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What Blood Tests Are There For Thyroid Health? 

Let’s start at the beginning. What is thyroid? Most people have heard of this mysterious organ but not many of us could give a clear explanation for what exactly it does. 

The thyroid is a gland located at the front of the neck. Glands are organs that secrete biologically-active chemicals, and the thyroid is part of an especially significant network of glands called the endocrine system. Organs in the endocrine system produce hormones – messenger molecules which transmit signals from one part of the body to another, and stimulate the recipient areas into action.

The thyroid secretes three hormones:

  • Thyroxine

The body makes this vital hormone from the mineral iodine, and is commonly referred to as T4, in reference to the number of iodine molecules present within its structure. T4 normally accounts for around 95% of the thyroid hormones in circulation.

  • Triiodothyronine

This hormone is a variant of T4, with one less iodine molecule. As you may have guessed, it’s therefore often referred to as T3. When more T3 is needed, the body can convert T4.

T4 – and to a lesser extent T3 – regulate multiple biological functions, including physical growth, sleep, brain function, digestion of glucose and fat, breathing, heart function, blood flow and metabolism – i.e. the rate at which our bodies conduct the different chemical processes which keep them alive. In short, the thyroid hormones affect the function of every cell in our bodies.

  • Calcitonin

This is a peptide hormone, meaning it’s created from amino acids, the building blocks of protein. Calcitonin has a number of functions within the body, including enabling the full absorption of phosphate and calcium: both minerals important to our health.

The production of T3, T4 and calcitonin is stimulated by a fourth hormone produced by another member of the endocrine system: the pituitary gland, located at the base of the brain. 

This has the self-explanatory name ‘thyroid stimulating hormone’ or TSH. It’s produced in response to changing levels of the thyroid hormones: if these drop, the pituitary will produce more TSH to stimulate the production of increased levels of T4, T3 and calcitonin. This ensures – at least in theory – that the body remains in balance and health is maintained.

Most common thyroid disorders

Like any other organ, the thyroid can malfunction, especially as a person ages. Women are especially prone to thyroid problems. Let’s take a look at some of the most common thyroid problems.

Hyperthyroidism and hypothyroidism

These two conditions are by some margin the most frequently seen thyroid disorders. Having an overactive thyroid – one which produces excessive levels of the three hormones – is known as hyperthyroidism. 

Hypothyroidism is an underactive thyroid – a much more common condition: around one in 20 people will experience it at some point in their lives.

Thyroid nodules

Thyroid nodules are lumps that sometimes form within this vital gland. They may be solid or filled with fluid. In most cases they cause no symptoms but in some instances they can become large enough to obstruct breathing, at which point surgery will be required. Rarely, thyroid nodules can become cancerous.

Hashimoto’s thyroiditis

Hashimoto’s thyroiditis is an autoimmune disorder. The patient’s immune system becomes wrongly sensitised and begins to produce antibodies to attack the gland.

It is also known as autoimmune thyroiditis or autoimmune thyroid disease.


Despite the name, hyperparathyroidism is a condition which does not directly affect the thyroid. The parathyroid glands are, in fact, four separate hormone-producing glands located directly behind the thyroid. These are also part of the endocrine system. 

Parathyroid hormone controls the levels of calcium in our blood and bones. If hyperparathyroidism develops, one or more of these glands has become overactive, producing too much of the hormone. This condition can require surgery or medication.

The different tests

There are a number of  blood tests available for suspected thyroid disorders. 

Typically, thyroid function tests are focused on the T4 within the subject’s bloodstream. These provide a good indication of the state of their thyroid and whether it’s functioning correctly. For example:

  • Unusually high levels of the hormones are likely to indicate hyperthyroidism
  • Unusually low levels generally suggest hypothyroidism

TSH tests are also routinely conducted. As we saw above, the levels of TSH in a particular person’s bloodstream are directly correlated to the levels of thyroid hormone, so measurement of the pituitary hormone provides a good indication of thyroid health. For example: high levels of TSH most likely indicate low levels of T4 and T3, and that may mean an underactive thyroid.

Testing for T3 specifically is more unusual but sometimes undertaken if T4 and TSH levels are found to be abnormal.

FT3 and FT4 – or free triiodothyronine and free thyroxine – are the forms of T3 and T4 not bound to protein cells. Only FT3 and FT4 are directly accessible to organs and cells so some thyroid function tests focus on these.

Thyroid function tests are blood tests. Samples may be taken in a medical setting or by a concerned person themselves using a home testing kit (see below for more on these). In either case, the sample will be analysed by professional medical testing equipment.

Which test is right for you?

Assess your symptoms to work out which test might be most suitable for you. If you regularly experience any of the following, you may have an underactive thyroid:

  • Fatigue
  • Feeling uncomfortably cold
  • Constipation
  • ‘Brain fog’ and memory lapses
  • Thinning, unhealthy hair
  • Unusually dry skin
  • Feeling uncharacteristically low or depressed

By contrast, typical symptoms of hyperthyroidism include:

  • Shakiness and physical weakness
  • Feeling uncomfortably hot
  • Insomnia
  • Weight gain
  • Hunger
  • Heart palpitations

Your GP will be able to provide meaningful guidance on the right testing route for you.

Thyroid testing from home

There is another option: a home testing kit – for example, the full thyroid function test from Health Hub.

These inexpensive and easy to administer kits will provide you a full breakdown of your current FT4, FT3 and TSH levels, along with two additional factors: the antibodies TPOAb and TGAb. The presence of the latter can indicate Hashimoto’s thyroiditis, as well as such conditions as type 1 diabetes, rheumatoid arthritis and anaemia.

Sian Baker

Medically reviewed by Sian Baker, Dip ION mBANT mCNHC – Written by Beth Giddings.

Updated on 1st December 2021