TSH test results: how to read your thyroid number

Written and evidence-checked by the VeriNourish Editorial Team against 12 named sources listed below. No licensed clinician reviews this content.

If you are holding a TSH result and trying to work out whether the number is a problem, the short version is this: TSH is the most sensitive first test for the thyroid, and it is also the one most easily thrown off. Both halves matter when you read your own report.

What TSH measures, and why it moves first

TSH stands for thyroid-stimulating hormone, and the first surprise is that it is not a thyroid hormone at all. It is made by the pituitary gland in your brain and works like a thermostat: when the thyroid is underactive, the pituitary raises TSH to push it harder, and when the thyroid is overactive, TSH drops. That inverse relationship is why the number goes up when you might expect it to go down.

The reason it is such a good first test has a shape worth knowing. The relationship between TSH and free T4 is log-linear, meaning a small fall in thyroid hormone produces a large rise in TSH (Thyroid, 2023). The alarm is louder than the thing it reports. That is exactly what you want from a screening test, and it is why the American Thyroid Association says the best way to initially test thyroid function is to measure TSH (American Thyroid Association).

Reading your result

Reference ranges vary by laboratory, so check the values printed on your own report. They are given in mIU/L.

A common range is roughly 0.3 to 4.5 mIU/L. The values are not spread evenly across it: the median in a population usually falls between 1 and 1.5, depending partly on iodine intake. The upper limit depends on the assay, typically landing around 4.2 to 4.5. Age shifts it: in people over 70 the upper limit of normal can reach about 6.0, so a mildly raised reading in an older woman does not carry the same meaning as the same number in a younger one (Thyroid Research, 2015).

The number on its own is only half the picture. What it means depends on free T4 beside it:

Your TSH	Free T4	What it usually suggests	Next step
High	Low	An underactive thyroid (hypothyroidism)	Treatment, usually levothyroxine
High	Normal	Subclinical hypothyroidism	Confirm on a repeat test before deciding on treatment
Low	High	An overactive thyroid	Find the cause; T3 helps judge how severe it is
Low	Normal	Subclinical hyperthyroidism if free T3 is also normal; an overactive thyroid if T3 is raised	Free T3, and a check for recent illness, medicines and biotin
Low	Low	A problem with the pituitary gland rather than the thyroid	Needs a doctor to work through
Within range	Normal	The thyroid is unlikely to explain your symptoms	Look at other causes

The pairings follow the American Thyroid Association's explanation of thyroid tests, including the pituitary pattern and the people whose T3 rises before their free T4 does (American Thyroid Association). In the UK, NICE suggests starting with TSH alone when a pituitary problem is not suspected, then adding free T4 from the same sample if TSH is high, and free T4 plus free T3 if it is low. When a pituitary problem is suspected, TSH and free T4 are measured together from the start (NICE NG145).

If the result does point to an underactive thyroid, the usual cause in places with enough iodine is autoimmune thyroiditis, and the treatment is levothyroxine (Cureus, 2023). NICE sets the starting dose by body weight for adults under 65 without heart disease, and starts lower and builds up slowly for those 65 and over or with heart disease (NICE NG145), so the dose is your doctor's call rather than a sum to do at home.

A low TSH points the other way. An overactive thyroid speeds the metabolism, so it is one of the causes worth checking behind weight loss you did not intend, usually alongside a fast pulse and heat intolerance.

Why TSH is read with free T4

TSH tells you something has shifted. Free T4 tells you what.

That is why the NHS describes the pair together as the blood tests used to check for an underactive thyroid (NHS). A raised TSH with a low free T4 confirms an underactive thyroid; a raised TSH with a normal free T4 is a much milder finding that is handled differently.

Thyroid antibodies answer a separate question, which is why the gland is struggling in the first place. Their presence points to Hashimoto's, the most common cause of an underactive thyroid in women. T3 has a narrower job: the American Thyroid Association notes that T3 testing rarely helps in an underactive thyroid, because it is the last result to become abnormal (American Thyroid Association). It earns its place when TSH is low: in some people with an overactive thyroid only T3 is raised while free T4 stays normal (American Thyroid Association). That is why NICE measures free T3 alongside free T4 whenever TSH is below range, and defines subclinical hyperthyroidism as a low TSH with both free T3 and free T4 in range (NICE NG145). Our guide to the full thyroid panel goes through the other tests, and when each one is worth ordering.

Subclinical hypothyroidism, the result that confuses everyone

A mildly high TSH with a normal free T4 is the single most common source of thyroid anxiety, and it sits in a genuine gray zone.

The label describes a blood test result, not how you feel, and it does not mean you need medication. NICE sets out when treatment is worth considering, and both routes start with two raised results taken 3 months apart (NICE NG145):

a TSH of 10 mIU/L or higher on both occasions, where levothyroxine can be considered;
a TSH above the range but below 10 mIU/L on both occasions, with symptoms of an underactive thyroid, in adults under 65, where a 6-month trial of levothyroxine can be considered.

NICE also suggests measuring thyroid peroxidase antibodies once in adults with a raised TSH, without repeating them. Their presence points to an autoimmune cause such as Hashimoto's. Pregnancy and trying to conceive are handled differently, and have their own section below.

The practical point: a TSH of 5.2 with a normal T4 and no symptoms is not the same finding as a TSH of 12, and treating them the same way is the mistake to avoid.

Before you act on the number: what can distort it

This is the part most result explainers skip, and it is where a wrong decision usually starts.

A biotin supplement. The most common avoidable error. Labs measure TSH with immunoassays that use biotin in the chemistry itself, so a large biotin intake corrupts the measurement rather than your thyroid. Doses of 5 mg or more raise blood levels enough to interfere, while ordinary multivitamin amounts up to 1 mg have not been reported to cause it. ADLM guidance recommends waiting at least 8 hours after a 5 to 10 mg dose before blood is taken. Some assays are more sensitive and may need a washout of up to 72 hours, and people prescribed high-dose biotin of 100 mg a day or more are advised to stop for at least 72 hours unless that is medically inadvisable. Clearance can also be slower when the kidneys do not work well (ADLM). The right interval depends on the assay your lab uses, so ask the lab or your doctor how long to wait. If a doctor prescribed your biotin, do not stop it on your own; ask them how to time the test.

The direction matters: the classic pattern is a falsely low TSH alongside falsely raised thyroid hormones, which reads like an overactive thyroid in a woman whose thyroid is fine. Biotin is the headline ingredient in hair, skin and nail supplements, often at 5 or 10 mg a serving, which is exactly the dose that causes trouble. Our biotin guide covers what those doses actually achieve.

The time of the blood draw. TSH is higher in the morning than in the afternoon. In a small 2013 study, 20 women with subclinical hypothyroidism had a median TSH of 5.83 mU/L when blood was drawn between 8 and 9 a.m. and 3.79 mU/L between 2 and 4 p.m., and in about half of them the afternoon result no longer met the diagnostic criteria (Endocrine Research, 2013). Twenty women is a small sample, so hold the exact figures loosely. The practical use is consistency: when a borderline result is repeated, ask for the draw at a similar time of day. Fasting is only needed if other blood tests are ordered alongside, so follow the instructions you are given (MedlinePlus).

Recent illness, and some medicines. NICE advises against thyroid testing during an acute illness unless the illness itself may be thyroid-related, because the illness can change the results (NICE NG145). Some medicines, including dopamine and glucocorticoids, lower TSH (Merck Manual). A result taken while you were unwell, or while on one of those medicines, needs that context. Do not stop a prescribed medicine to change a test result.

Low iron. A shortfall can worsen thyroid function, so your doctor may check ferritin and correct low iron alongside rather than treating the thyroid in isolation.

When one result is not a diagnosis

TSH varies between draws in the same person even when nothing about the thyroid has changed. Add the daily rhythm above, and a single borderline number carries much less weight than it appears to.

That is why a borderline result is confirmed before anyone starts treatment. For subclinical hypothyroidism, NICE's treatment criteria use two results taken 3 months apart, and when symptoms get worse or new ones appear it suggests repeating the tests no sooner than 6 weeks after the last one (NICE NG145). If your result sits just outside the range and you feel well, a repeat is the expected next step rather than a prescription.

Take two details to the appointment that will not be printed on your report: what time your blood was drawn, and whether you take any supplement containing biotin. Both change how the number should be read.

Thyroid, pregnancy and trying to conceive

Pregnancy shifts the normal ranges, so results in pregnancy are read against pregnancy-specific values rather than the usual ones. If you are pregnant or planning to be, say so, because it changes how your result is interpreted.

Planning a pregnancy does not by itself mean a mildly raised TSH gets treated. ASRM's 2024 guideline found moderate evidence that levothyroxine for subclinical hypothyroidism does not reduce pregnancy loss or improve pregnancy or live birth rates, and recommends against treating it for that purpose in women trying to conceive or already pregnant. It also says a woman trying to conceive should be read against the usual nonpregnant ranges until she is pregnant (ASRM, 2024).

That guideline does not cover recurrent pregnancy loss, which ASRM addresses in a separate 2026 opinion (ASRM, 2026). If you have had repeated losses, ask your specialist how your thyroid results apply to you. A high TSH with a low free T4 is a different situation: that is an underactive thyroid, and it is treated in pregnancy (ASRM, 2026). If you already take levothyroxine, agree a plan for pregnancy with your prescriber rather than changing the dose yourself.

Symptoms that make a thyroid test worthwhile

Thyroid problems are far more common in women, and the signs build so slowly they are easy to attribute to something else:

Fatigue and feeling run down
Weight gain and feeling cold
Low mood, dry skin and hair thinning
Constipation and brain fog

When to see a doctor

Ask for a thyroid function test if several of these symptoms fit you, or if thyroid disease runs in your family. If your TSH comes back abnormal, expect your doctor to confirm with free T4, often repeat the test, and then decide between treatment and monitoring.

Interpreting a thyroid result is a job for your doctor rather than a home calculation, particularly when the number sits near the edge of the range. What you can usefully bring is the context the lab does not have: your symptoms, the time of the draw, any recent illness, and what is in your supplement cupboard.

Frequently asked questions

What is a normal TSH level?

A common reference range is roughly 0.3 to 4.5 mIU/L, and the median value in a population usually falls between 1 and 1.5. The upper limit depends on the assay, and it rises with age: in people over 70 it can reach about 6.0, so a mildly raised reading at 75 does not mean what the same number means at 30. Read your result against the range printed on your own report.

What does a high TSH mean?

Usually an underactive thyroid: the pituitary is pushing harder because the gland is not keeping up. A high TSH with a low free T4 confirms it. A high TSH with a normal free T4 is the milder picture called subclinical hypothyroidism, which is often monitored rather than treated straight away.

What does a low TSH mean?

Usually an overactive thyroid, where the gland is making too much hormone and the pituitary has stopped asking for more. It needs free T4 and free T3 from the same sample. If both are in range, the pattern is called subclinical hyperthyroidism; in some people with an overactive thyroid only T3 is raised, so a normal free T4 alone does not settle it. Acute illness, medicines such as glucocorticoids, and a biotin supplement can also push the reading down.

Does the time of day change my TSH result?

It can. TSH is higher in the morning than in the afternoon. In a small study of 20 women with subclinical hypothyroidism, median TSH was 5.83 mU/L in the morning and 3.79 mU/L in the afternoon, and in about half of them the afternoon result no longer met the diagnostic criteria. When a result is repeated, have the blood drawn at a similar time of day.

Can biotin supplements affect a thyroid test?

Yes, and this is the most common avoidable error. Doses of 5 mg or more interfere with the immunoassays most labs use, and the classic pattern is a falsely low TSH with falsely high thyroid hormones, which reads like an overactive thyroid in someone whose thyroid is fine. Ordinary multivitamin amounts up to 1 mg have not been reported to cause it. ADLM guidance recommends waiting at least 8 hours after a 5 to 10 mg dose; some assays need up to 72 hours, so ask the lab or your doctor how long to wait. If biotin was prescribed, do not stop it without talking to the prescriber.

Should one abnormal TSH be repeated?

Usually yes, before anyone starts treatment. TSH varies between draws in the same person even when nothing about the thyroid has changed. For subclinical hypothyroidism, NICE's treatment criteria rely on two raised results taken 3 months apart, and when symptoms change it suggests repeating the tests no sooner than 6 weeks after the last one.

Does trying to conceive mean a mildly raised TSH must be treated?

No. ASRM's 2024 guideline recommends against treating subclinical hypothyroidism with levothyroxine in women trying to conceive or already pregnant, because treatment has not been shown to reduce pregnancy loss or improve pregnancy or live birth rates. It does not cover recurrent pregnancy loss, which needs a specialist's view. A high TSH with a low free T4 is a different situation and is treated.

References

Reference ranges for TSH and thyroid hormones · Thyroid Research, 2015
Thyroid Function Tests · American Thyroid Association
Thyroid Stimulating Hormone and Thyroid Hormones: An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status · Thyroid, 2023
Clinical significance of TSH circadian variability in patients with hypothyroidism · Endocrine Research, 2013
Guidance Document on Biotin Interference in Laboratory Tests · ADLM (formerly AACC)
Underactive thyroid (hypothyroidism): diagnosis · NHS
Thyroid disease: assessment and management (NICE guideline NG145) · NICE, via NCBI Bookshelf
Subclinical hypothyroidism in the infertile female population: a guideline · American Society for Reproductive Medicine, Fertility and Sterility, 2024
Recurrent pregnancy loss: a committee opinion · American Society for Reproductive Medicine, Fertility and Sterility, 2026
Euthyroid sick syndrome · Merck Manual Professional Version
TSH (thyroid-stimulating hormone) test · MedlinePlus / NIH National Library of Medicine
Epidemiology, types, causes, clinical presentation, diagnosis and treatment of hypothyroidism · Cureus, 2023