What the thyroid blood test checks, how to read your report and how to prepare
Published: 16 October 2026 | Updated: 16 October 2026 | By MCT Clinic and Diagnostic Centre, a unit of Millat Charitable Trust
A "thyroid test" usually means a blood test for TSH, often together with T4 and sometimes T3. Patients often come to us with a report in hand and one question: is my value normal? This guide explains what each part of the test measures, the typical ranges used by laboratories, why your own report may show a slightly different range, and how to prepare so that the result is reliable.
The information below comes from MedlinePlus (U.S. National Library of Medicine), the U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and the NHS. Links are listed at the end of the page.
The thyroid is a small, butterfly-shaped gland at the front of the neck. It makes two hormones, thyroxine (T4) and triiodothyronine (T3), which control how the body uses energy. Because of this, they affect almost every organ, including the heart, digestion, weight, mood and periods.
The thyroid is controlled by the pituitary gland in the brain. The pituitary releases thyroid stimulating hormone (TSH), which tells the thyroid how much hormone to make. When thyroid hormone in the blood is low, the pituitary releases more TSH; when it is high, the pituitary releases little or none. This feedback loop is why TSH is usually the first and most useful number to check.
At our centre the thyroid profile and TSH are listed under Thyroid Test and Hormonal Tests in the tests catalogue.
The MedlinePlus Medical Encyclopedia gives these typical adult ranges:
| Test | Typical normal range (adults) | Notes |
|---|---|---|
| TSH | 0.4 to 4.8 microunits per mL (µU/mL, same as mIU/L) | Some labs use a higher upper limit for older people, as high as 7. Different in pregnancy. |
| Free T4 | 0.8 to 1.9 ng/dL (10.3 to 24.5 pmol/L) | Free T4 is less affected than total T4 by pregnancy, birth control pills and steroids. |
Three points matter more than the exact numbers:
High TSH is most often caused by an underactive thyroid. MedlinePlus lists symptoms such as tiredness, weight gain, feeling cold, dry skin, thinning hair, constipation, low mood, joint and muscle pain, and heavy or irregular periods. Hypothyroidism develops slowly, so many people do not notice it for months or years.
Low TSH usually means an overactive thyroid. Symptoms can include weight loss despite eating more, a fast or irregular heartbeat, nervousness, poor sleep, shaky hands, sweating and frequent bowel movements. Too much thyroid medicine, some other medicines and a pituitary problem can also lower TSH.
NIDDK advises that when TSH is abnormal, at least one more test is needed to find the cause. TSH can also be temporarily out of range during a serious illness that has nothing to do with the thyroid, and it may be higher in people over 80 without any thyroid disease.
According to MedlinePlus, a TSH test on its own needs no special preparation. Fasting is only needed if your doctor has ordered other tests on the same sample, such as fasting blood sugar or a lipid profile. In that case, follow the longest fasting time; our guide on how to prepare for a fasting blood test explains the usual times.
Pregnancy changes thyroid hormone levels. TSH is often slightly low in the first three months, and doctors use pregnancy-specific targets. A doctor may advise checking TSH when a woman is planning a pregnancy, and women with a history of thyroid disease should mention it early. Our gynaecologist can advise during antenatal visits.
Newborn babies are screened with a TSH test to detect congenital hypothyroidism.
People who take levothyroxine for an underactive thyroid have regular TSH tests to check that the dose is right. The NHS notes that being on the wrong dose can cause side effects, so keep your follow-up tests even when you feel well, and bring your previous reports to each visit so the trend can be seen.
Thyroid symptoms overlap with many other conditions, including anaemia and diabetes. A doctor may therefore order a CBC test or a HbA1c sugar test with your thyroid profile. Our physician can review your results; see the list of our doctors.
The MedlinePlus Medical Encyclopedia gives 0.4 to 4.8 microunits per millilitre as a typical normal range for adults. Ranges differ slightly between labs, some labs use a higher upper limit for older people, and the range is different in pregnancy, so always read your result against the range printed on your own report.
A TSH test on its own needs no special preparation according to MedlinePlus. If your doctor has ordered other tests on the same sample, such as fasting sugar or a lipid profile, you may need to fast for those. TSH varies during the day, so an early morning sample is preferred.
A high TSH most often means the thyroid is underactive (hypothyroidism), because the pituitary gland releases more TSH to push the thyroid to work harder. Your doctor will usually check T4 and sometimes thyroid antibodies to confirm the cause.
Yes. MedlinePlus notes that biotin (vitamin B7) can affect TSH and free T4 results. Tell your doctor if you take biotin or a hair, skin and nail supplement that contains it before you have thyroid tests.
Yes. Pregnancy changes TSH, which is often a little low in the first three months. Doctors use pregnancy-specific targets, so a pregnant woman should discuss her result with her gynaecologist rather than compare it with the general adult range.
For information only, not medical advice. This page does not replace advice from your doctor.