Thyroid Function Blood Test, from our experts to you.
Dr Sam Rodgers MBBS, MRCGP

Chief Medical Officer

Expert Image

What is
thyroid function test?

Our Thyroid Function Blood Test is a simple finger-prick test to check and monitor your thyroid hormones: TSH, FT3, and FT4. Your thyroid gland produces the hormones T3 and T4, which bind to proteins as they transport around your body. As the hormones become bound, they are not readily available to your cells. Our thyroid function test measures the level of free or unbound thyroid hormones, which are biologically active. These hormones are a better measure than measuring total levels of T4 and T3 and are more likely to explain your symptoms.

What does a blood test
for thyroid function show?

Many routine thyroid tests that only include TSH and FT4 don't identify if you are not converting FT4 to the more biologically active FT3 easily. It is possible to have optimal levels of TSH and FT4 but still experience symptoms of an underactive thyroid if your levels of FT3 are low. Testing FT3 is helpful if you take T3 medication, like liothyronine or natural desiccated thyroid (NDT). TSH, when functioning well, stimulates the thyroid gland to produce thyroid hormones (mainly T4). High levels of TSH can signify that your thyroid is struggling to make enough T4 for your body's needs. Testing TSH alone will not necessarily give a picture of your thyroid function, which is why we test the other thyroid hormones, too.

What are the early warning
signs of a thyroid condition?

Symptoms of a thyroid-related condition include feeling tired all the time or experiencing changes in mood, weight, or energy levels. That's because your thyroid hormones control the speed of the cells in your body, governing how fast your heart beats and how quickly your intestines process food. The symptoms of thyroid conditions can be debilitating but are often difficult to diagnose - especially if they are mild or confused with something else.

What's Included?

Thyroid hormones
Select profile for more information

TSH Thyroid stimulating hormone (TSH) is produced in the pituitary gland in order to regulate the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3) by the thyroid gland. If thyroid hormones in the blood are low, then more TSH is produced to stimulate the thyroid gland to produce more of them. If thyroid hormone levels are high, then the pituitary produces less TSH to slow the production of thyroid hormones. If TSH is too high or too low, it normally signifies that there is a problem with the thyroid gland which is causing it to under or over produce thyroid hormones. Sometimes a disorder of the pituitary gland can also cause abnormal TSH levels.
Free T3 Triiodothyronine (T3) is the more active of the two thyroid hormones produced by the thyroid gland. Most T3 is bound to protein in the blood. Free T3 measures the level of T3 that is free, or unbound to protein, and is available to regulate metabolism.
Free thyroxine Thyroxine (T4) is one of two hormones produced by the thyroid gland. It works to speed up the rate of your metabolism. Most T4 is bound to carrier proteins in the blood - it is only the free, or unbound, T4 that is active in the body, which is measured in this test. Free T4 is the less active of the two main thyroid hormones. To have an impact on your cells it needs to convert to the more active T3 when your body needs it.

How to prepare
for your test

Special Instructions

Prepare for your Thyroid Function Blood Test by following these instructions. Please take your sample before 10am. You should take this test before you take any medication or vitamin/mineral supplements. Do not take biotin supplements for two days before this test, discuss this with your doctor if it is prescribed.

Frequently asked questions

What is the thyroid?

The thyroid is a butterfly shaped gland that sits under your Adam’s apple. It produces thyroid hormones which help to regulate your metabolism. Sometimes, the thyroid produces too many hormones, leading to an overactive thyroid (hyperthyroidism), or too few, leading to an underactive thyroid (hypothyroid).

Have a look at our Thyroid Health Guide for how your thyroid could hold the answers to better skin, losing weight and regaining your energy.

What is thyroid disease and who is at risk?

As many as 1 in 20 people are thought to have a thyroid disorder. There are a number of different conditions that can affect the normal functioning of the thyroid, including an underactive thyroid (hypothyroid), an overactive thyroid (hyperthyroidism)autoimmune disease, thyroid swelling (or a goitre), nodules or thyroid cancer.

Although anyone can develop a thyroid disorder, factors including being female or over the age of 50, and people with a family history of thyroid problems, can increase an individual’s risk of developing thyroid disease.

How often should I get a thyroid test?

How often you should take a thyroid test will depend on whether you have been diagnosed with a thyroid disorder and, importantly, whether you are experiencing symptoms.You may want to take a thyroid test if you are experiencing symptoms of an overactive or underactive thyroid or if you are managing a thyroid condition. Our Thyroid Health Guide explains what frequency of testing may be right for you, depending on your results and symptoms.

Can I test my thyroid during pregnancy?

You can test your thyroid during pregnancy as it’s important to have a well-controlled thyroid for the healthy development of the baby. If you have a thyroid condition and are planning to conceive, or when find out you’re pregnant, let your GP know so they can monitor you or take your thyroid test results to your doctor.

Thyroid results during pregnancy:

  • Underactive thyroid during pregnancy – TSH level should be less than 2.5 mU/l in the first trimester and less than 3.0 mU/l in the second and third trimesters.
  • Overactive thyroid during pregnancy - doses may be adjusted through regular monitoring.