Your thyroid gland produces hormones that help control your metabolism. If you don’t make enough of these hormones it’s called hypothyroidism (underactive thyroid). If you make too much of these hormones it’s called hyperthyroidism (overactive thyroid). Thyroid disorders are sometimes caused by an autoimmune disease. If your thyroid isn’t working properly it can cause tiredness, weight gain or loss, and mood problems. A blood test can check for a thyroid disorder and autoimmune thyroid disease.


What is a thyroid function test?

A thyroid function test is a blood test that will check how your thyroid is working. It does this by measuring your thyroid hormones.

What does your thyroid gland do?

Your thyroid gland is a gland found at the front of your neck, just below your Adam’s apple. Your thyroid gland produces hormones that play a key role in regulating your metabolism and affects things like your heart rate, body temperature, and breathing. These hormones are:

  • thyroxine (T4)
  • triiodothyronine (T3)

The levels of these hormones are controlled by thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH).

Signs you should do a thyroid function test

If any of these hormones are out of balance it can lead to a thyroid disorder and cause a wide range of symptoms. If you’re experiencing any of these symptoms it’s really important to check your thyroid function. It’s also common for thyroid disorders to run in families so it’s particularly important to test your thyroid function if you think you might be at risk.

Thyroid disorders and symptoms

Underactive thyroid (hypothyroidism)

If you don’t produce enough thyroid hormones your bodily processes start to “slow down”. Signs of an underactive thyroid include:

  • tiredness
  • weight gain
  • depression
  • constipation
  • dry skin
  • brittle hair
  • sensitivity to cold
  • memory problems
  • muscle aches
  • hoarse voice

Overactive thyroid (hyperthyroidism)
If you produce too much thyroid hormones it over-stimulates your metabolic functions. Signs of an overactive thyroid include:

  • weight loss
  • increased appetite
  • excessive sweating
  • sensitivity to heat
  • twitching
  • fatigue
  • irritability and anxiety
  • heart palpitations
  • difficulty sleeping

Autoimmune thyroid diseases
An autoimmune disease is when your immune system mistakenly attacks your body — in this case, thyroid antibodies attack your thyroid. This can lead to a thyroid disorder.
Types of autoimmune thyroid diseases include Graves’ disease (can cause an overactive thyroid) or Hashimoto’s disease (can cause an underactive thyroid).

How to test your thyroid function

You can test your thyroid function using a blood sample. The two main ways to collect a blood sample are:

  • a venous blood test — a trained professional will use a needle to puncture a vein, usually in your arm, to collect a blood sample.
  • a finger-prick blood test — using a lancet, you can prick your own finger and collect a small blood sample.

Where to get a thyroid function test

NHS thyroid function test
The NHS offer blood tests which can be done at your GP’s or a local hospital. A trained nurse or doctor will take your blood, usually a venous sample.

Home thyroid function test
It’s possible to order a home thyroid function test kit online and do it yourself. For this test, you’ll use a lancet to collect a finger-prick blood sample. Your results will be reviewed by a GP and are available for you to view online.

What’s included in a thyroid function test?

A thyroid function test will usually measure your levels of:

  • T4
  • T3
  • TSH

A more advanced test will also test for the presence of any thyroid antibodies that could be causing an autoimmune disease. This might check your levels of:

  • thyroid peroxidase (TPO/PEX)
  • thyroglobulin antibodies (TgAb)
  • TSH receptor antibodies (TRAb)

It’s important to note that the presence of these antibodies in your blood doesn’t automatically mean you have an autoimmune thyroid disease — but it can indicate an increased risk of developing one in the future.

Some vitamins and minerals are essential for your thyroid to function properly — like vitamin D, vitamin B12, folate (vitamin B9), and iron. A nutrient deficiency can sometimes mimic the symptoms of a thyroid disorder. If possible, it’s a good idea to test your thyroid hormones and these nutrients as well.

Things that can affect your thyroid function results

Generally, you don’t need to fast before doing a thyroid function test. However, not fasting is sometimes linked to a lower TSH level. This means your results might not pick up on mild (subclinical) hypothyroidism — where your TSH levels are only mildly elevated.

Some medications can interfere with your results so it’s important to tell your doctor about any drugs you take.

Being pregnant can also influence your results. So if you’re pregnant make sure to discuss this with your doctor.

Understanding your results

TSH (thyroid-stimulating hormone) results
The normal range for TSH is:

  • between 0.27-4.3 mIU/L

If these levels are low, it could indicate an overactive thyroid. If these levels are raised, it could indicate an underactive thyroid.

T4 (thyroxine) results
T4 levels can be tested in two different ways — ‘total’ T4 or ‘free’ T4 (FT4). FT4 is preferable as it’s the amount of T4 that’s available for your body to use. The normal range for:

  • total T4 is between 66-181 nmol/L
  • FT4 is between 12-22 pmol/L

If these levels are low, it could indicate an underactive thyroid. If these levels are raised, it could indicate an overactive thyroid.

T3 (triiodothyronine) results
Like T4, T3 can be broken down into ‘total’ T3 and ‘free’ T3 (FT3). The normal range for:

  • T3 is between 100-200 ng/dL
  • FT3 is between 3.1-6.8 pmol/L

If these levels are low, it could indicate an underactive thyroid. If these levels are raised, it could indicate an overactive thyroid.

TPOAb (anti-thyroid peroxidase) results
The normal range for TPOAb is:

  • between 0-34 kIU/L

Raised TPOAb levels are a sign of Hashimoto's disease, also called autoimmune thyroiditis. These antibodies attack your thyroid and cause an underactive thyroid (hypothyroidism). Up to 95% of people with Hashimoto’s will have TPO antibodies in their blood which makes it a good marker for diagnosing this disease.

TgAb (anti-thyroglobulin antibody) results
The normal range for TgAb is:

  • between 0-115 kU/L

TgAb might be present if you’re suffering from an autoimmune thyroid disease — about 60-70% of people with Hashimoto’s disease will have raised TgAb levels. It’s very rare for a patient to have raised TgAb levels and TPOAb levels in normal range. For this reason, a TPOAb test is more widely used to diagnose an autoimmune thyroid disease.

TRAb (TSH receptor antibodies) results
The normal range for TRAb is:

  • between 0-0.81 IU/L

TRAb is the antibody associated with Graves' disease. It acts like TSH, the hormone that stimulates your thyroid to release more hormones — which can cause an overactive thyroid.

Treating thyroid disorders

If your levels indicate you have a thyroid disorder, your doctor will work out if there are any underlying causes and come up with a treatment plan.

An underactive thyroid is usually treated with a medication called levothyroxine — a hormone replacement tablet.

An overactive thyroid is usually treated with either medication, radiation, or surgery.


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Aisling Moran BSc (Hons)

Written by Aisling Moran BSc (Hons)

16th Jun 2021 • 8 min read