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What is the TSH Blood Test?

Medically approved by
Dr Earim Chaudry
Last updated
3rd June 2021

In 30 seconds…

What is the TSH blood test? Well, the TSH stands for thyroid-stimulating hormone. It’s a hormone, produced by the pituitary gland, that helps your thyroid produce the other hormones your body needs to control your heart rate and metabolism (such as thyroxine and triiodothyronine).

The TSH test is just one of the thyroid function tests. Together, these can tell you if you have an underactive thyroid (known as hypothyroidism), an overactive thyroid (hyperthyroidism), or if you’re perfectly healthy. You should take the test if you have symptoms of these conditions, or if you have a swollen neck, an irregular heartbeat, or fertility problems.

Normal TSH results will be between 0.4 and 4.0 mU/L (milliunits per litre). If your TSH result is high and thyroxine low, it suggests you may have an underactive thyroid. If your TSH is low and thyroxine high, you may have an overactive thyroid.

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Introducing the Thyroid

The thyroid is a complex little gland in your neck that doesn’t usually receive a lot of attention. However, if you’re experiencing changes to your weight, having trouble sleeping, or you have excessive sensitivity to temperature changes, it might be your thyroid that is responsible.

Complex, yes, but crucial too. The thyroid plays a big role in controlling the speed of your metabolism and the speed of your heart, which in turn control the way that you use energy. And when something is wrong with the gland, it can make you feel either tired or restless and can make you lose or gain weight dramatically.

So, it’s not to be overlooked in any health check. A TSH blood test – which checks for thyroid-stimulating hormone – is essential for investigating the health of this gland. Let’s explore this test in more detail.

What is the TSH Blood Test?

The TSH blood test is one of the tests that monitor the health of your thyroid. It’s very unlikely that you will take this test without doing the other thyroid function tests (TFTs) too.

That’s because TSH is one hormone in a chain of interlinked processes that involve your thyroid gland. Produced in the pituitary gland, TSH stimulates the thyroid to produce other hormones. Hence the name. So, while the TSH blood test can let you know important things about your thyroid, it can’t do it all by itself. The results of this test need to be cross-referenced with the measurements of the other hormones in the chain. These are:

  • Thyroxine. This is also known as T4. It’s produced by the thyroid itself and then secreted into the bloodstream. It then goes to the liver and kidneys, where it’s converted to its active form, triiodothyronine.
  • Triiodothyronine. Known as T3, this hormone does the heavy lifting. It is responsible for your heart rate and digestion, your brain development, and your muscle and bone maintenance. Your full TFT will need to check this too.

Without looking at these other hormones, your TSH blood test results will not give you any clear information about your thyroid. Seen alone, it will be difficult to determine whether the results show a problem with the thyroid itself or with the pituitary gland. Cross-referencing results with the other tests will clear up any confusion.

What Are Normal TSH Levels?

So, you’ve received your TSH test results. But how do you know if your hormone levels are healthy or not? For that, you need to consider the normal range (or reference range).

Normal levels for TSH are between 0.4 and 4.0 mU/l, according to the British Thyroid Foundation. If your results are above or below this range, it might suggest that you’re suffering from a thyroid disease. However, as we mentioned, to know this for sure, your results will need to be compared to your T4 and T3 readings.

Normal readings for those hormones are as follows:

  • T4 (Thyroxine) – between 9.0 and 25.0 pmol/l. You may see this referred to as “free T4”, the specific name for the amount of T4 in your bloodstream.
  • T3 (Triiodothyronine) – between 3.5 and 7.8 pmol/l. Again, you may see this referred to as “free T3”.

If you have a low TSH reading and a high T4, it might suggest an overactive thyroid (hyperthyroidism). Meanwhile, high TSH with low T4 suggests an underactive thyroid, primary hypothyroidism resulting from a problem with the thyroid itself. However, low TSH and low T4 suggests that your hypothyroidism is secondary, or is due to a problem with the pituitary gland.

On the other hand, high TSH and low T3 will usually be interpreted as hypothyroidism. And, in the interests of completeness, low TSH and high T3 suggests hyperthyroidism.

What Causes Thyroid Problems?

We know it’s all a little complicated. Your doctor or healthcare provider should be able to explain, when you receive your results, precisely what your situation involves. However, thyroid conditions can be serious – and you must know what to expect.  

Thyroid problems can be caused by a range of different conditions and medical interventions. For example, hypothyroidism can be the result of the following:

  • Hashimoto’s thyroiditis. The most common cause of hypothyroidism. It’s an autoimmune disorder that tends to run in families. It causes the inflammation of your thyroid, which prevents the gland from producing T4. In this case, your TSH will be high.
  • Removed thyroid. Some people need to have part or all of their thyroid removed if they have had thyroid nodules, thyroid cancer or Graves’ disease. This will result in too much TSH and too little T4.

Meanwhile, an overactive thyroid – hyperthyroidism – can be caused by:

  • Graves’ disease – in which your immune system attacks your thyroid.
  • Nodules – if there is a lump on your thyroid, the extra tissue will produce more hormones.
  • Medication, such as amiodarone, can cause hyperthyroidism as a side effect.

The Symptoms of Thyroid Disorders

These thyroid disorders have a wide range of symptoms, affecting almost every part of your body. If you’re yet to receive a TSH blood test, you must look out for the following symptoms:

  • Changes to your weight. Radical weight loss or gain can result from changes to your body’s metabolism due to different levels of thyroid hormones.
  • Restlessness or fatigue. While high levels of T4 and T3 will make you restless and will affect your sleep, low levels will make you chronically tired.
  • Sensitivity to heat. An overactive thyroid can make you sensitive to heat, while hypothyroidism can make it difficult to bear the cold.
  • Changes to the menstrual cycle. In women, thyroid problems can cause changes to your periods. They might be very heavy or they may stop completely.
  • Goitre (thyroid swelling). One of the most obvious signs of an overactive thyroid is the goitre – a swollen thyroid that’s visible in your neck.
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Key Takeaways

The TSH blood test is one of the crucial elements of checking for thyroid problems. Alongside the tests for thyroxine and triiodothyronine, the test will be able to pinpoint any problem your thyroid might be experiencing.

For TSH, the important numbers to remember are 0.4 and 4.0 mu/l. That’s the normal range. Anything above or below this may signal problems.

References

  1. Nussey S, Whitehead S. (2001). Endocrinology: An Integrated Approach. – https://www.ncbi.nlm.nih.gov/books/NBK27/

  2. The British Thyroid Foundation – Thyroid function tests: https://www.btf-thyroid.org/thyroid-function-tests

  3. NHS – Division of Laboratory Medicine – free thyroxine, FT4 – https://mft.nhs.uk/app/uploads/2020/01/Free-T4.pdf

  4. E MartinoL BartalenaF BogazziL E Braverman (2001). The effects of amiodarone on the thyroid: https://pubmed.ncbi.nlm.nih.gov/11294826/

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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