TSH blood test: what it measures and what your result means

Medically reviewed by Dr Lebeth Jacob · 21 October 2026

Quick answer: TSH (thyroid-stimulating hormone) is made by the pituitary gland in your brain. It tells your thyroid how much thyroid hormone to make. A high TSH usually means your thyroid is underactive. A low TSH usually means it is overactive, or that a thyroid medicine dose is too high. TSH is usually tested with free T4 to give the full picture.

What is TSH?

Your thyroid is a gland in your neck. It makes the hormones T4 and T3, which control how fast your body uses energy. The pituitary gland keeps check: when thyroid hormone levels drop, it releases more TSH to push the thyroid harder, and when they're high, it releases less.

So TSH works in reverse. A high TSH means the body is asking for more thyroid hormone. A low TSH means there's already plenty, or too much.

Why is TSH tested?

  • You feel tired, cold, low or have gained weight without a clear reason
  • You feel anxious or shaky, have a racing heart or have lost weight without trying
  • Your periods have changed, or you're trying to get pregnant
  • You have a family history of thyroid problems, or another autoimmune condition such as type 1 diabetes
  • You take levothyroxine and need to check your dose

What does a high TSH result mean?

A high TSH usually means your thyroid isn't making enough hormone (an underactive thyroid, or hypothyroidism). Common reasons are:

  • Hashimoto's thyroiditis, where the immune system attacks the thyroid. This is the most common cause in the UK.
  • A levothyroxine dose that's too low, or not being taken regularly
  • Recovering from a recent illness
  • Some medicines, such as amiodarone and lithium
  • Previous thyroid surgery or radioactive iodine treatment

If TSH is raised but free T4 is normal, it's called subclinical hypothyroidism. A GP will often repeat the test after a few months before deciding on treatment. Thyroid antibody tests can show whether an autoimmune cause is likely.

What does a low TSH result mean?

A low TSH usually means there's too much thyroid hormone (an overactive thyroid, or hyperthyroidism). Common reasons are:

  • Graves' disease, an autoimmune condition
  • Lumps (nodules) in the thyroid that make extra hormone
  • A levothyroxine dose that's too high
  • Inflammation of the thyroid (thyroiditis), which can be temporary
  • Early pregnancy, when a slightly low TSH can be normal

What can affect your result?

  • Biotin (in many hair, skin and nail supplements) can interfere with thyroid tests. Stop taking it for at least two days before your test, unless a doctor has prescribed it.
  • TSH is higher in the early morning. Try to test at the same time of day each time.
  • If you take levothyroxine, have your blood taken before that day's dose
  • A recent illness, pregnancy and some medicines, such as steroids, can change TSH

When to see a GP

Your results report shows the laboratory's normal range and flags any result outside it. Speak to your GP if:

  • Your TSH is flagged high or low
  • You have symptoms of an underactive or overactive thyroid
  • You notice a lump or swelling in your neck
  • You're pregnant or planning a pregnancy and have a thyroid condition (speak to them promptly)

Call 999 or go to A&E if you have a very fast or irregular heartbeat with chest pain, a high temperature, confusion or severe agitation.

How we test TSH

You can have TSH tested with a blood sample taken at one of our 250+ partner clinics, or with a finger-prick kit at home. Samples are analysed by a UKAS-accredited laboratory. Results are usually ready in 3–5 working days, with a doctor's comments on every result.

Tests that include TSH

Prices for in-clinic tests include your appointment. Compare every test with TSH in the biomarker explorer

Frequently asked questions

What is a dangerously high TSH level?

There is no single dangerous number. The higher the TSH, the more underactive the thyroid is likely to be, and the more likely treatment is needed. A result flagged high should be discussed with your GP, and promptly if you're pregnant or feel very unwell.

What does a low TSH mean in a woman?

The same as in a man: usually an overactive thyroid or too high a levothyroxine dose. In early pregnancy a slightly low TSH can be normal. Your GP will look at your free T4 and symptoms too.

Does biotin affect a thyroid blood test?

Yes. Biotin can make results look higher or lower than they really are. Stop biotin supplements for at least two days before testing, unless they're prescribed.

Does a routine blood test show thyroid problems?

Only if it includes thyroid tests. A full blood count doesn't check your thyroid. You need TSH, and ideally free T4.

Can a thyroid blood test show cancer?

No. Most people with thyroid cancer have normal thyroid blood tests. A lump in the neck needs to be examined by a doctor, who may arrange an ultrasound scan.

My TSH is normal but I still have symptoms. What now?

Tiredness and weight changes have many causes, such as low iron, vitamin D or B12, or poor sleep. Speak to your GP about your symptoms. A wider blood test can help rule other causes in or out.