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What Is A TFTs Blood Test? Thyroid Function Tests Explained

As we get older, many people discover that a gland we once took for granted is no longer working as well as it should. The thyroid, located in the lower part of the neck, produces hormones vital to the basic functioning of our bodies, but as time passes, they can become overactive – producing too much of these hormones – or underactive, producing too little. These imbalances can trigger a number of unpleasant symptoms and generally damage our health. 

The thyroid produces three hormones: triiodothyronine, thyroxine and calcitonin. 

The first two are synthesized by the body from the mineral iodine. Calcitonin, meanwhile, is a so-called ‘peptide’ hormone – meaning it’s made from an arrangement of amino acids: molecular chains that form the basis of protein. Calcitonin is used by the body to control levels of calcium and phosphate in the bloodstream.

Triiodothyronine is more commonly referred to as T3, and thyroxine as T4, in reference to the number of iodine molecules in each. T3 and T4 are sometimes referred to as the ‘thyroid hormones’. They are involved in multiple biological functions. For example:

The cardiovascular system

T3 and T4 boost the heartbeat, and rate and strength of breathing, increasing blood flow around the body as well as internal temperature.


These hormones charge the growth of the brain and body in children and are especially important in the growth of neurons.


Our metabolism is the rate at which the multiple chemical processes which sustain life occur. Metabolic rate varies between individuals – that is why, for example, some people put on weight quickly and easily while others do not. The thyroid hormones increase the metabolic rate, and also keep our digestive systems active and healthy, aiding in the digestion and absorption of glucose, fat and other nutrients.


T3 and T4 are involved in the maintenance of healthy sleep cycles.

In short, the thyroid hormones affect every organ in the body and even help to generate the cellular energy that keeps us alive. 

What are thyroid function tests?

As the name implies, thyroid function tests, or TFTs, are conducted to assess the health of a person’s thyroid gland.

Thyroid problems typically develop in later life, although children can also be affected. Common triggers include a diet low in iodine (an iodine deficiency), infection by a virus (e.g. flu, shingles, the common cold), and autoimmune disorders – for example rheumatoid arthritis, type 1 diabetes, or psoriasis.

Sometimes, however, the thyroid begins to misbehave for no discernible reason. Women have a significantly higher risk of developing thyroid problems than men – perhaps because they are also more prone to developing autoimmune disorders.

Doctors refer to an overactive thyroid as ‘hyperthyroidism’. Around one in 100 individuals, on average, will develop hyperthyroidism at some point in their lives. Typical symptoms are associated with the body being ‘sped up’ by the excess hormones. For example:

  • A racing heart
  • Shaky hands
  • Muscular weakness
  • Feeling too hot
  • Difficulty sleeping
  • Weight loss
  • Feeling irritable, having a short temper

However, an underactive thyroid – known as hypothyroidism – is around five times more common, affecting around one in 20 people over the age of 12. As you might expect, hypothyroidism has the opposite effect to hyperthyroidism, effectively slowing down physical functions. Individuals vary, but the symptoms often include:

  • Feeling disproportionately tired
  • Dry skin
  • Hair thinning
  • A poor memory
  • Constipation
  • Low moods
  • Feeling cold
  • Putting on disproportionate amounts of weight

What can they TFTs reveal? Thyroid Disease Assessment

TFTs blood tests are conducted on blood samples. When analysed in a medical laboratory, varying levels of T4 will be assessed to determine the health of the patient’s thyroid. As you might expect, higher levels can suggest hyperthyroidism, and low levels the opposite.

When circulating in the bloodstream, T4 takes two forms: bound to carrier proteins or free. The latter form is referred to as free thyroxine, or FT4. As only this form is accessible to cells, thyroid function tests typically focus on FT4.

It’s also common to measure levels of a third substance: thyroid stimulating hormone, or TSH. This is secreted by another highly significant organ – the pituitary gland, located in the central part of the brain. TSH is produced by the pituitary in response to fluctuating levels of T3 and T4. 

When the amounts circulating in the bloodstream drop below a certain level, TSH is produced to trigger the thyroid into synthesising more hormones in order to maintain equilibrium and keep the body healthy. Therefore, if a thyroid function test detects high levels of TSH, it can be inferred that T3 and/or T4 levels are low and therefore, the person’s thyroid may be underactive: in other words, they may have hypothyroidism.

T4/FT4 and TSH are the two metrics most commonly used in standard medical thyroid tests. T3 or FT3 (free triiodothyronine) levels may also be measured if results indicate a likely thyroid disorder.

One autoimmune condition directly targets the thyroid: this is known as autoimmune thyroid disease, autoimmune thyroiditis or Hashimoto’s thyroiditis. When this condition develops, a misfiring immune system produces antibodies which then attack the thyroid. The presence of such antibodies is a good indication of problems with the gland.

Understanding the results

Thyroid hormones can be influenced by a variety of factors, from a person’s current state of health to, as we saw above, whichever prescription medications they may be taking at the time. Genetics also play a role: someone with a family history of thyroid problems is more likely to experience hypothyroidism or hyperthyroidism themselves.

However, extensive testing has established the following baseline amounts for health:

  • TSH: 0.5 to 5 milli-international units per litre
  • T3: 80-220 nanograms per decilitre
  • T4: 5.0 to 12.0 micrograms per decilitre

The benefits of home-to-lab thyroid testing

If your GP suspects that a poorly functioning thyroid may lie behind bothersome symptoms, he or she will most likely refer you for a thyroid function test. Typically these will be conducted at the local hospital, with the samples then despatched for analyses to a laboratory or specialist hospital elsewhere.

But, unless you have a severe case – and of course, some people do – your test results may not be regarded as a processing priority. But why wait weeks or even months? A home-to-lab kit will allow you to jump the queue and receive your results in a matter of a few days.

The full thyroid function test from Health Hub is a simple-to-administer home testing kit which works via a painless finger prick procedure. It will provide you with a detailed breakdown of your current levels of FT3, FT4 and TSH, plus any detectable thyroid antibodies.

Armed with this knowledge, you will then be able to discuss available treatments with your doctor – and with any luck, soon start to feel better.

Sian Baker

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

Updated on 1st December 2021