Lab result explainedThyroidLast updated November 1, 2026

TSH blood test: what high and low TSH levels mean

Thyroid-stimulating hormone (TSH) · Thyroid panel · reported in mIU/L (same as µIU/mL)

Thyroid-stimulating hormone (TSH) is the signal your pituitary gland sends to your thyroid to tell it how much thyroid hormone to make, and a TSH blood test is the single most sensitive way to check whether the thyroid is working properly. The normal range on most US reports is about 0.4 to 4.0 mIU/L (some labs go to 4.5 or 5.0). The two most common reasons it is out of range are Hashimoto's thyroiditis, the autoimmune cause of an underactive thyroid that pushes TSH up, and Graves' disease or an overactive nodule, which push it down.

TSH moves in the opposite direction to the thyroid itself. When thyroid hormone (T4 and T3) falls, the pituitary responds by releasing more TSH; when thyroid hormone is excessive, TSH is suppressed, often to nearly zero. Because the pituitary reacts to tiny changes, TSH shifts outside the normal range long before T4 does, which is why it is the first-line test and why a mildly abnormal TSH with a normal T4 (subclinical disease) is so common.

A single TSH is a snapshot. It varies through the day, rises during recovery from illness, is affected by biotin supplements and several medicines, and runs higher in older people and lower in early pregnancy. Guidelines therefore recommend repeating an abnormal TSH with a free T4 after 2 to 3 months before diagnosing anything, unless the result is markedly abnormal or you are pregnant.

TSH at a glance

What it is: the pituitary hormone that drives the thyroid; it rises when thyroid hormone is low and falls when it is high.
Normal: roughly 0.4 to 4.0 mIU/L in adults (lab ranges vary; some use 0.45 to 4.5). Older adults can be normal up to about 5 to 7.
High TSH means the thyroid is underactive (hypothyroidism); above 10 mIU/L usually needs treatment, 4.5 to 10 is subclinical and often watched.
Low TSH means the thyroid is overactive (hyperthyroidism) or you are taking too much thyroid hormone; below 0.1 mIU/L is significant.
Pregnancy: lower is normal in the first trimester (hCG effect); the upper limit is about 4.0 or 0.5 below the lab's non-pregnant limit.
Next step: repeat TSH with free T4 in 6 to 12 weeks, plus TPO antibodies if high and T3 or TSI if low.

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What TSH measures

The hypothalamus releases TRH, which tells the pituitary to release TSH, which tells the thyroid to make thyroxine (T4) and triiodothyronine (T3). Circulating T4 and T3 then feed back on the pituitary and hypothalamus to switch TSH down. The relationship is log-linear: a halving of free T4 roughly causes a tenfold rise in TSH. That amplification is what makes TSH such a sensitive detector of thyroid failure or excess.

Laboratories measure TSH with a two-site immunoassay (third-generation assays detect down to about 0.01 mIU/L, which is what separates true suppression from a low-normal value). Results are in milli-international units per litre (mIU/L), which is identical to micro-international units per millilitre (µIU/mL).

Because the test relies on antibody binding, substances that interfere with antibodies can distort it. High-dose biotin (common in hair and nail supplements, 5 to 10 mg or more) can make TSH read falsely low on many platforms, and heterophile antibodies in some people can make it read falsely high. Stop biotin for at least 48 hours before a thyroid test.

Normal range for TSH

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (mIU/L (same as µIU/mL))	Notes
Adults (typical lab range)	0.4–4.0 mIU/L	Some labs report 0.45–4.5 or 0.35–5.0. Interpret with your lab's printed range.
Adults over 70 to 80	Up to about 5–7 mIU/L may be normal	TSH rises with age; a value of 5 to 7 in an older adult without symptoms often needs no treatment.
Pregnancy, first trimester	About 0.1–2.5 mIU/L (lower limit) to 4.0 (upper)	hCG stimulates the thyroid, so TSH dips in weeks 8 to 14. Use trimester-specific ranges; the ATA suggests an upper limit of 4.0 or 0.5 below the lab's non-pregnant limit.
Subclinical hypothyroidism	4.5–10 mIU/L with normal free T4	Affects up to 10 percent of adults; treatment depends on symptoms, antibodies, age and pregnancy plans.
Overt hypothyroidism	Above 10 mIU/L, or any high TSH with low free T4	Levothyroxine is usually recommended.
Subclinical hyperthyroidism	0.1–0.4 mIU/L with normal free T4 and T3	Below 0.1 carries higher risk of atrial fibrillation and bone loss, especially over 65.
Overt hyperthyroidism	Below 0.1 (often below 0.01) mIU/L with high free T4 or T3	Needs assessment for Graves' disease, toxic nodules or thyroiditis.
Target on levothyroxine	About 0.5–2.5 mIU/L (or within the lab range)	Lower end (0.1 to 0.5) is used after thyroid cancer; pregnancy targets below 2.5.

High TSH

A high TSH means the pituitary is working harder to get the thyroid to respond, which almost always means the thyroid is underproducing: primary hypothyroidism. If free T4 is also low, this is overt hypothyroidism and treatment with levothyroxine is standard. If free T4 is still normal, it is subclinical hypothyroidism; with a TSH under 10 and no symptoms, many clinicians simply repeat the test in a few months, because up to half of mildly raised values normalise on their own.

The cause in most of the world where iodine is adequate is Hashimoto's thyroiditis, an autoimmune attack on the thyroid identified by TPO antibodies. In people already taking levothyroxine, a high TSH usually means the dose is too low or the tablet is not being absorbed, most often because it is being taken with food, coffee, calcium, iron or a proton-pump inhibitor.

Rarely, a high TSH is not thyroid failure at all. The pituitary can overproduce TSH (a TSH-secreting adenoma), the body can be resistant to thyroid hormone, and the assay can be fooled by heterophile antibodies or by lab timing, since TSH peaks around midnight and is up to 50 percent higher on an early-morning draw than in the afternoon.

Common causes

Hashimoto's thyroiditis (autoimmune hypothyroidism)
Antibodies against thyroid peroxidase (TPO) and thyroglobulin slowly destroy thyroid tissue, so T4 output falls and TSH climbs. The most common cause in the US, affecting women five to eight times more often than men.
Subclinical hypothyroidism
Mildly raised TSH (4.5 to 10) with a normal free T4; an early or mild version of the above, or a transient rise after illness. Often found on routine testing with few or no symptoms.
Levothyroxine dose too low or poorly absorbed
Taking the tablet with food, coffee, calcium, iron, soy or antacids can cut absorption by a third or more; celiac disease and proton-pump inhibitors do the same. A rising TSH on a stable dose is a prompt to check timing before increasing the dose.
Recovery phase of thyroiditis
After subacute, postpartum or silent thyroiditis, the thyroid passes through a low phase lasting weeks to months during which TSH overshoots before recovering.
Medications
Amiodarone, lithium, interferon, tyrosine kinase inhibitors and immune checkpoint inhibitors (pembrolizumab, nivolumab) frequently cause hypothyroidism; high-dose iodine and some antithyroid drugs push TSH up if the dose is too high.
Prior thyroid surgery or radioactive iodine
Removing or ablating thyroid tissue for Graves' disease, nodules or cancer removes hormone output; TSH rises until replacement is adequate.
Iodine deficiency or iodine excess
Too little iodine means the thyroid cannot build hormone; a sudden excess (kelp supplements, contrast dye) can also shut hormone synthesis down for weeks (the Wolff-Chaikoff effect).
Non-thyroidal illness recovery and lab timing
During recovery from a serious illness TSH can transiently rise to 10 to 20 before settling. TSH also follows a daily rhythm, highest late at night, so a first-thing-in-the-morning sample reads higher than an afternoon one.
Pituitary TSH-secreting tumour or thyroid hormone resistance
Rare causes in which TSH is high or inappropriately normal while free T4 is high rather than low; the pattern of a high TSH with a high T4 is the clue.
Congenital hypothyroidism and heterophile antibody interference
Newborns are screened because untreated congenital hypothyroidism harms brain development. In adults, antibodies that cross-react with the assay reagents can produce a falsely high TSH that does not match symptoms or T4.

Symptoms that can go with it

Tiredness and sleepiness that rest does not fix
Feeling cold when others are comfortable
Weight gain of a few pounds, or difficulty losing weight
Constipation
Dry skin, coarse or thinning hair, brittle nails
Low mood, slowed thinking, poor memory
Heavy or irregular periods, trouble conceiving
Muscle aches, cramps, puffiness around the eyes, hoarse voice
Slow heart rate, raised LDL cholesterol on blood tests
Often no symptoms at all when TSH is only mildly raised

Low TSH

A low TSH means the pituitary is being told to stand down, which usually means there is too much thyroid hormone in the blood. If free T4 or T3 is high, this is overt hyperthyroidism. If both are normal, it is subclinical hyperthyroidism, which is still worth attention when TSH is below 0.1 mIU/L because of the raised risk of atrial fibrillation and bone loss, particularly in people over 65.

The most common causes are Graves' disease (an antibody that mimics TSH), an overactive thyroid nodule or multinodular goitre, inflammation of the thyroid that leaks stored hormone (thyroiditis), and simply taking too much levothyroxine. In the first trimester of pregnancy a mildly low TSH is normal because hCG stimulates the thyroid.

A low TSH with a low free T4 is a different problem: it points to the pituitary or hypothalamus rather than the thyroid (central hypothyroidism) and needs further pituitary hormone testing. Severe illness, high-dose steroids, dopamine and biotin supplements can all lower TSH without any thyroid disease.

Common causes

Graves' disease
An autoimmune antibody (TSI or TRAb) binds the TSH receptor and switches the thyroid on continuously. The most common cause of hyperthyroidism in people under 50, often with eye symptoms and a smooth enlarged thyroid.
Too much levothyroxine (over-replacement)
A dose that was right at one weight or age becomes too much after weight loss, menopause or ageing, or a brand switch changes absorption. One of the most common reasons for a low TSH in primary care.
Toxic nodule or toxic multinodular goitre
One or several thyroid nodules make hormone independently of TSH. More common after 50 and in areas of past iodine deficiency; TSH is low but there are no Graves' antibodies.
Thyroiditis (postpartum, subacute, silent, drug-induced)
Inflammation damages thyroid cells and releases stored hormone, suppressing TSH for weeks before a low phase and recovery. Postpartum thyroiditis affects about 5 to 10 percent of women within a year of delivery.
Early pregnancy
hCG is structurally similar to TSH and stimulates the thyroid directly; TSH is often 0.1 to 0.4 between weeks 8 and 14 and recovers by the second trimester. Hyperemesis gravidarum exaggerates this.
Medications and supplements
Glucocorticoids, dopamine, octreotide and bexarotene suppress TSH; amiodarone and iodine contrast can trigger thyrotoxicosis; immune checkpoint inhibitors cause thyroiditis. High-dose biotin gives a falsely low TSH on many assays.
Central (secondary) hypothyroidism
Pituitary tumours, surgery, radiation, head injury or Sheehan syndrome reduce TSH output, so TSH is low or inappropriately normal and free T4 is low. The pattern, not the TSH alone, makes the diagnosis.
Severe non-thyroidal illness
During hospitalisation for serious illness, TSH often falls below normal and T3 drops (euthyroid sick syndrome) without thyroid disease; it should be rechecked after recovery.
Thyroid hormone in supplements or weight-loss products
Some over-the-counter thyroid support and desiccated glandular products contain active T3 and T4 and suppress TSH; taking more than the prescribed levothyroxine dose does the same.

Symptoms that can go with it

Racing or pounding heartbeat, palpitations, atrial fibrillation
Anxiety, irritability, restlessness, poor sleep
Weight loss despite a good appetite
Feeling hot, sweating, heat intolerance
Tremor of the hands, muscle weakness in the thighs or upper arms
Frequent or loose bowel movements
Lighter or missed periods
Eye bulging, grittiness or double vision (Graves' eye disease)
Often no symptoms with subclinical hyperthyroidism

Read it alongside

Free T4 — the hormone the thyroid actually makes; separates subclinical (normal T4) from overt (abnormal T4) disease and reveals central hypothyroidism (low TSH with low T4).
Free or total T3 — rises first in Graves' disease and toxic nodules; a low TSH with normal T4 but high T3 is T3 toxicosis.
TPO antibodies — confirm Hashimoto's as the cause of a high TSH and predict progression from subclinical to overt hypothyroidism (about 4 percent per year when positive).
TSI or TRAb antibodies — confirm Graves' disease as the cause of a low TSH without needing a radioactive uptake scan.
Lipid panel — hypothyroidism raises LDL; a new high cholesterol with fatigue is a classic reason a TSH gets ordered.
CBC and hemoglobin — hypothyroidism causes mild anemia; Hashimoto's travels with pernicious anemia and celiac disease.
Prolactin and cortisol — checked when a pituitary cause is suspected, because untreated adrenal insufficiency must be corrected before starting thyroid hormone.
Thyroid ultrasound — not a blood test, but ordered for a low TSH with a lumpy gland or a high TSH with a goitre.

What usually happens next

A mildly abnormal TSH (4.5 to 10, or 0.1 to 0.4) with a normal free T4 is repeated in 6 to 12 weeks, at the same time of day, with biotin stopped for 48 hours beforehand, before any diagnosis is made.
A high TSH is followed by free T4 and TPO antibodies; a low TSH by free T4, T3 and TSI/TRAb antibodies, sometimes an ultrasound or uptake scan.
If you take levothyroxine, review how you take it (empty stomach, 30 to 60 minutes before food and coffee, 4 hours away from calcium and iron) before the dose is changed; TSH is rechecked 6 to 8 weeks after any change.
Ask whether your result needs treatment at all: a TSH of 5 to 7 in an older adult without symptoms often does not, while any abnormal TSH in pregnancy or when trying to conceive is treated promptly.
Ask about symptoms that would change the plan: palpitations, eye changes, a neck lump, heavy periods or infertility.

Diet, lifestyle and natural levers with evidence

What can move TSH once the cause is known. Nothing here replaces finding the cause.

Take levothyroxine correctly and consistently

Strong evidence

Not a herb, but the lever with the biggest proven effect on TSH in treated people. Taking levothyroxine on an empty stomach 30 to 60 minutes before breakfast and coffee, or at bedtime 3 hours after the last meal, and separating it by 4 hours from calcium, iron, soy and antacids, improves absorption enough to lower TSH by several points in crossover studies. Switching brands or generics can shift TSH too, so stay on one product and recheck 6 to 8 weeks after any change.

Selenium

Moderate evidence

A meta-analysis of 16 randomised trials found selenium 200 mcg a day lowered TPO antibody levels in Hashimoto's thyroiditis at 3, 6 and 12 months, and some trials reported better mood and wellbeing, but it did not consistently change TSH or reduce the levothyroxine dose needed. Most trials ran 3 to 12 months and used selenomethionine or sodium selenite.

Do not exceed 200 mcg a day; chronic intake above 400 mcg causes hair loss, nail changes and nerve damage. Brazil nuts vary wildly in selenium content.

Adequate, not excessive, iodine

Moderate evidence

Both deficiency and excess raise TSH. Adults need about 150 mcg a day (220 to 290 mcg in pregnancy and breastfeeding), easily met with iodised salt, dairy, eggs and fish. Trials and population studies show that high-dose iodine from kelp or supplements (over 500 mcg a day) can trigger or worsen hypothyroidism in people with Hashimoto's, and in regions with marginal intake, iodised salt programmes normalise TSH in the population.

Avoid kelp, bladderwrack and high-dose iodine drops if you have autoimmune thyroid disease unless your clinician recommends them.

Vitamin D repletion if deficient

Preliminary evidence

Low vitamin D is more common in Hashimoto's and Graves' disease, and small randomised trials of vitamin D supplementation (typically 2,000 to 4,000 IU a day for 3 to 6 months) have reduced TPO antibody titres by 20 to 30 percent and produced small TSH reductions in people with subclinical hypothyroidism, though trial quality is mixed and the effect on hormone levels is modest.

Check a 25-hydroxyvitamin D level first; supplementing when already replete adds nothing and high doses raise calcium.

Ashwagandha

Preliminary evidence

One 8-week randomised, placebo-controlled trial in 50 adults with subclinical hypothyroidism found ashwagandha root extract 600 mg a day lowered TSH by about 17 percent (from roughly 7.3 to 6.0 mIU/L) and raised T4 and T3 modestly. It is a single small study, and case reports describe ashwagandha triggering thyrotoxicosis in people with overactive thyroids.

Do not use with hyperthyroidism, a low TSH or in pregnancy. Can push TSH too low if combined with levothyroxine without monitoring. Rare reports of liver injury.

Weight loss in obesity

Preliminary evidence

Obesity itself mildly raises TSH (leptin stimulates TRH), and studies after bariatric surgery and structured weight loss show TSH falls by about 0.5 to 1 mIU/L as weight comes down, with mildly raised values often normalising. This matters because a TSH of 4.5 to 6 in someone with obesity may not be thyroid disease at all.

When to see a clinician promptly

TSH above 10 mIU/L, or any raised TSH with a low free T4: book an appointment within a few weeks; treatment is usually recommended.
TSH below 0.1 mIU/L, or a low TSH with palpitations, tremor, weight loss or eye changes: prompt review for hyperthyroidism, including a heart rhythm check.
Any abnormal TSH if you are pregnant, breastfeeding or trying to conceive: both high and low values affect fertility and the baby's development and are treated promptly.
Fever, confusion, a very fast heart rate, vomiting or agitation with a low TSH: possible thyroid storm, an emergency.
Extreme drowsiness, low body temperature, slow breathing or a very slow pulse with a very high TSH: possible myxedema coma, an emergency.
A new lump in the neck, difficulty swallowing or a hoarse voice, regardless of the TSH value.

Frequently asked questions

Is a TSH of 5 bad?

A TSH of 5 is mildly raised and almost always falls into subclinical hypothyroidism, meaning free T4 is still normal. In an adult under 65 with symptoms or positive TPO antibodies, treatment may be considered; in an older adult with no symptoms it is often simply watched. Most guidelines advise repeating the test in 2 to 3 months first, because about a third to a half of values between 4.5 and 7 return to normal on their own.

What TSH level is dangerous?

There is no single danger line, but a TSH above 10 mIU/L reliably indicates hypothyroidism that benefits from treatment, and a TSH below 0.1 mIU/L indicates clinically significant hyperthyroidism with risks to heart rhythm and bone. Values above 50 to 100, or a suppressed TSH with fever and confusion, need urgent assessment. The accompanying free T4 and your symptoms matter more than the number alone.

My TSH is high but my T4 is normal. What does that mean?

That pattern is subclinical hypothyroidism: the pituitary is working harder, but the thyroid is still keeping up. It affects up to 10 percent of adults. Whether to treat depends on how high the TSH is (above 10 usually yes), whether TPO antibodies are positive, your age, symptoms and pregnancy plans. Many people with a TSH of 4.5 to 7 are rechecked rather than treated.

Do I need to fast for a TSH test, and does time of day matter?

Fasting is not required, although some studies show TSH reads slightly lower after a meal. Time of day matters more: TSH peaks around midnight and is lowest in the late afternoon, so an 8 a.m. sample can be 20 to 50 percent higher than a 4 p.m. one. For borderline results, repeat at the same time of day, and stop biotin supplements for 48 hours beforehand.

Can stress raise TSH?

Day-to-day stress does not materially change TSH. Severe physical stress such as a serious illness, surgery or hospitalisation can lower TSH during the illness and raise it, sometimes to 10 to 20, during recovery, which is why thyroid tests are best avoided or repeated after an acute illness. Chronic stress and poor sleep can shift TSH slightly within the normal range but do not cause thyroid disease.

Does high TSH cause weight gain?

Overt hypothyroidism typically causes a modest gain of 5 to 10 pounds, much of it fluid, and treatment reverses that amount, not more. Subclinical hypothyroidism has little effect on weight. Obesity itself can mildly raise TSH, so the relationship sometimes runs the other way; weight loss lowers TSH slightly.

What does low TSH mean when I am on levothyroxine?

It usually means the dose is now too high for you, which can happen after weight loss, with age, after menopause or when a new medicine improves absorption. A persistently suppressed TSH on treatment raises the risk of atrial fibrillation and osteoporosis, so the dose is normally reduced and TSH rechecked in 6 to 8 weeks. The exception is after thyroid cancer, where a low TSH is sometimes the intended target.

Why is my TSH normal but I still have thyroid symptoms?

Fatigue, weight gain, hair loss and low mood have many causes, and a normal TSH with a normal free T4 makes thyroid disease unlikely as the explanation. Worth checking instead are iron and ferritin, vitamin B12, vitamin D, sleep apnea, depression, perimenopause and medications. If TPO antibodies are positive you may be in the early stage of Hashimoto's, and a TSH recheck every 6 to 12 months is sensible.

References

Jonklaas J, Bianco AC, Bauer AJ, et al.. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid (2014)
Ross DS, Burch HB, Cooper DS, et al.. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid (2016)
Alexander EK, Pearce EN, Brent GA, et al.. 2017 guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid (2017)
Biondi B, Cappola AR, Cooper DS. Subclinical hypothyroidism: a review. JAMA (2019)
Wilson SA, Stem LA, Bruehlman RD. Hypothyroidism: diagnosis and treatment. American Family Physician (2021)
Thyroid function tests. American Thyroid Association (2023)View source
TSH (thyroid-stimulating hormone) test. MedlinePlus, US National Library of Medicine (2023)View source
The FDA warns that biotin may interfere with lab tests: FDA safety communication. US Food and Drug Administration (2019)View source
van Zuuren EJ, Albusta AY, Fedorowicz Z, Carter B, Pijl H. Selenium supplementation for Hashimoto's thyroiditis: summary of a Cochrane systematic review. European Thyroid Journal (2014)
Sharma AK, Basu I, Singh S. Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: a double-blind, randomized placebo-controlled trial. Journal of Alternative and Complementary Medicine (2018)
Hypothyroidism (underactive thyroid). Mayo Clinic (2024)View source
Hypothyroidism. Merck Manual Professional Version (2024)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.