TSH test: normal range and what a high or low result means

A TSH test measures thyroid-stimulating hormone, the signal your pituitary gland sends to your thyroid. It is usually the opening test when thyroid function is in question, because TSH tends to move early and by a lot when thyroid hormone levels change. A high TSH usually points to an underactive thyroid and a low TSH to an overactive one, but the result is read with free T4, and often free T3, before anyone draws a conclusion.

What TSH measures

The pituitary gland, at the base of the brain, releases TSH to tell the thyroid how much hormone to make. When thyroid hormone in the blood falls, the pituitary releases more TSH; when it rises, the pituitary releases less. The relationship is steep: a small change in free T4 produces a large change in TSH, which is why TSH often shifts before the thyroid hormones leave their own reference ranges.

This works when the pituitary itself is healthy. If the problem lies in the pituitary or the hypothalamus above it, TSH can look normal or low even when the thyroid is underactive, and free T4 becomes the more useful number.

Normal range

TSH is reported in mIU/L, sometimes written mU/L or µIU/mL, which give the same number. Published adult ranges from three reference laboratories:

Ranges vary with the laboratory and its assay, with age and sex, and in pregnancy, so read your result against the range printed on your own report. TSH runs higher with age: in a large US survey of people without thyroid disease, the 97.5th centile was 3.56 mIU/L at ages 20 to 29 and 7.49 mIU/L over 80. The same survey found TSH slightly higher on average in women than in men. In pregnancy, laboratories use separate ranges for each trimester, and children have their own ranges too.

What a high TSH usually points to

A TSH above the range usually means the thyroid is not making enough hormone, so the pituitary is pushing harder. What a doctor typically does next depends on free T4.

• High TSH with low free T4 is the usual pattern of an underactive thyroid (primary hypothyroidism), most often from autoimmune thyroiditis, also called Hashimoto's disease.
• High TSH with normal free T4 is the pattern called subclinical hypothyroidism. Doctors usually confirm that TSH stays raised on a repeat test and may check thyroid peroxidase antibodies, because positive antibodies point to a higher chance of the thyroid becoming underactive over time.

A raised TSH does not always mean thyroid disease: illness, age and assay interference can all raise it, as covered below. In people who take thyroid hormone medicine, TSH is the number used to check the dose.

What a low TSH usually points to

• Low TSH with high free T4 or free T3 is the usual pattern of an overactive thyroid (hyperthyroidism), commonly Graves' disease or an overactive nodular thyroid.
• Low TSH with normal free T4 and free T3 is the pattern called subclinical hyperthyroidism. It is usually repeated, and NICE suggests specialist advice when TSH stays below 0.1 mIU/L on two readings at least 3 months apart with signs of thyroid disease.
• Low TSH with low free T4 is uncommon. It can point to the pituitary gland, or to the effect of a recent serious illness.

TSH can also be low without thyroid disease: during serious illness, in the early months of pregnancy, and when the dose of thyroid hormone medicine is higher than needed.

What moves TSH

• Time of day. TSH peaks at night and is lowest between about 10 a.m. and 4 p.m. Mayo Clinic Laboratories notes that this daily swing does not usually change how a daytime result is read.
• Illness. An acute illness can shift TSH for a while, which is why NICE advises against testing during one unless the thyroid is the suspected cause.
• Medicines. Amiodarone and lithium can change thyroid function, and thyroid hormone medicine moves TSH directly.
• Biotin. High-dose biotin, common in hair and nail supplements, can make some TSH assays read falsely low. In one study, 10 mg a day for a week lowered TSH on two widely used assays, on one of them to below the reference range. Newer versions of some assays are built to tolerate biotin, so the effect depends on the laboratory's method.
• Assay interference. Rarely, antibodies in a person's blood interfere with the assay and give a falsely high or low TSH, so a result that does not fit the rest of the picture is worth querying with the laboratory.

Preparing for the test

A TSH test needs no fasting on its own. If other tests in the same draw need fasting, follow those instructions. Tell the person ordering the test about every medicine and supplement you take, especially thyroid medicine and biotin, and do not stop a prescribed medicine unless your doctor tells you to. NICE asks doctors to check biotin intake when thyroid problems are suspected, and some laboratories and the American Thyroid Association ask for a gap between the last dose and the sample.

How Clare's panel handles it

In Clare's panel, Thyroid-Stimulating Hormone (TSH) is measured by the lab, together with Free T4 (FT4) and Free T3 (FT3) in the same blood draw. Having all three at once means a TSH near the edge of its range can be read against the hormones it controls.

Why it matters in the UAE

For people who take levothyroxine and fast in Ramadan, TSH often rises in the weeks after the month ends. In 481 patients at a UAE endocrine centre, median TSH went from 2.0 mIU/L before Ramadan to 2.9 mIU/L one to two weeks after, and was back in the normal range 3 to 6 months later. A second UAE study, of 112 patients, found a similar rise. A TSH taken soon after Ramadan is read with that timing in mind.

Price in the UAE

When we read public prices on 11 October 2026, a TSH test on its own cost from 35 to 90 at 2 UAE providers. TSH is also sold within a thyroid function test with free T4 and free T3, which cost from 99 to 300 at 6 providers. VAT was rarely stated, some prices were offers, and home collection can add a fee.

Related tests

TSH is read with Free T4 and Free T3, and the three together make up the thyroid test. For how we read them as a set, read How we read a thyroid panel: TSH, free T4 and free T3.

Why the number means more over time

Each person's thyroid hormone levels stay close to their own set point, while population reference ranges are broad. So a TSH that has drifted upward over several tests can say more than one value inside the range. NICE suggests repeating thyroid tests no sooner than 6 weeks after the last one when symptoms change, and for adults with untreated subclinical hypothyroidism, checking TSH and free T4 once a year if there are signs of underlying thyroid disease, or every 2 to 3 years if not. Clare is opening with a waitlist at clare.health.