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TSH blood test

Thyroid-stimulating hormone

In short

A typical adult TSH result is 0.3 to 4.2 mIU/L. TSH works backwards: a high TSH usually goes with an underactive (slow) thyroid. Small rises are common and often settle, so the test is often repeated first. Always read your result against the range printed on your own report.

A brain hormone that tells the thyroid how hard to work.

TSH is usually the first thyroid test doctors check. It comes from the pituitary, a small gland at the base of the brain, and rises when thyroid hormone runs low. When thyroid hormone runs high, TSH falls, so it moves the opposite way to the thyroid.

Is my TSH high or low?

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Typical range

ForRangeUnit
Adults0.3–4.2mIU/L

Typical ranges for adults. Labs differ a little, so the range printed next to your result is the one to go by.

If your TSH is high

TSH works backwards: a high TSH usually goes with an underactive (slow) thyroid. Small rises are common and often settle, so the test is often repeated first.

Common reasons

  • Recovery from a recent illness, when TSH can rise for a while
  • Being over 80, when TSH tends to sit a little higher
  • Hashimoto's, a common immune condition that gradually slows the thyroid
  • For people on thyroid medicine, a dose below what the body needs
  • Some medicines, including some for heart rhythm or mood
  • Less often: a rare problem in the pituitary gland itself

If your TSH is low

TSH works backwards: a low TSH usually goes with an overactive (fast) thyroid. A slightly low result often settles on a repeat test, especially after an illness.

Common reasons

  • For people on thyroid medicine, a dose above what the body needs
  • A recent serious illness or high-dose steroid medicines, which lower TSH for a while
  • Early pregnancy, when TSH often dips a little
  • Graves' disease, an immune condition that makes the thyroid overactive
  • Thyroid nodules, small lumps that make extra hormone on their own
  • Thyroiditis, an inflamed thyroid that leaks stored hormone for a few weeks
  • Less often: a pituitary problem, when free T4 is low as well

What can throw it off

Biotin, a hair-and-nail supplement, can make TSH look falsely low on some lab tests, and a recent illness can shift it for a while.

Questions to take to your appointment

  1. 1Is this small enough to just repeat the test?
  2. 2What does my free T4 show alongside it?
  3. 3Could my medicines, supplements or a recent illness explain it?

Read together with

Doctors rarely read TSH alone. These usually sit next to it.

Quick answers

What is the TSH blood test?

TSH (Thyroid-stimulating hormone): A brain hormone that tells the thyroid how hard to work. TSH is usually the first thyroid test doctors check. It comes from the pituitary, a small gland at the base of the brain, and rises when thyroid hormone runs low. When thyroid hormone runs high, TSH falls, so it moves the opposite way to the thyroid.

What is a normal TSH level?

For most adults, TSH is typically 0.3 to 4.2 mIU/L. Labs differ a little, so the range printed next to your result is the one to go by.

What does a high TSH mean?

TSH works backwards: a high TSH usually goes with an underactive (slow) thyroid. Small rises are common and often settle, so the test is often repeated first. Common reasons include recovery from a recent illness, when TSH can rise for a while; being over 80, when TSH tends to sit a little higher; Hashimoto's, a common immune condition that gradually slows the thyroid; and for people on thyroid medicine, a dose below what the body needs. A doctor reads it together with your other results and how you feel.

What does a low TSH mean?

TSH works backwards: a low TSH usually goes with an overactive (fast) thyroid. A slightly low result often settles on a repeat test, especially after an illness. Common reasons include for people on thyroid medicine, a dose above what the body needs; a recent serious illness or high-dose steroid medicines, which lower TSH for a while; early pregnancy, when TSH often dips a little; and Graves' disease, an immune condition that makes the thyroid overactive.

What can affect my TSH result?

Biotin, a hair-and-nail supplement, can make TSH look falsely low on some lab tests, and a recent illness can shift it for a while.

What should I ask my doctor about my TSH?

Questions worth taking to the appointment: Is this small enough to just repeat the test? What does my free T4 show alongside it? Could my medicines, supplements or a recent illness explain it?

What this page can’t do

It reads each number on its own.

Doctors read results together — ALT with AST, ferritin with hemoglobin, TSH with free T4. A pattern says more than any single flag.

It doesn't know you.

Your age, your medicines, how you feel and your last result all change what a number means.

It doesn't remember.

Close this page and your results are gone. That's on purpose: nothing was ever sent to us.

We also make Vita: Health Coach, an app for those parts. It reads your whole report with your medicines and history in mind, answers follow-up questions in plain words, and keeps your reports in one place — for you and for the people you look after.

Vita: Health Coach on the App Store

Sources

Stuck on a word? See the medical words. How we choose our sources. More about Vita Health Coach.