7 min - BioSense Team - 2025-01-09
- level:understanding
- TSH
- thyroïde
- hypothyroïdie
- T4 libre
- anti-TPO
TSH test: normal range, high and low levels, ultrasensitive
The TSH test, sometimes listed as "ultrasensitive TSH" on lab reports, is the first blood test used to check the thyroid. In adults, a normal level usually falls around 0.4 to 4.5 mIU/L, and the range printed on your own report is the one that applies. A high TSH suggests an underactive thyroid; a low TSH suggests an overactive thyroid or too high a dose of thyroid medication. A slightly abnormal value does not always mean disease or a need for treatment. Here is how to read your result in light of US guidance, with UK guidance where it differs.
What is TSH?
TSH (thyroid-stimulating hormone) is made by the pituitary, a small gland at the base of the brain. It tells the thyroid to make its hormones, T4 and T3, which help the body use energy, stay warm and keep the brain, heart and muscles working as they should. The system works like a thermostat:
- when the thyroid makes too little hormone, the pituitary raises TSH to stimulate it: a high TSH suggests an underactive thyroid (hypothyroidism);
- when the thyroid makes too much, TSH falls: a low TSH suggests an overactive thyroid (hyperthyroidism).
This is why the American Thyroid Association (ATA) calls TSH the best way to initially test thyroid function. Changes in TSH often appear before thyroid hormone levels become too high or too low. Free T4 is usually added when TSH is abnormal. Rarely, a pituitary problem makes TSH misleading, which is one reason a doctor reads it alongside your symptoms.
What is a normal TSH level?
There is no single US number. MedlinePlus (National Library of Medicine) lists 0.4 to 4.8 mIU/L as a typical normal range, while the US Preventive Services Task Force (USPSTF) notes that the upper limit is commonly, but arbitrarily, set at 4.5 mIU/L and the lower limit at about 0.4 mIU/L. The two units you may see, mIU/L and µIU/mL, give the same number. Experts do not fully agree on the upper limit, and the range also changes with your situation:
- with age: some labs use a higher upper limit in older people, even as high as 7 mIU/L according to MedlinePlus;
- during pregnancy: the ATA's 2026 pregnancy guideline prefers ranges specific to the lab and the trimester. When none is available, it suggests 0.1 to 4.0 mIU/L in the first and second trimesters;
- during an acute illness or a hospital stay: TSH can shift temporarily;
- with the time of day: TSH varies during the day, so MedlinePlus says early morning is best for the test.
High TSH: overt or subclinical hypothyroidism?
A TSH above the normal range is not enough to reach a conclusion. The next step is usually a free T4 test:
- Overt hypothyroidism combines a high TSH with a low free T4. The standard treatment is levothyroxine, according to the ATA.
- Subclinical hypothyroidism combines a high TSH with a normal T4. It often causes few or no symptoms. The USPSTF describes two groups: TSH between 4.5 and 10 mIU/L, and TSH above 10 mIU/L.
A single, slightly high TSH is usually checked again first, because many such values return to normal by themselves. The USPSTF notes that professional groups often recommend repeat testing over 3 to 6 months in people without symptoms, unless TSH is above 10 or below 0.1 mIU/L.
If the rise persists, a doctor may test for thyroid peroxidase (TPO) antibodies, which are present in most people with Hashimoto's disease. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), Hashimoto's is the most common cause of hypothyroidism in the US. It is 4 to 10 times more common in women than in men and most often develops between ages 30 and 50.
Should subclinical hypothyroidism be treated?
Not automatically, and guidance varies by country. Attribution matters here, because the recommendations are not identical:
- In the US, the USPSTF states that consensus is lacking on when to treat, especially when TSH is below 10 mIU/L. Decisions are made case by case with your doctor.
- In the UK, NICE suggests considering levothyroxine for adults when TSH is 10 mIU/L or higher on two tests 3 months apart. Below 10 mIU/L, it suggests considering a 6-month trial for adults under 65 who have symptoms.
- An international BMJ panel (2019) made a strong recommendation against thyroid hormones for most adults with subclinical hypothyroidism. It does not apply to people trying to become pregnant or to those with a TSH above 20 mIU/L, and it may not apply to people with severe symptoms or young adults.
Without treatment, follow-up relies on repeat blood work. NICE suggests a check once a year if there are signs of underlying thyroid disease, such as raised antibodies, and every 2 to 3 years otherwise.
Extra caution applies to older adults. The TRUST trial, in people aged 65 and over with subclinical hypothyroidism, found no improvement in symptoms or fatigue with levothyroxine.
What USPSTF says about thyroid screening
The USPSTF (2015, I statement) found the evidence insufficient to assess the benefits and harms of screening nonpregnant adults who have no symptoms. That is not a recommendation for or against: it means the question is open. It does mean a yearly TSH is not a test that is recommended for everyone. Possible harms include false-positive results, labeling and overtreatment.
Low ultrasensitive TSH: what does a low result mean?
TSH is low when it falls below your lab's range. The USPSTF describes two levels:
- Low but detectable, about 0.1 to 0.4 mIU/L: usually rechecked first if you have no symptoms, along with T4 and T3 if it stays low;
- Clearly low or undetectable, below 0.1 mIU/L: more likely to lead to prompt follow-up and, where appropriate, treatment.
Overt hyperthyroidism means a low TSH with high thyroid hormone levels. Subclinical hyperthyroidism means a low TSH with normal T4 and T3. A low TSH is not always due to the thyroid. MedlinePlus lists certain medicines among the possible causes, including steroids, dopamine, some chemotherapy drugs and opioids such as morphine, as well as a pituitary disorder or too much iodine.
Low TSH on levothyroxine
On levothyroxine (Synthroid, Levoxyl or a generic), a low TSH may indicate too high a dose. The ATA's goal is a TSH back in the normal range with symptoms resolved, and it warns against overtreatment, particularly a TSH below 0.1 mIU/L in older people and after menopause, because of the risks of atrial fibrillation and osteoporosis. NIDDK describes a blood test about 6 to 8 weeks after starting the medicine or changing the dose. Palpitations, nervousness or insomnia call for contacting your doctor quickly, without changing the treatment yourself.
What are the symptoms of abnormal TSH?
The signs of hypothyroidism are not specific when taken one by one: fatigue, feeling cold, constipation, moderate weight gain, slowing down, low mood, hair changes, dry skin, cramps, menstrual problems. It is the combination of several that usually prompts a TSH test, and NIDDK notes that a diagnosis cannot be based on symptoms alone.
Conversely, a low TSH with palpitations, weight loss, tremor, nervousness or excessive sweating may suggest hyperthyroidism. It calls for prompt medical advice.
What can distort a TSH result
- Biotin (vitamin B7), found at high doses in some hair and nail supplements, can interfere with certain lab tests, as the FDA has warned. The ATA notes that it most often produces a falsely low TSH and falsely high T4 and T3, which can look like hyperthyroidism. ATA patient publications advise stopping it for at least 2 days, and in a later article 3 to 5 days, before testing. Tell the lab and your doctor, and follow their instructions.
- Some medicines affect thyroid function, notably amiodarone and lithium, which are often monitored with regular testing.
- Estrogen, in birth control pills or during pregnancy, raises total T4 and T3 by raising their binding proteins. The ATA recommends relying on TSH and free T4 in this situation. Starting or stopping estrogen can also change the levothyroxine dose someone needs.
- When you take your treatment: if you take levothyroxine, ask whether to have your blood drawn before that day's dose, and keep the same routine from one test to the next.
- A recent illness can make TSH vary temporarily: it is usually better to recheck later.
The ATA notes that T3 testing is rarely helpful for hypothyroidism, since it is the last test to become abnormal, and that free T3 is often unreliable. Reverse T3 is not clinically useful for this question.
TSH, pregnancy and planning a pregnancy
The need for thyroid hormones rises from the start of pregnancy. The ATA's 2026 guideline finds insufficient evidence to recommend routine testing of everyone planning a pregnancy or pregnant. It suggests offering TSH testing to people with risk factors, such as a history of thyroid disease, autoimmune disease or certain medications. Anyone already on levothyroxine should talk to their doctor as soon as they plan a pregnancy or get a positive test, since the dose often needs to increase. NIDDK also says to contact your doctor right away if you find out you're pregnant.
Thyroid: diet and supplements
The thyroid needs iodine, supplied by iodized salt, seafood and dairy products. NIDDK notes that iodine deficiency is extremely rare in the US, and that large amounts of iodine from kelp, seaweed or supplements may cause or worsen hypothyroidism in people with Hashimoto's disease. Pregnancy and breastfeeding are an exception: the ATA recommends a 150 mcg daily iodine supplement before conception, in pregnancy and during lactation, unless intake is already high.
The FDA does not approve dietary supplements before they are sold. No "thyroid support" supplement replaces a prescribed treatment.
Frequently asked questions
What is an ultrasensitive TSH test?
An ultrasensitive TSH test measures the same hormone as a standard TSH test, with a technique able to detect very low values. This helps separate a low but detectable TSH from an undetectable one. Read it like any TSH, using the range printed on your report, often close to 0.4 to 4.5 mIU/L.
What does a high TSH level mean?
A high TSH usually suggests an underactive thyroid, but one slightly high result does not confirm it. A doctor typically repeats the test and checks free T4. The result is then read with your symptoms, your age and any medications you take.
Is a low TSH serious?
Not necessarily: a slightly low TSH without symptoms is usually rechecked first. It may point to hyperthyroidism, too high a dose of thyroid medication or a temporary cause. Palpitations, weight loss or tremor call for prompt medical advice.
Is a TSH of 5 mIU/L hypothyroidism?
Not necessarily. A TSH of 5 mIU/L is slightly above most ranges and is usually retested, with free T4 added if the rise persists. Many slightly high values return to normal or stay stable without treatment.
My TSH is normal but I'm still tired: is it my thyroid?
In most healthy people, a normal TSH means the thyroid is working properly, according to the ATA. Persistent fatigue has many other causes to explore with your doctor, such as iron deficiency, anemia, poor sleep or depression. See our article on persistent fatigue after 40.
To go further, read also why a result "within the normal range" does not tell the whole story, our article on perimenopause biomarkers and the one on ferritin.
*This article is for information only and does not replace medical advice. Never change a thyroid treatment without talking to your doctor.*
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Sources and methodology
International
- Bekkering GE, et al. Thyroid hormones treatment for subclinical hypothyroidism: a clinical practice guideline. BMJ, 2019;365:l2006. PubMed
United States
- US Preventive Services Task Force. Thyroid Dysfunction: Screening. Final recommendation statement (I statement), 2015. USPSTF
- American Thyroid Association. Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism. Thyroid, 2014;24(12):1670-1751. PMC
- American Thyroid Association. Korevaar TIM, et al. 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Thyroid, 2026;36(5):481-544. DOI 10.1177/10507256261445624. ATA guidelines list
- American Thyroid Association. Thyroid Function Tests. Patient information. ATA
- American Thyroid Association. Biotin supplement use is common and can lead to the false measurement of thyroid hormone in commonly used assays. Clinical Thyroidology for the Public, December 2018. ATA
- American Thyroid Association. Biotin use can interfere with the management of thyroid diseases, including thyroid cancer. Clinical Thyroidology for the Public, January 2022. ATA
- MedlinePlus, National Library of Medicine. TSH test. MedlinePlus
- NIDDK. Hypothyroidism (Underactive Thyroid). NIDDK
- NIDDK. Hyperthyroidism (Overactive Thyroid). NIDDK
- NIDDK. Hashimoto's Disease. NIDDK
- FDA. Biotin Interference with Troponin Lab Tests. 2022. FDA
- FDA. Questions and Answers on Dietary Supplements. FDA
United Kingdom
- National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE guideline NG145, 2019. NICE
Europe
- Stott DJ, et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism (TRUST). New England Journal of Medicine, 2017;376:2534-2544. PubMed
*BioSense methodology: our content is based on guidance from US authorities and medical societies (such as the USPSTF, CDC, NIH, FDA, ACC/AHA and ADA), UK bodies (NICE, NHS), European societies and the WHO. Sources are listed by country, and differences between countries are stated when they matter.*