, which TSH range should we use?
- Should we repeat my TSH and check free T4 in a couple of months?
- Would thyroid antibodies help explain this result?
- Could any medicine or supplement I take, including biotin, be affecting it?
Questions people ask
What is a normal TSH?
Most US labs print an adult range of roughly 0.3 to 4.5 mIU/L, and guidelines quote 0.4 to 4.0. The exact limits depend on the lab and the test method, and the upper limit is looser in older adults. Use the range on your own report.
Does a TSH just above the range mean I have a thyroid problem?
Not necessarily. Mildly raised TSH is common, can be temporary, and is more common with age. Guidelines usually suggest repeating TSH with a free T4 (and sometimes TPO antibodies) after 2 to 3 months before drawing any conclusions. Your clinician can put it in context.
Does the time of day matter for a TSH test?
A little. TSH is higher overnight and lower in the afternoon. For comparing results over time, try to have blood drawn at about the same time of day, and at the same lab if you can.
Can biotin supplements change my TSH?
Yes, on some tests. High-dose biotin can make TSH look falsely low, and has caused results that mimic an overactive thyroid. Labcorp asks patients to stop biotin at least 72 hours before the draw. Check with your clinician or lab about your own situation first.
Is TSH different in pregnancy?
Yes. The usual range is lower in pregnancy and differs by trimester, and pregnancy-specific ranges are used. If a lab does not have its own, the American Thyroid Association suggests using an upper limit of about 4.0 mIU/L. Ask your prenatal clinician which range applies to you.
Show the science and sources
Units
- 1 mIU/L = 1 mIU/L
- 1 mIU/L = 1 µIU/mL
Is a change real?
TSH naturally varies about 17.9% from test to test in the same healthy person. Between two results from the same lab, a rise of 69% or more, or a fall of 41% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Biological variation estimates of thyroid related measurands - meta-analysis of BIVAC compliant studies (Fernandez-Calle P et al., EFLM WG-BV).
For clinicians and researchers
TSH (thyrotropin) is a glycoprotein made of the alpha subunit shared with LH, FSH and hCG and a hormone-specific beta subunit. Pituitary thyrotrophs secrete it in pulses under TRH drive, with negative feedback from T4 (converted locally to T3) and T3. Because the TSH-to-free-T4 relationship is roughly log-linear, small shifts in free T4 produce large shifts in TSH, which is why the AACE/ATA 2012 guideline calls a serum TSH the single best screening test for primary thyroid dysfunction in most outpatient situations. That relationship fails in central hypothyroidism, where TSH may be low, normal or mildly raised with a low free T4, and during non-thyroidal illness, recovery, and treatment with some drugs. LOINC has separate codes for sensitive assays with detection limits of 0.05 and 0.005 mIU/L. In NHANES III the reference population mean was about 1.4 mIU/L (geometric), with higher values in older people and lower values in Black Americans (Hollowell 2002), and the 97.5th percentile rises from 3.56 mIU/L at ages 20 to 29 to 7.49 at 80 and over (Surks 2007). In a single-assay study of about 465,000 results, Ehrenkranz (2015) found the 97.5th percentile rose from 6.45 to 7.55 mIU/L with age, mainly because the nocturnal TSH surge grows larger, while the lower limit stayed constant through the day, the year and across ages. Within-person variation is small, about a third of the width of the population reference range (ETA 2013). Reference intervals in print vary: 0.4 to 4.0 (ATA 2014, ETA 2013), 0.45 to 4.5 (Labcorp), 0.3 to 4.2 (Mayo), 0.27 to 4.20 (ARUP). Pregnancy: the ATA 2017 guideline recommends trimester-specific ranges from the local population and, where these are unavailable, an upper limit of about 4.0 mIU/L, which for most assays is about 0.5 mIU/L below the non-pregnant upper limit. Interferences include biotin on streptavidin-biotin sandwich assays (falsely low), heterophile antibodies, and macro-TSH (falsely high). Subclinical hypothyroidism is a raised TSH with free T4 in range, confirmed on repeat 2 to 3 months later; the AACE/ATA guideline says the decision to treat when TSH is below 10 mIU/L should be tailored to the individual patient.
LOINC 3016-3 (primary), 11580-8, 11579-0. Specimen: serum-or-plasma.
Sources
- Thyroid Stimulating Hormone (TSH) (test 004259), Labcorp
- Thyroid Function Cascade, Monitor (without Thyroid Peroxidase Antibody), Serum (test THCSM), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
- Thyroid Stimulating Hormone (test 0070145), ARUP Laboratories
- TSH test (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine
- Thyroid function tests (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine
- Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid (2014)
- 2013 ETA Guideline: Management of Subclinical Hypothyroidism, European Thyroid Journal (European Thyroid Association) (2013)
- Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association, Endocrine Practice (2012)
- 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum, Thyroid (2017)
- The Need for Dynamic Clinical Guidelines: A Systematic Review of New Research Published After Release of the 2017 ATA Guidelines on Thyroid Disease During Pregnancy and the Postpartum, Frontiers in Endocrinology (2020)
- Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): NHANES III, Journal of Clinical Endocrinology & Metabolism (2002)
- Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population: implications for the prevalence of subclinical hypothyroidism, Journal of Clinical Endocrinology & Metabolism (2007)
- Circadian and Circannual Rhythms in Thyroid Hormones: Determining the TSH and Free T4 Reference Intervals Based Upon Time of Day, Age, and Sex, Thyroid (2015)
- Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks, Endocrine Practice (2017)
- Biotin Treatment Mimicking Graves' Disease, New England Journal of Medicine (2016)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.