What is a thyroid function test, how to test it & what does it show?

The thyroid is a small, butterfly-shaped gland situated at the bottom of the neck, underneath the Adam’s apple. It is part of the endocrine system, which is a complex network of glands responsible for coordinating many of your body’s activities. The thyroid gland itself produces hormones that regulate the body’s metabolism.

A number of different disorders may arise when the thyroid produces an excessive amount of hormones (hyperthyroidism) or not enough (hypothyroidism). It is estimated that approximately 1 in 50 people (rising to 1 in 20 for people over 60) have thyroid function problems – most frequently an underactive thyroid gland – and women are more commonly affected.

This guide highlights what a thyroid function test is, how it is undertaken and what the results may show.

What is a thyroid function test?

A thyroid function test is a simple blood test involving drawing a small sample of blood from a person’s inner arm with a needle. This is commonly undertaken by a nurse or phlebotomist, and the process is quick and usually only produces mild discomfort for a short time . The sample is then sent to a lab to be examined. A blood sample can also be taken with a simple finger prick, which is generally an option taken by those who would prefer to take their own sample from the comfort of their own home.

The sample is then sent to a laboratory for analysis for measurement of the levels:

  • Thyroid Stimulating Hormone (TSH): a pituitary hormone that stimulates the thyroid to secrete triiodothyronine and thyroxine.
  • Thyroxine (T4): a hormone regulating the body’s metabolism and growth. The thyroid function test assesses the ‘free T4’ (FT4) levels, as free T4 relates to what is active and available for the body to use.

Thyroid problems can be identified when the levels of such hormones are above or below the recommended ranges. High levels of TSH combined with low FT4 suggest an underactive thyroid (hypothyroidism), whilst low TSH levels combined with high FT4 suggest an overactive thyroid (hyperthyroidism). Getting tested is particularly helpful as most cases are symptomless (particularly at the beginning and in mild cases) or mimic the symptoms of other ailments.

What does a thyroid function test show?

A thyroid function test shows the levels thyroid stimulating hormone (TSH) and free Thyroxine (FT4). If the levels are abnormal, it could be an indication of thyroid disease, including:

  • Hyperthyroidism occurs when the thyroid gland is overactive. This condition is about ten times more frequent in women than men. The most common cause of hyperthyroidism is Graves’ disease, which affects about 70% of people with an overactive thyroid. Thyroid nodules can also trigger the gland to produce excess amounts of hormones.
  • Hypothyroidism is the opposite of hyperthyroidism. This condition is caused by an underactive thyroid that cannot produce a sufficient amount of hormones. Hypothyroidism may be caused by Hashimoto’s disease, surgery to remove the thyroid gland, or damage from radiation treatment.
  • Hashimoto’s disease, also known as chronic lymphocytic thyroiditis, can arise at any age, however it is more frequently seen in middle-aged women. Hashimoto’s disease happens when the body’s immune system mistakenly attacks the thyroid and damages its ability to generate hormones.
  • Graves’ disease is the most frequent cause of hyperthyroidism in the UK, affecting approximately 3 in 4 people with an overactive thyroid. Graves’ disease is an autoimmune disorder where the body’s immune system mistakenly attacks the thyroid, which causes the gland to produce excessive amounts of the hormone responsible for regulating the body’s metabolism. This condition is hereditary and can arise at any age. However, stress and smoking may increase the risk of the disease developing.
  • Goitre is a noncancerous enlargement of the thyroid, which causes a visible swelling in front of the neck. It is most frequently caused by iodine deficiency in the diet, and can affect anyone at any age, particularly in parts of the world where foods rich in iodine are scarce. However, goitre is commoner after the age of 40 and in women. Other risk factors include family medical history, certain medications, pregnancy, and exposure to radiation.
  • Thyroid nodules are growths that form inside or on the thyroid. The causes may include iodine deficiency and Hashimoto’s disease, although it may be caused by other unknown reasons. The nodules may be solid or filled with fluid, and can be cancerous although they are generally found to be benign in most cases. As per most thyroid ailments, nodules are more typically found in women than men, and the risk increases with age for both sexes.

What is the normal range for thyroid levels?

The recommended ranges of Thyroid Stimulating Hormone (TSH) and Free thyroxine (FT4) in the blood are the same for adult males and females, and are as follows:

Hormone Units Recommended range
Thyroid Stimulating Hormone (TSH) mIU/L 0.27-4.2
Free thyroxine (FT4) pmol/l 12-22

 

Levels that above or below the average levels can suggest an over- or under-active thyroid:

Low High
Thyroid Stimulating Hormone (TSH) TSH levels are commonly decreased in hyperthyroidism (an overactive thyroid).

Causes may include Graves’ disease or thyroid nodules, which can be treated with antithyroid drugs to prevent the thyroid from producing its hormones. In more serious cases, surgery may be performed to remove the thyroid gland.

TSH levels are commonly increased in hypothyroidism (an underactive thyroid).

Causes may include Hashimoto’s disease, surgery to remove the thyroid gland, or damage from radiation treatment. Most cases of hypothyroidism are  treated easily by taking thyroid hormone tablets.

Free thyroxine (FT4) Free thyroxine (FT4) levels are typically decreased in hypothyroidism (an underactive thyroid).

Causes may include Hashimoto’s disease, surgery to remove the thyroid gland, or damage from radiation treatment. Most cases of hypothyroidism are treated easily by taking thyroid hormone tablets.

Free thyroxine (FT4) levels are typically increased in hyperthyroidism (an overactive thyroid).

Causes may include Graves’ disease or thyroid nodules, which can be treated with drugs or radioactive iodine to prevent the thyroid from producing its hormones. Occasionally  surgery may be performed to remove the thyroid gland.

 

If your levels are above or below the average, you should bring it to the attention of your GP, who will consider possible causes to explain the result in the context of your medical history, current treatments (including thyroid medications) and any previous thyroid tests.

What can affect thyroid test results?

The results of a thyroid function test can be affected by a number of factors, including:

  • Serious illnesses: heart attacks, trauma, infection, liver disease or kidney failure.
  • Thyroid medications: medicine used to treat thyroid disorders, especially when the dosage is incorrect.
  • Other medicines: steroid hormones, anticonvulsants, contraceptive pills, anti-inflammatory drugs, lithium and amiodarone.

Therefore, it is essential that the above are considered in the interpretation of your results to establish the correct reason for abnormal results.

How can I get a thyroid function test?

At Bluecrest, we offer a range of packages that include thyroid function tests, so you can easily assess your thyroid health and act accordingly.

At a clinic

You can book a health check with one of our Certified Healthcare professionals at one of over 2,000 mobile clinics nationwide. Our Complete Health MOT Package includes a thyroid function test, in addition to a wide array of health markers for a detailed view of your health. You can choose between 3 levels of testing to suit your own personal needs.

You can also book a thyroid test separately if you do not wish to have any other readings.

At Home

Alternatively, you can get a thyroid function test from the comfort of your own home with our Total Home Test Kit, which includes a thyroid function test and also assess a variety of other key readings pertaining to your general health. All you need to do is order your kit, take your own sample, send it back in our pre-addressed return envelope, and wait to receive your results online within 8 days.

Sources:

https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/diagnosis/

https://www.btf-thyroid.org/thyroid-function-tests