TSH Levels: What Your Result Means (Normal Range, High & Low)
TSH (thyroid-stimulating hormone) is your pituitary gland’s signal to your thyroid. Most adult labs treat roughly 0.4–4.5 mIU/L as normal. Many UK labs use about 0.27–4.2. A high TSH usually means the thyroid is underactive. A low TSH usually means it is overactive. The range printed on your own report is the one to go by.
Units: mIU/L, µIU/mL and mU/L are the same number. A TSH of 2.1 µIU/mL is 2.1 mIU/L.
Where does my TSH sit?
Type the number from your report. You can also add your lab’s own range and a free T4 result. Everything is worked out on your device, and nothing is saved or sent anywhere.
TSH levels chart
TSH covers such a wide range that it reads best on a log scale. The bands below are the ones most guidelines use. The bars show how the “normal” range moves between labs, in early pregnancy and with age.
Sources: Labcorp test 004259; Exeter Clinical Laboratory; ATA 2026 pregnancy guideline; Surks & Hollowell, JCEM 2007 (US NHANES III data). Full references at the end of this page.
| TSH (mIU/L) | How it’s usually described | What it often points to |
|---|---|---|
| Below 0.1 | Very low (“suppressed”) | An overactive thyroid, or a dose of thyroid tablets that’s too high. This needs a prompt review. |
| 0.1 to 0.4 | Low | A mildly overactive thyroid, early pregnancy, some medicines, or a passing change |
| 0.4 to 4.5* | Within the usual adult range | The pituitary and thyroid are usually signalling normally |
| 4.5* to 10 | Mildly raised | Often a mild or early underactive thyroid. Can also follow recovery from illness, or simply come with older age. |
| Above 10 | Clearly raised | An underactive thyroid (hypothyroidism) is more likely |
*Upper limits differ between labs, typically somewhere from 4.0 to 4.5. The 0.1 and 10 cut-offs are the ones guidelines use when deciding how urgently to act (ETA 2015, NICE NG145).
What TSH actually tells you
A good detective doesn’t question the suspect first. They talk to the informant. TSH is the informant. Your pituitary gland, at the base of the brain, keeps watching how much thyroid hormone (T4 and T3) is in your blood, and adjusts TSH to match:
- Too little thyroid hormone means the pituitary “shouts” louder, so TSH goes up.
- Too much thyroid hormone means the pituitary goes quiet, so TSH goes down.
That’s why the numbers seem back to front: high TSH means an underactive thyroid. The pituitary reacts to small changes, so TSH usually moves before free T4 does. That makes it the best first test. UK guidance (NICE NG145) starts with TSH and adds free T4 to the same sample when TSH is outside the range.
The one exception: if the problem is in the pituitary or hypothalamus itself (“central” hypothyroidism), TSH can look low or normal even though thyroid hormone is low. This is rare. It’s the main reason a doctor sometimes checks free T4 even when TSH looks fine (ETA 2018).
What your TSH number means
Find your band below. Each one is set out like a case file: the clues you’ll usually see, the usual suspects behind them, the next lead your doctor is likely to follow, and the questions worth asking.
TSH below 0.1 mIU/L
A TSH this low means the pituitary has almost switched off its signal. Usually that’s because there is already more than enough thyroid hormone in the blood.
- TSH under 0.1
- Free T4 and/or free T3 often high
- Sometimes a racing heart, weight loss, tremor, heat intolerance
- An overactive thyroid, such as Graves’ disease, thyroid nodules or thyroiditis
- A dose of thyroid tablets that’s too high
- Less often: strong pregnancy-hormone effects (for example with severe sickness) or certain medicines
Free T4 and free T3 on the same or a repeat sample. Depending on the result, thyroid antibodies (TRAb) or a thyroid scan may follow.
Below 0.1 is the level at which treatment is considered more readily, especially over 65 or with heart or bone risks (ETA 2015).
Ask: Are my free T4 and free T3 high? Could any of my medicines or supplements explain this? Do I need treatment, or a repeat test first?
TSH between 0.1 and 0.4 mIU/L
Slightly below the usual range. If free T4 and free T3 are normal, doctors call this subclinical hyperthyroidism. Mild low results can be temporary, so the first step is usually to repeat the test.
- TSH just under your lab’s range
- Free T4 usually normal
- Often no symptoms at all
- A mildly overactive thyroid (nodules, early Graves’, thyroiditis)
- A thyroid tablet dose that’s a little high
- The first trimester of pregnancy (a normal effect)
- Medicines such as steroids, dopamine-type drugs or somatostatin analogues
A repeat TSH with free T4 and free T3. Free T3 can be raised on its own.
Between 0.1 and 0.39, treatment is usually reserved for older adults or those with specific risks. Many people are simply monitored (ETA 2015).
Ask: Should we repeat this with free T3? Could pregnancy, a recent illness or my medicines explain it?
TSH between 0.4 and 4.5 mIU/L (or within your lab’s range)
A TSH inside your lab’s range usually means the pituitary and thyroid are talking normally. There’s no need to “optimise” a normal TSH. See the optimal TSH myth below.
- Pituitary or hypothalamic disease (rare)
- A recent change in thyroid medicine dose, because TSH takes weeks to settle
- Biotin supplements, which can distort results (see what can throw off TSH)
Other common suspects are worth ruling out: low iron stores (ferritin), vitamin B12, vitamin D, anaemia, blood sugar, sleep and mood. Our “Always tired?” case file walks through them.
Ask: If my TSH is normal, what else could explain how I feel?
TSH between 4.5 and 10 mIU/L
Above the usual upper limit, but under 10. If free T4 is normal, this is called subclinical hypothyroidism: the thyroid is keeping up, but the pituitary has to push harder to make it.
- TSH above range, under 10
- Free T4 usually normal
- Symptoms often mild or absent
- Early autoimmune thyroiditis (Hashimoto’s)
- Recovery from a recent illness, when TSH can briefly overshoot
- Older age, as TSH naturally drifts up (see age)
- Medicines such as amiodarone (early on) or lithium
A repeat TSH with free T4, and often TPO antibodies (NICE NG145 1.5.1). A positive antibody test makes progression more likely.
NICE: for adults under 65 with symptoms, consider a 6-month trial of levothyroxine if TSH is above range on 2 tests 3 months apart. Otherwise, monitor: yearly if there are features of thyroid disease, or every 2–3 years if not (NG145 1.5.4 and 1.5.7). For people over 80, the ETA generally favours “wait and see”.
Ask: Should we check TPO antibodies? When should we repeat this? Do my age, symptoms or plans for pregnancy change the plan?
TSH above 10 mIU/L
A TSH above 10 makes an underactive thyroid (hypothyroidism) much more likely. Free T4 tells your doctor whether it’s overt (free T4 low) or a more marked subclinical pattern (free T4 still normal).
- TSH above 10
- Free T4 low or low-normal
- Possibly tiredness, feeling cold, weight gain, constipation, dry skin, low mood
- Hashimoto’s thyroiditis, the most common cause where iodine intake is adequate
- Previous thyroid surgery or radioactive iodine
- Medicines such as lithium or amiodarone
- The underactive phase of a thyroiditis, for example after pregnancy
Free T4 and TPO antibodies. If free T4 is normal, a repeat test to confirm it.
NICE: consider levothyroxine when TSH is 10 or higher on 2 tests 3 months apart, even with a normal free T4 (NG145 1.5.3). The ETA recommends treatment for adults under about 65–70 with TSH above 10, even without symptoms.
Ask: Is this overt or subclinical? Do I need treatment now? When should my TSH be rechecked?
TSH and free T4 together: the patterns
TSH is the informant and free T4 is the evidence. Read together, they point to a pattern. Free T4 comes in two units: pmol/L (UK and most of the world) and ng/dL (US). To convert, 1 ng/dL = 12.87 pmol/L. A typical UK range is about 12–22 pmol/L, though some labs use 9–25. A typical US range is about 0.8–1.8 ng/dL.
| TSH | Free T4 | Usual pattern |
|---|---|---|
| High | Low | Overt underactive thyroid (primary hypothyroidism) |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Overt overactive thyroid (hyperthyroidism) |
| Low | Normal | Subclinical hyperthyroidism. Check free T3 too. |
| Low or normal | Low | Possible pituitary cause (central hypothyroidism) or a severe non-thyroid illness. Needs a doctor’s review. |
| High or normal | High | Unusual. Can come from the timing of a levothyroxine dose, lab interference (for example biotin) or rare causes. Needs a doctor’s review. |
| Normal | Normal | Thyroid function looks normal on these tests |
Want the full picture? Read thyroid hormones explained, then free T4 and free T3.
What can throw off a TSH result
Before blaming the thyroid, a careful detective rules out a planted clue. These things can move TSH or free T4 without any thyroid disease:
- Biotin supplements. Biotin is common in hair, skin and nail products, and it can make some lab tests read falsely high or falsely low (FDA). The American Thyroid Association advises stopping biotin at least 2 days before a thyroid blood test. Labcorp asks for at least 72 hours. Tests vary in how sensitive they are, so always tell whoever orders yours.
- Time of day. TSH is highest overnight and lowest roughly between 10 a.m. and 4 p.m. (Mayo Clinic Laboratories). For a daytime test this rarely changes the reading. For a borderline result, though, it makes sense to repeat it at a similar time of day.
- A recent illness. During a significant illness TSH can drop. During recovery it can rebound briefly above the range before settling.
- Medicines. Steroids (glucocorticoids), dopamine and dopamine-type drugs, and somatostatin analogues can lower TSH. Amiodarone can raise it early in treatment. Lithium can cause a genuinely underactive thyroid.
- Levothyroxine timing. Taking your dose shortly before the blood draw can push free T4 up for several hours, while TSH barely moves. UK guidance says this isn’t clinically significant, but many clinicians prefer you to take that day’s dose after the test. Ask what your clinic prefers.
- Pregnancy and age. Both shift what “normal” looks like. See the next two sections.
TSH in pregnancy and when trying to conceive
In early pregnancy, the hormone hCG gently stimulates the thyroid, so TSH naturally dips. That’s why pregnancy uses its own ranges.
- Best: your lab’s own trimester-specific range.
- If there isn’t one: the American Thyroid Association’s 2026 guideline suggests about 0.1–4.0 mIU/L in the first trimester. That’s the usual range shifted down by about 0.4 at the bottom and 0.5 at the top. After that, values gradually return towards the non-pregnant range.
- Outdated targets: older advice used a cut-off of 2.5 in the first trimester and 3.0 later. Current guidelines no longer recommend those fixed targets.
If you already have a thyroid condition or take levothyroxine, contact your care team as soon as you know you’re pregnant, because dose needs often rise early. The same applies if you’re planning a pregnancy.
TSH as you get older
TSH naturally drifts upward with age. In a large US study of people without thyroid disease (Surks & Hollowell, 2007), the top 2.5% cut-off rose from 3.56 mIU/L at age 20–29 to 7.49 mIU/L at age 80 and over. About 70% of older people with a TSH above 4.5 were still within the range for their age.
Most labs don’t print age-adjusted ranges, so a mildly raised TSH in your 70s or 80s may be your normal. European guidance generally recommends “wait and see” for people over 80 with a TSH up to 10 (ETA 2013).
Is there an “optimal” TSH of 1 to 2?
You’ll find claims online that anything above 2 or 2.5 is “suboptimal”. Mainstream guidelines don’t diagnose that way:
- US endocrinology guidance (AACE/ATA 2012) looked at lowering the upper limit to 2.5. It concluded this would add more than 10 million new diagnoses without clear evidence of benefit. When a lab gives no range, it uses about 0.45–4.12.
- For people already on treatment, advice differs. The European Thyroid Association suggests aiming for the lower half of the range (about 0.4–2.5) for most adults, while US and UK guidance accept anywhere within the range.
Bottom line: a TSH of 3 in an untreated adult is a normal result, not a hidden problem.
Reading TSH on your lab report or app
- Names: TSH, thyroid-stimulating hormone, or thyrotropin.
- Units: mIU/L, µIU/mL and mU/L are the same number. No conversion is needed.
- Flags: “H” or “L” only means outside that lab’s range. It isn’t a diagnosis.
- Apps and portals: in the US, results often appear in your patient portal before your clinician has commented. In the UK, results in the NHS App may come with a short note from your GP practice. Either way, the range column on your report is the one to use.
When to get help urgently
- A very low TSH and chest pain, a very fast or irregular heartbeat, breathlessness, or feeling acutely unwell.
- A very high TSH and unusual drowsiness or confusion, a very slow pulse, or feeling extremely cold.
- Pregnant with an out-of-range result: contact your midwife, obstetrician or GP promptly.
TSH questions people ask
What is a normal TSH level?
For most adults, roughly 0.4–4.5 mIU/L. Many UK labs use about 0.27–4.2. Upper limits differ between labs and are lower in early pregnancy, so always use the range printed on your own report.
Is a TSH of 5 bad?
Usually not an emergency. It’s mildly above most labs’ range. The usual next step is a repeat test with free T4, often with TPO antibodies. Many people with a TSH between 4.5 and 10 are monitored rather than treated, and in older adults it can be normal for age. Treatment is considered sooner if you’re pregnant or trying to conceive.
Is a TSH of 0.3 a problem?
It’s slightly low. It’s often rechecked with free T4 and free T3, because mild low results can be temporary. Medicines, early pregnancy or a slightly high thyroid-tablet dose can also explain it.
Why is my TSH normal but I still feel tired?
Tiredness has many suspects besides the thyroid: low iron stores, B12, vitamin D, anaemia, blood sugar, sleep and mood. See the “usual range” section for links to each.
Is mIU/L the same as µIU/mL?
Yes. They give the same number. Some UK labs write mU/L, which is also the same.
Should I stop biotin before a thyroid blood test?
Yes, if your clinician agrees. The American Thyroid Association advises stopping at least 2 days before. Some labs, including Labcorp, ask for 72 hours.
Can I have an underactive thyroid with a normal TSH?
Rarely. If the problem is in the pituitary or hypothalamus (central hypothyroidism), TSH can look normal while free T4 is low. That’s why free T4 matters when symptoms don’t fit.
How we wrote this page
Written and medically reviewed by Dr. Mustafa Sakr, MBBCh. Last reviewed: October 2026. The numbers here come from current guidelines (NICE, ATA, ETA, AACE) and published lab reference intervals, listed below. We update this page when guidance changes.
How the checker works: it compares your TSH with your lab’s range, or with 0.4–4.5 if you don’t enter one. It uses 0.1 and 10 as the guideline cut-offs, and adds a free T4 pattern if you enter one. Everything runs on your device. Nothing is stored or sent, and the checker can’t take account of your symptoms, medicines or history.
References
- NICE. Thyroid disease: assessment and management (NG145), 2019. nice.org.uk/guidance/ng145
- Korevaar TIM, Leung AM, et al. 2026 American Thyroid Association Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Thyroid 2026;36(5):481–544. doi:10.1177/10507256261445624
- Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults (AACE/ATA). Thyroid 2012;22(12):1200–1235. doi:10.1089/thy.2012.0205
- Pearce SHS, et al. 2013 ETA Guideline: Management of Subclinical Hypothyroidism. Eur Thyroid J 2013;2:215–228. doi:10.1159/000356507
- Biondi B, et al. 2015 ETA Guidelines for the Management of Endogenous Subclinical Hyperthyroidism. Eur Thyroid J 2015;4:149–163. doi:10.1159/000438750
- Persani L, et al. 2018 ETA Guidelines on the Diagnosis and Management of Central Hypothyroidism. Eur Thyroid J 2018;7:225–237. Summary (Endo-ERN)
- Surks MI, Hollowell JG. Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population. J Clin Endocrinol Metab 2007;92(12):4575–4582. doi:10.1210/jc.2007-1499
- Association for Clinical Biochemistry, British Thyroid Association, British Thyroid Foundation. UK Guidelines for the Use of Thyroid Function Tests, 2006. british-thyroid-association.org (PDF)
- American Thyroid Association. Thyroid Function Tests (biotin advice). thyroid.org
- US Food and Drug Administration. Testing for Biotin Interference in In Vitro Diagnostic Devices (guidance). fda.gov
- Labcorp. Thyroid-stimulating Hormone (TSH), test 004259: reference interval and biotin note. labcorp.com
- Exeter Clinical Laboratory. TSH (thyroid stimulating hormone): adult reference range. exeterlaboratory.com
- Mayo Clinic Laboratories. TSH, sensitive (STSH): daily rhythm; and SI unit conversion table (free T4 × 12.87). Test overview · Conversion table
- Merck Manual Professional Version. Euthyroid Sick Syndrome. merckmanuals.com
This page is for education only and isn’t medical advice, a diagnosis or a substitute for your own doctor, who knows your history. Reference ranges vary between laboratories. Always interpret results with a qualified clinician. See our medical disclaimer.
