TSH Levels Checker & Chart

TSH levels at a glance

TSH (thyroid-stimulating hormone) is made by the pituitary gland at the base of the brain. It acts like a thermostat signal for the thyroid: when thyroid hormone levels fall, the pituitary releases more TSH to push the thyroid harder; when thyroid hormone levels are high, TSH falls. Because the pituitary is extremely sensitive to small changes in thyroid hormone, TSH is the single best first test of thyroid function for most people, and it is usually the first number doctors look at.

Key points about TSH levels:

• A typical adult reference interval is about 0.4 to 4.0 mIU/L (milli-international units per litre). The NBME reference values used in US medical exams list 0.4–4.0 µU/mL. Some laboratories set the upper limit at 4.5 or even 5.0 mIU/L, depending on their method and population.
• mIU/L and µIU/mL are the same number: 1 µIU/mL = 1 mIU/L. A result of 2.1 µIU/mL is 2.1 mIU/L.
• High TSH usually means the thyroid is underactive (hypothyroidism) – the pituitary is shouting at a sluggish gland.
• Low TSH usually means the thyroid is overactive (hyperthyroidism), or that someone is taking more thyroid hormone than they need.
• TSH normally rises with age. A TSH of 6 mIU/L can be unremarkable in an 85-year-old and significant in a 30-year-old.
• Pregnancy changes TSH, especially in the first trimester.

The checker above places your TSH in context for adults, older adults and pregnancy, and, if you add a free T4 result, describes the pattern the two make together. It does not diagnose thyroid disease; it helps you understand your report before talking to your clinician.

TSH levels chart

These bands are a general guide. Your laboratory's own reference interval, printed next to your result, is the one to use for your report.

Normal TSH levels by age

TSH reference intervals were originally built from healthy adults of working age. A large analysis of the US National Health and Nutrition Examination Survey (NHANES III) by Surks and Hollowell showed that, in people with no thyroid disease and no thyroid antibodies, TSH distribution shifts upwards with every decade of age. The 97.5th centile – the value only 2.5% of thyroid-healthy people exceed – was about 3.6 mIU/L at age 20–29 but about 7.5 mIU/L at age 80 and over.

What this means in practice:

• Older adults with a mildly raised TSH and a normal free T4 often have no thyroid disease at all; their pituitary simply runs at a slightly higher set point. European guidelines on subclinical hypothyroidism discuss age-specific interpretation for exactly this reason.
• Younger adults with a TSH at the top of the standard range may deserve a repeat test, particularly if they have symptoms or a family history.
• Children have their own reference intervals, and newborns have very high TSH for the first few days of life. Use your child's laboratory range, not adult charts.

TSH levels in pregnancy

During pregnancy, the hormone hCG – the same hormone pregnancy tests detect – has a weak TSH-like effect on the thyroid. hCG peaks towards the end of the first trimester, and as it stimulates the thyroid, TSH falls. Many healthy pregnant people have a first-trimester TSH below the usual non-pregnant lower limit.

The 2017 American Thyroid Association pregnancy guideline recommends that laboratories use population- and trimester-specific reference ranges where possible. When those are not available, it suggests an upper limit of about 4.0 mIU/L in early pregnancy, with a lower limit of roughly 0.1 mIU/L in the first trimester.

Thyroid function matters for both parent and baby in pregnancy, and thresholds for further assessment are lower. If you are pregnant or planning pregnancy and your TSH is outside the range on your report, contact your maternity team or clinician promptly rather than waiting for a routine appointment.

Reading TSH with free T4

TSH tells you how hard the pituitary is pushing; free T4 (FT4) tells you how much active thyroid hormone is actually circulating. Reading them together is how thyroid problems are classified:

A typical adult free T4 range is about 0.9–1.7 ng/dL (roughly 12–22 pmol/L), though ranges vary by assay. To convert, multiply ng/dL by 12.87 to get pmol/L; the free T4 converter does it in both directions. When TSH is low, a free T3 is often added, because some overactive thyroids make mainly T3 – see the free T3 converter.

What a high TSH can mean

A raised TSH most often reflects primary hypothyroidism, in which the thyroid gland itself is not making enough hormone. In iodine-sufficient countries such as the US and UK, the commonest cause is Hashimoto's (autoimmune) thyroiditis, which is usually confirmed with thyroid peroxidase (TPO) antibodies. Other causes include previous thyroid surgery or radioactive iodine, some medicines (for example lithium and amiodarone), and iodine deficiency in some regions.

Not every high TSH is thyroid disease:

• Recovery from a significant illness can cause a temporary rise in TSH as the pituitary resets.
• Age, as discussed above.
• Obesity is associated with slightly higher TSH values.
• Assay interference from certain antibodies is uncommon but possible.
• Missed doses or timing of thyroid medicine, or taking it with calcium or iron, can raise TSH in people already on treatment.

Symptoms of an underactive thyroid can include tiredness, feeling cold, weight gain, constipation, dry skin, hair thinning, low mood and heavier periods – but these are non-specific and very common in people with normal thyroid function too.

Subclinical hypothyroidism

"Subclinical" means TSH is raised while free T4 is still in the normal range. It is common, particularly in women and older adults. Guidelines from the American Association of Clinical Endocrinologists/ATA and the European Thyroid Association advise repeating TSH with free T4 after a couple of months before drawing conclusions, because a substantial proportion of mildly raised results return to normal on their own. Guidelines distinguish between TSH values below 10 mIU/L and 10 mIU/L or above, and consider age, symptoms, antibodies and pregnancy plans. Decisions about treatment belong to you and your clinician.

What a low TSH can mean

A low or suppressed TSH most often reflects too much thyroid hormone, from an overactive gland or from medication. Causes include:

• Graves' disease, an autoimmune condition that stimulates the thyroid.
• Toxic multinodular goitre or a single toxic nodule.
• Thyroiditis – inflammation (after a viral illness, after childbirth, or painless) that releases stored hormone temporarily.
• Excess thyroid hormone medicine.
• First-trimester pregnancy (hCG effect), and hyperemesis gravidarum.
• Medicines such as high-dose glucocorticoids and dopamine, and severe non-thyroidal illness, which can lower TSH without thyroid disease.

Symptoms of an overactive thyroid can include palpitations, a fast heart rate, weight loss despite a good appetite, tremor, heat intolerance, anxiety and loose stools. A persistently low TSH matters even without symptoms, particularly in older adults, because it is associated with atrial fibrillation and loss of bone density. Seek prompt advice if a suppressed TSH comes with chest pain, palpitations or breathlessness.

How the TSH test is done

TSH is measured in a standard blood sample taken from a vein in the arm, usually as part of a "thyroid function test" panel that may also include free T4, free T3 and sometimes thyroid antibodies. Modern TSH tests are called third-generation assays because they can measure very low concentrations – down to about 0.01 mIU/L – which is what makes it possible to tell a mildly low TSH from a fully suppressed one.

Because TSH is released in pulses and follows a daily rhythm – higher overnight and in the early morning, lower in the afternoon – a result taken at 8 a.m. can be noticeably higher than one taken at 4 p.m. on the same day. The difference is usually small compared with the gap between normal and abnormal results, but it matters when you are comparing borderline values. Having repeat tests at a similar time of day makes trends easier to read.

Why TSH changes before T4

The relationship between TSH and free T4 is log-linear: a small, two-fold change in free T4 produces a roughly one-hundred-fold change in TSH. This amplification is why TSH becomes abnormal early – often while free T4 is still inside its reference range – and why "subclinical" thyroid conditions are defined by an abnormal TSH with a normal free T4. It is also why TSH alone can be misleading when the pituitary itself is not working normally, for example after pituitary surgery, in which case clinicians rely on free T4 instead.

TSH and thyroid antibodies

When TSH is raised, a test for thyroid peroxidase (TPO) antibodies helps show whether autoimmune thyroiditis is the likely cause. In the NHANES III data, TPO antibodies were found in a substantial proportion of people with higher TSH values and were more common in women and with increasing age. A raised TSH with positive TPO antibodies is more likely to progress to overt hypothyroidism over the following years than a raised TSH with negative antibodies. When TSH is low, TSH-receptor antibodies (TRAb) help identify Graves' disease. Antibody tests do not change the TSH number itself, but they change what the number most likely means.

TSH when you already take thyroid medicine

For people taking thyroid hormone replacement, TSH becomes a monitoring test. Clinicians usually check it several weeks after any change, because TSH takes time to settle into a new steady state, and then less often once things are stable. Targets are set individually and can differ – for example in pregnancy, in older adults, and after treatment for thyroid cancer, where a deliberately lower TSH may be intended. This page does not give dosing advice; if your TSH on treatment is outside the target you agreed with your clinician, the next step is a conversation with them.

Factors that can affect a TSH result

Units and conversions

TSH is almost always reported in mIU/L or µIU/mL, which are numerically identical. You may also see "mU/L" – the same unit again. There is no molar conversion for TSH because it is a glycoprotein measured against an international reference standard. Free T4 and free T3 have different units in different countries:

For a broader view of thyroid tools, see the thyroid calculators hub.

When to see a doctor

• Your TSH is below 0.1 or above 10 mIU/L, or outside your lab's range with symptoms.
• You are pregnant, trying to conceive or recently gave birth and your TSH is outside the range given on your report.
• You have palpitations, chest pain, marked weight loss or a swelling in the neck.
• You take thyroid medicine and your TSH has moved outside the target your clinician set.

A single slightly abnormal TSH in someone who feels well is usually repeated rather than acted on immediately.