What is TSH?
TSH (thyroid stimulating hormone) is made by the pituitary gland at the base of the brain. It tells the thyroid how much thyroid hormone to make, mainly T4 and a smaller amount of T3. The two work as a feedback loop: when thyroid hormone runs low, the pituitary pushes TSH up; when there is too much, TSH falls.
That is why TSH moves in the opposite direction to thyroid function, and why the American Thyroid Association (ATA) calls it the best first test. Changes in TSH often show up before thyroid hormone levels themselves leave the normal range.
TSH normal range
| Source | Adult TSH range | Note |
|---|---|---|
| MedlinePlus | 0.4 to 4.8 mIU/L | Some labs allow up to 7 in older people |
| ARUP Laboratories | 0.27 to 4.20 mIU/L | Adults 20 and over |
| NHANES III (US population) | Average 1.5 mIU/L | People without thyroid disease |
Experts do not fully agree on where the upper limit should sit, which is why labs differ. Always compare your result with the range on your own report. mIU/L and µU/mL are the same number, so no conversion is needed.
TSH normal range for women and men
MedlinePlus and ARUP give one TSH range for women and men. In the large US NHANES III survey, average TSH was a little higher in women than in men and thyroid antibodies were more common in women, but the reference range itself does not change by sex.
TSH normal range by age
TSH rises with age. MedlinePlus notes some labs use an upper limit as high as 7 mIU/L for older people, and that TSH may be higher in people over 80 without any thyroid problem. Children have their own ranges; these are ARUP's:
| Age | TSH (mIU/L) |
|---|---|
| 1 to 24 months | 0.62 to 8.05 |
| 2 to 6 years | 0.54 to 4.53 |
| 7 to 11 years | 0.66 to 4.14 |
| 12 to 19 years | 0.53 to 3.59 |
| 20 and over | 0.27 to 4.20 |
ARUP Laboratories reference intervals. Some labs allow up to 7 mIU/L in older adults (MedlinePlus).
Pregnancy: TSH is often a little low in the first three months, and the normal range is different in pregnancy, so pregnant women should read results with their doctor.
What a high TSH means
A TSH above the range most often means the thyroid is underactive (hypothyroidism) and the pituitary is working harder to push it. The most common cause is Hashimoto's disease, an autoimmune condition picked up by TPO antibodies. Doctors read TSH together with free T4:
| TSH | Free T4 | Usual meaning |
|---|---|---|
| High | Low | Hypothyroidism (underactive thyroid) |
| High | Normal | Mild (subclinical) hypothyroidism |
| Low | High | Hyperthyroidism (overactive thyroid) |
| Low | Normal | Mild (subclinical) hyperthyroidism, or sometimes only T3 is raised |
| Low | Low | Possible pituitary problem (secondary hypothyroidism) |
In NHANES III, 4.6% of the US population had hypothyroidism (0.3% overt and 4.3% mild) and 1.3% had hyperthyroidism, many without knowing it. A single mildly high TSH is usually repeated with free T4 before anyone talks about treatment. If yours is high, talk to your doctor.
Worked example: TSH 6.2 with a normal free T4
Say a 38-year-old woman gets a TSH of 6.2 mIU/L with a free T4 of 1.1 ng/dL. TSH is above both the MedlinePlus (4.8) and ARUP (4.2) upper limits, while free T4 sits inside the 0.8 to 1.9 ng/dL range. On the table above, that pattern is mild (subclinical) hypothyroidism.
Before reading much into it, check the basics: was the sample taken in the morning, had she stopped biotin for 2 days, had she been ill recently, and could she be pregnant, which changes the range? A single mildly high TSH is usually repeated.
Questions for the doctor: Can we repeat TSH with free T4 in the morning? Should we check TPO antibodies to look for Hashimoto's? If I plan to get pregnant in the next year, does that change the plan? At what TSH would you consider treatment for me?
What a low TSH means
A TSH below the range usually means the thyroid is making too much hormone. MedlinePlus lists these causes:
- Graves' disease
- Toxic nodular or multinodular goiter
- Too much iodine, for example from contrast dye used in CT scans
- Taking too much thyroid hormone medicine, or supplements that contain thyroid hormone
- Medicines such as glucocorticoids (steroids), dopamine, some chemotherapy drugs and opioid painkillers
Less often, a low TSH comes from the pituitary itself not making enough, in which case free T4 is low too. Serious illness unrelated to the thyroid can also lower TSH for a short time.
What can change your TSH
- Time of day. TSH varies through the day, so MedlinePlus advises testing early in the morning.
- Biotin. This common hair and nail supplement can make thyroid tests look abnormal when they are not. The ATA advises stopping biotin for 2 days before a thyroid blood test.
- Medicines. Amiodarone, dopamine, lithium, potassium iodide and prednisone or other steroids can affect TSH. Do not stop any medicine without asking your doctor.
- Illness, age and pregnancy, as above.
- Thyroid medicine dose. If you take thyroid hormone, TSH is the number your doctor uses to adjust the dose. Dose changes are for your doctor to make.
How to test TSH
- Test in the morning, and at the same time of day each time you retest.
- Stop biotin for 2 days first (ATA).
- No fasting is needed for TSH, but other tests on the same draw may require it.
- If TSH is out of range, repeat it with free T4 rather than acting on one result.
Common questions about TSH
How soon is TSH rechecked after a dose change?
The ATA says your doctor will check your blood 6 to 8 weeks after starting or changing a thyroid hormone dose, and more often if you are pregnant. It also lists antacids, calcium or iron supplements, biotin, estrogen (including birth control) and some seizure medicines as things that affect thyroid pills, so starting or stopping one is a reason to tell your doctor.
What should be tested with TSH?
If TSH is out of range, MedlinePlus says the next tests are usually T4, T3 or thyroid antibodies. For a high TSH, add TPO antibodies or thyroglobulin antibodies to look for Hashimoto's. Our TSH levels by age chart covers age-specific limits in more detail.
Which at-home tests include TSH
SiPhox Health includes TSH on five of its six panels: Longevity Essentials, Heart & Metabolic, Thyroid Focus, Ultimate 360 and GLP Optimization. Hormone Focus is the only panel without it.
If thyroid is your main concern, Thyroid Focus is the one to pick. It adds free T4, free T3, TPO antibodies and thyroglobulin antibodies to TSH, and it costs the same $124 a test as the other core panels. Longevity and Heart & Metabolic give you TSH only, which is fine as a screen. Our at-home thyroid test guide compares SiPhox with other kits, and the SiPhox review covers accuracy and turnaround.
Thyroid Focus
A full thyroid workup including both thyroid antibodies, plus core lipids.
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Longevity Essentials
The default panel: heart, metabolic, hormone and inflammation markers in one draw.
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Ultimate 360
Everything on the four core panels plus iron studies, B12 and folate, GGT and PSA.
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Frequently asked questions
What is a normal TSH level for a woman?
The same as for a man: about 0.4 to 4.8 mIU/L on the MedlinePlus range, or 0.27 to 4.2 mIU/L at ARUP. Women run slightly higher on average and are more likely to have thyroid disease, but the range does not change by sex. It does change in pregnancy.
Is a TSH of 5 bad?
It is above most lab ranges. With a normal free T4, that pattern is called mild or subclinical hypothyroidism. Doctors usually repeat the test with free T4 and TPO antibodies before deciding anything, and some labs allow up to 7 mIU/L in older people. Discuss it with your doctor.
What TSH level indicates hypothyroidism?
A TSH above your lab's upper limit (around 4 to 5 mIU/L) with a low free T4 indicates hypothyroidism. A high TSH with a normal free T4 is mild hypothyroidism.
Can biotin affect a TSH test?
Yes. Biotin can make thyroid tests look abnormal when they are not. The American Thyroid Association advises stopping biotin for 2 days before thyroid blood tests.
Should I test TSH in the morning?
Yes. TSH varies during the day and MedlinePlus advises an early morning test. Retest at the same time of day so results are comparable.
Sources
- MedlinePlus Medical Encyclopedia: TSH test
- MedlinePlus Lab Tests: TSH (thyroid stimulating hormone) test
- ARUP Laboratories Test Directory: Thyroid stimulating hormone
- American Thyroid Association: Thyroid function tests
- American Thyroid Association: Hypothyroidism (underactive thyroid)
- Hollowell JG et al. Serum TSH, T4 and thyroid antibodies in the United States population (NHANES III). J Clin Endocrinol Metab 2002
- SiPhox Health: panel comparison (checked 29 Sep 2026)
- SiPhox Health: Thyroid Focus panel
- SiPhox Health: Longevity Essentials panel