Understanding Thyroid Function Test Results

Written by
The Manual Team
Medically approved by
Dr Earim Chaudry
Last updated
18th February 2021

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The Thyroid Function Test, or TFT, is a type of blood test that monitors the health of your thyroid. That’s the gland that produces and regulates the hormones your body needs to turn food into energy.

But what do the thyroid function test results mean? The TFT specifically measures the levels of thyroid-stimulating hormone (or TSH), thyroxine, and triiodothyronine in your blood. And when their levels are outside of the normal range, it can indicate a problem with the thyroid gland.

Low TSH and high thyroxine signals hyperthyroidism, or an overactive thyroid. On the other hand, low thyroxine suggests hypothyroidism, although the severity of this condition depends on the levels of TSH as well.

The Thyroid Function Test

The thyroid function test – or TFT – is one of the better known blood tests around. It will be used if you have – or if doctors suspect that you have – a problem with the thyroid, the gland in the neck that looks a bit like a butterfly.

The thyroid produces the hormones that control some crucial processes in your body: the speed of your metabolism, for example, the speed with which you burn calories, and the speed at which your heart beats. If you suffer from some sort of thyroid disorder – that may make it over- or underactive – all of these things can be affected. And, as a result, you may gain or lose weight, or feel tired or restless.

Knowing this, it’s not hard to see why the TFT is a pretty important test. But how do you understand the thyroid function test results? That’s what we’re covering here – alongside some of the other crucial things you’ll need to know.

What Does the TFT Test For?

What’s crucial to know about the thyroid function test to begin with is that it is not just one test. Rather, it is made up of a number of different tests that look at the levels of each of the specific hormones produced by the gland. However, the doctors will only need to take one blood sample from you.

There are three most significant hormones that will be measured in the test. Combined, they give you a picture of what’s going on inside that gland.

Thyroid Stimulating Hormone. Actually produced by the pituitary gland, TSH gets the thyroid going. It stimulates the thyroid to produce hormones, such as the thyroxine and triiodothyronine. When TSH levels are either too high or too low, it can indicate that there is a problem with your thyroid or your pituitary gland.

Thyroxine. Known alternatively as T4, thyroxine is the main hormone produced by the thyroid. It is secreted into the bloodstream, through which it travels to the liver and kidney. There, it is converted into its active form, triiodothyronine. 

Triiodothyronine. Also sometimes known as T3, triiodothyronine is the crucial hormone responsible for heart and digestive function, metabolic rate, bone maintenance, muscle control, and brain development. This one is produced from thyroxine – and too much or too little can cause some quite serious problems.

Note that you may see thyroxine and triiodothyronine referred to as free T4 and free T3 during your test. This is the name given to the hormones present in your blood (rather than in your organs).

How to Interpret the Thyroid Function Test Results

The hormones themselves are one thing. However, to understand the health of that gland, you need to know the levels of these hormones too. That’s what your thyroid function test results show you.

So, what are the reference ranges for thyroid hormone levels? For adults, the normal results, according to the NHS, are as follows:

Thyroid-Stimulating Hormone (TSH): 0.2 – 5.5 miU/L

Thyroxine (Free T4): 10 – 24.5 pmol/L

Triiodothyronine (Free T3): 3.1 – 6.8 pmol/L  

For those who are interested, miU/L is shorthand for milli-international units per litre. Pmol/L, meanwhile, is picomoles per litre. You don’t need to know this, however. Knowing the ranges themselves is enough.

Spotting Any Abnormalities

Anything outside of these ranges might mean that you are suffering from a thyroid disease such as hyperthyroidism or hypothyroidism – when the thyroid is overactive or underactive respectively. Yet, understanding these results is not as easy as these values being too high or too low. Instead, they need to be cross-referenced with each other to be interpreted correctly.

Specifically, a low TSH and a high T4 is usually a sign of hyperthyroidism.

Low T4 is an indication of hypothyroidism, yet its type depends on the level of TSH. A high TSH (combined with low total T4) suggests primary hypothyroidism, meaning that there is a problem with the thyroid gland itself. However, low amount of T4 and low TSH indicates secondary hypothyroidism, meaning that the problem lies in the organs that feed the thyroid.

Similarly, high TSH with low T3 generally signals hypothyroidism, while low TSH with high T3 tends to be a sign of hyperthyroidism.

Something that is really crucial to know, however, is that the thyroid function test alone is rarely considered sufficient for a diagnosis. There have been questions over the reliability of the TSH test in particular – although its record is pretty good. In the long run though, you’ll probably need some more tests to confirm.

Some Common Thyroid Disorders

So, we’ve mentioned hypo and hyperthyroidism quite enough here. But what do they actually mean, and what causes them?

What is Hypothyroidism?

Hypothyroidism – or an underactive thyroid – can result from a number of different conditions. Yet, in essence, it is the name given to the problem in which your thyroid produces less hormones than it should.

It can be the result of thyroid problems such as thyroid cancer, or autoimmune diseases such as Hashimoto’s thyroiditis, in which an antibody attacks the TSH receptor on the gland. Alternatively, it can be the consequence of a problem upstream, such as in the pituitary gland or hypothalamus, which are crucial to the production of TSH.

Regardless, hypothyroidism can cause a low heart rate, weight gain, and reduced appetite – as it slows down your metabolism.

What is Hyperthyroidism?

On the other hand, an overactive thyroid – or hyperthyroidism – is the name given to the thyroid condition in which the gland produces too many hormones. It too can be caused by autoimmune problems, such as Graves’ disease, but also by inflammation or a tumour or nodules on the gland.

You can often recognise a hyperthyroid condition by the presence of a goitre, or an enlarged thyroid that is visible in the neck. Its symptoms are often the opposite: weight loss, increased appetite, heart rate, and bowel movements, and tiredness. Yet, it can cause hair loss and irregular menstrual cycles in women as well.

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Key Takeaways

If your thyroid function test results are normal, then that’s a really good sign. However, if they are not – or, if you feel like you have the symptoms of a thyroid dysfunction – you should organise a follow up with an endocrinologist.

They will be able to take other tests – such as antibody tests – and consider your symptoms with a more specialist eye.

References

  1. www.gloshospitals.nhs.uk – Thyroid Function Tests (TSH, FT4, FT3): https://www.gloshospitals.nhs.uk/our-services/services-we-offer/pathology/tests-and-investigations/thyroid-function-tests-tsh-ft4-ft3/

  2. C Ling, Q Sun, J Khang, M Felipa Lastarria, J Strong, B Stolze, X Yu, TP Parikh, MA Waldman, K Welsh, J Jonklaas, L Masika, and SJ Soldin (2017). Does TSH Reliably Detect Hypothyroid Patients? : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5847294/

While we've ensured that everything you read on the Health Centre is medically reviewed and approved, information presented here is not intended to be a substitute for professional medical advice, diagnosis, or treatment. It should never be relied upon for specific medical advice. If you have any questions or concerns, please talk to your doctor.

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