Thyroid function tests and scans: all you need to know

29th April, 2022 • 9 min read

Feeling tired? Noticed changes in your weight? Feeling more anxious and wound up than usual? Or sluggish and low? If you’re talking to your doctor about symptoms like this, they might want to test you to check for thyroid problems, such as an underactive or overactive thyroid.

More women than men tend to get a thyroid condition, with some estimates suggesting about 1 in 8 women will be affected during their lifetime. It can lead to a variety of issues, including period problems, trouble getting pregnant and difficulties during pregnancy. And if it comes on around the menopause, it can mean a double whammy of symptoms.

So what tests can your doctor use to find out if your thyroid is causing your symptoms? Read on to learn what to expect.

Why your thyroid is being investigated

Your thyroid gland is a butterfly-shaped gland that sits in the front of your neck, near your Adam’s apple. Its job is to make 2 hormones – thyroxine (T4) and triiodothyronine (T3).

A hormone called thyroid stimulating hormone (TSH), made in the pituitary gland in your brain, tells your thyroid gland how much T4 and T3 to make.

Thyroid hormones help you to use energy and stay warm by controlling your body’s metabolism. They also make sure your organs work properly. So if you have too much or too little of them, it can affect you in lots of different ways.

If your doctor thinks a thyroid issue could be causing your symptoms, they’ll do tests to check your levels of these hormones, and other tests if needed.

Thyroid blood tests

What happens: your doctor, nurse or another healthcare professional will draw some blood from you and send it to the local hospital laboratory for testing.

Blood tests
to check thyroid function usually measure levels of TSH, T4 and sometimes T3.

In most cases, TSH is measured first, because if this is normal it’s unlikely that you have a thyroid problem. Many labs have a system where they then measure your thyroid hormones if your TSH level is found to be abnormal:

  • if your TSH is above normal, T4 is measured
  • if your TSH is below normal, T4 and T3 are measured

This means you only need to give 1 sample of blood, instead of going back for follow-up tests. The results are usually available within a week.

TSH test

If you’re found to have abnormal levels of TSH, you’ll need at least 1 other test to find out what’s causing this. Here’s what your TSH levels mean:

  • high levels of TSH usually mean your thyroid gland isn’t making enough thyroid hormone. This is known as
    underactive thyroid (hypothyroidism)
  • low levels of TSH usually mean your thyroid gland is making too much thyroid hormone, which is called
    overactive thyroid (hyperthyroidism)
  • your level of TSH can also act as an early warning sign that your thyroid hormone levels are changing, but haven’t become too high or low yet

T4 test

T4 is the main thyroid hormone in your blood. Here’s what your T4 levels mean:

low levels of T4 with high TSH suggest you have an underactive thyroid that needs treatment (read about

underactive thyroid treatment
high levels of T4 with low TSH suggest you have an overactive thyroid that needs treatment (read about
overactive thyroid treatment
normal levels of T4 with slightly high TSH suggest mild thyroid failure, known as subclinical hypothyroidism. This may slowly develop into an underactive thyroid over many years. You might need other tests to look for the cause, such as antibody tests (see below)

Very occasionally, you can get other combinations of results. For example, low T4 with low TSH can be caused by your pituitary gland not making enough TSH. High T4 with high TSH could mean you have a tumour in your pituitary gland.

T3 test

T3 is measured less often than TSH and T4. It isn’t as helpful for checking for underactive thyroid, as it’s usually the last hormone to become abnormal. It might be measured if your doctor:

  • thinks you may have an overactive thyroid, even though your T4 level is normal
  • wants to find out how severe your overactive thyroid is

Antibody tests

Some thyroid problems happen because your immune system mistakenly makes thyroid antibodies to attack your thyroid gland.

This can cause an ‘autoimmune’ thyroid disorder, such as Graves’ disease – the most common cause of overactive thyroid – or Hashimoto’s

– the most common cause of underactive thyroid.

Here’s what you need to know about antibody tests:

  • for overactive thyroid, TSH-receptor antibodies, that can stimulate TSH production, are often measured
  • for underactive thyroid, thyroid peroxidase antibody and thyroglobulin antibody may be measured
  • for subclinical hypothyroidism, antibodies will sometimes be checked to help give an idea of how likely you are to develop an underactive thyroid and how often you should have blood tests
  • antibody tests are less helpful once you have a diagnosis – your TSH and T4 levels will show how your thyroid gland has been affected by the autoimmune problem, and how it’s responding to treatment

Doctor’s tip

Dr Roger Henderson says:

A thyroid function blood test doesn’t need much preparation, but it can be helpful to know that:

  • you don’t need to fast beforehand
  • you should take any thyroid medication as normal
  • tell your doctor if you’ve recently had an X-ray where a contrast dye was used as this may affect your results
  • as levels of thyroid hormones can change during pregnancy, tell your doctor if you’re pregnant
  • some medicines can affect how well your body absorbs thyroid medication, including
    non-steroidal anti-inflammatory drugs (NSAIDs)
    , so make sure your doctor knows about any medication you’re taking
  • biotin can affect thyroid function tests, so don’t take supplements containing biotin for 2 days beforehand

Find useful information on other areas of thyroid health with our

complete Guide

Imaging tests for thyroid problems

Your doctor may order 1 or more of these

tests to help find the cause of your thyroid problem.

Ultrasound scan

What happens: in a thyroid

ultrasound scan
, a specialist runs a small probe over your neck, which uses high-frequency sound waves to create a detailed image of your thyroid. It’s a painless and simple test, which usually takes 20 to 30 minutes.

What you need to know:

  • it’s commonly used to check for
    thyroid nodules
    , or to examine them
  • it can help check if any thyroid nodules are more likely to become cancerous

Radioactive iodine uptake (RAIU) test

What happens: also known as a thyroid uptake test, a RAIU test checks how well your thyroid is working, or helps find the cause of overactivity.

You swallow radioactive iodine in liquid or capsule form. A specialist then measures the amount of radioactivity your thyroid gland absorbs, which shows how well your thyroid gland is working.

Measurements are usually taken 4 to 6 hours after you swallow the radioactive iodine, and again after 24 hours. The test usually only takes a few minutes.

What you need to know:

  • a very high RAIU means you have an overactive thyroid
  • a very low RAIU means you have an underactive thyroid
  • food high in iodine can stop your thyroid gland absorbing the radioactive iodine. You should avoid foods such as seafood, fish and raw green vegetables, and limit cakes or biscuits made with eggs and butter, for a week before the test
  • the dose of radiation is very small and there are no side effects, but you shouldn't have this test if you’re
    or breastfeeding

Thyroid scan

What happens: a thyroid scan is similar to a RAIU test, but involves creating images of your thyroid gland, which show its size, shape and position, and which parts absorb the radioactive iodine.

You’re given a small injection of a radioactive iodine, or a liquid or capsule to swallow. You then have a scan, where a special camera takes pictures of your thyroid, either 30 minutes after the injection or up to 24 hours after the liquid or capsule. The scan usually takes less than 30 minutes.

What you need to know:

  • if the radioactive iodine shows up over all your thyroid, this suggests you have Graves’ disease
  • certain types of thyroid nodule (which make thyroid hormone) will also show up on this test
  • as with a RAIU test, you should avoid foods high in iodine in the week before the scan, and avoid having this test if you’re pregnant or breastfeeding

Fine-needle aspiration biopsy

What happens: if your doctor or the tests above find a lump or nodule in your thyroid gland, you may have a fine-needle aspiration

to check for abnormal cells (such as cancer).

Your doctor or specialist will numb an area on your neck with

local anaesthetic
, then insert a very thin needle into your thyroid gland to remove some cells and fluid. They’ll use an ultrasound to guide the needle to the right area. The test usually takes less than 30 minutes.

Another specialist will then look at these cells under a microscope to see if there are any abnormalities. The results are sent to your doctor, usually within a week.

What you need to know:

  • if the lump or nodule is non-cancerous (benign), your doctor will talk to you about what treatment is needed, if any
  • thyroid cancer is rare, but if it’s found, you’ll be referred to a cancer specialist to discuss treatment, which usually involves surgery. Read more about thyroid cancer treatment

When to see a doctor

You should make an appointment with your doctor if you think you might have a thyroid problem, so they can arrange the above tests if necessary.

Symptoms of thyroid problems can include:

  • a swelling at the front of your neck
  • weight changes that have no obvious cause
  • feeling either too hot or too cold all the time
  • mood changes for no reason
  • feeling tired all the time
  • irregular or missing periods
  • a fast or irregular heart beat (palpitations) – get emergency medical help if you also feel dizzy, faint or have chest pain

Read more about symptoms of an


If you’re not sure if you need to see a doctor, use our

Smart Symptom Checker
to get more information and help you decide your best next step.

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.