All you need to know about thyroid-stimulating hormone (TSH)

Thyroid blood results can be hard to decipher. Find out all you need to know about the thyroid biomarker TSH.

If you’ve just received your blood test results or been told that you need a test for TSH, you may be wondering what it is and what it does. We’ve put together a complete guide on all you need to know about thyroid-stimulating hormone (TSH).

In this article, we include:

What is TSH?

TSH (also known as thyroid-stimulating hormone) is a hormone produced by the pituitary gland. Its main function is to stimulate the thyroid gland to produce and release thyroid hormones.

If you have an abnormal level of TSH, it can indicate thyroid disorders such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), and cause several unwanted symptoms – from fatigue to mental health conditions.

Why do I need a TSH blood test?

You may need a TSH blood test if you or your doctor suspect you have a thyroid condition. Your TSH level can be monitored either alone or alongside other hormones.

You can test your TSH level from the comfort of your own home with an at-home Thyroid Function Blood Test.

What is a normal TSH level?

As with many biomarkers, a normal range of TSH can vary depending on the laboratory that tested the sample (you can read more about how laboratory ranges are set in our blog: understanding your blood test results).

However, in general, a TSH level between 0.4 and 4.0 mIU/L is considered normal for most adults [1].

TSH level can also be influenced by lifestyle and other factors such as age, gender, and overall health. And so, in some cases, a TSH level outside of the normal range doesn’t always indicate a thyroid disorder.


What is considered to be a dangerous level of TSH?

There isn’t a specified dangerous level of TSH. When receiving your blood results back from the lab, your doctor will indicate whether your TSH level is too low or high.

For some people, a level of 4.2 mIU/L may be normal, whilst, for others, this may indicate subclinical hypothyroidism.

The same goes for people who have a level below the normal range – it could indicate hyperthyroidism.

Because TSH levels can vary so much, they should be interpreted by a doctor alongside your overall health and medical history. Further testing such as ultrasounds or further blood tests may be needed before any diagnoses are made.


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What causes a raised TSH level?

A raised TSH level can be caused by several factors, including:

  1. An underactive thyroid gland – when the thyroid underproduces hormones, the pituitary gland compensates by producing more TSH.
  2. A pituitary gland disorder – the pituitary gland produces TSH, so in some cases may be the cause of a raised TSH level [2].
  3. Iodine deficiency – as your iodine level drops, so can thyroid hormone production. In the case of iodine intake dropping below 100mcg a day, TSH production increases [3].
  4. Medications – certain medications can affect your TSH level, especially ones that contain iodine [4].
  5. Pregnancy – pregnancy can occasionally lead to postpartum thyroiditis and abnormal TSH levels. 

What causes a low TSH level?

A low TSH level can be caused by several factors, including:

  1. An overactive thyroid gland – when the thyroid doesn't make enough T3 and T4, the pituitary gland compensates by producing less TSH.
  2. A pituitary gland disorder – an issue with the pituitary gland can lead to reduced TSH production [2].
  3. Medications – certain medications like steroids can affect your TSH level, so be sure to let your doctor know of any medication you are currently taking.
  4. Pregnancy – when you are pregnant, your hCG level naturally increases, which can reduce levels of TSH [5].
  5. Ageing and genetic factors – as you get older, your TSH level may naturally drop [6]. Genetic factors, such as the presence of autoimmune thyroid disease, could also cause low TSH levels.

How can I correct my TSH level?

In some cases, you may not need to correct your TSH level – some thyroid conditions correct themselves. The underlying cause of your abnormal level will determine how or if it needs treatment. For example, if the underlying cause is hypothyroidism, your doctor will likely treat this with levothyroxine. This should bring your TSH level back within a normal range. There are lots of different treatments for thyroid conditions from medication to surgery.

You can find more about how to treat different thyroid disorders in our Thyroid Hub.

Is TSH alone enough to test the thyroid?

In most instances, if you are experiencing symptoms that indicate a thyroid disorder and your doctor wants to investigate, they will usually order a test that measures only TSH.

TSH can give a great indication of thyroid function, however, it is not always enough to fully evaluate thyroid health on its own. This is because TSH levels don’t always reflect thyroid hormones T4 and T3. And in some cases, T4 and T3 (tested alongside TSH) give a greater insight into thyroid health and function.

TSH by itself can also not determine whether a thyroid condition is down to an autoimmune disease, such as Graves’ disease or Hashimoto’s thyroiditis. For this, you would need to test your thyroid antibodies — thyroid peroxidase and thyroglobulin.

Can I test my TSH level at home?

Yes. Our range of Thyroid Blood Tests all include TSH and can be done from the comfort of your home. We will send you everything you need and your results, alongside doctor’s comments, will be displayed in MyMedichecks. You can also use the tracker function to monitor your TSH level over time.

Can diet and lifestyle affect TSH levels?

Technically, yes. Diet and lifestyle can affect TSH level, but you cannot always correct your TSH level with lifestyle fixes – this will depend on the underlying cause of your abnormal TSH level.

Diet and lifestyle factors that can affect TSH levels include:

  1. Exercise – exercise can help to improve TSH levels. Even if your TSH level is high, exercise is still encouraged as it can help reduce symptoms of a thyroid condition. You can read more in our blog on thyroid conditions – exercise dos and don’ts.
  2. Diet – eating a healthy and balanced diet is the first step, but you should also ensure a healthy iodine intake. Iodine is essential for producing thyroid hormones, and too little iodine in the diet can lead to a low TSH level and iodine deficiency-induced hypothyroidism. Iodine isn’t the only nutrient you need to be aware of when it comes to your thyroid. Other nutrients such as selenium can affect your TSH level. Learn more about what foods are good for thyroid health.

Leading a healthy lifestyle is an integral part of looking after your thyroid as a whole, but sometimes additional treatment like medication or surgery may be necessary to bring your TSH level into the normal range.   

What should I do if my TSH level is abnormal?


If your TSH level is abnormal, your doctor will provide you with next steps. This could be a follow-up blood test, further testing such as testing your other thyroid hormone levels (T3 and T4) or antibodies, or even an ultrasound.

The underlying cause of your abnormal TSH level will determine what treatment (if any) is recommended.

Where can I get more information on TSH and thyroid health?

There are plenty of resources available online for further information on thyroid health.

  • Thyroid Hub – from information on thyroid conditions to answering questions about whether thyroid conditions can go away by themselves, our Thyroid Hub is here to support you.
  • Thyroid UK – Thyroid UK is a charity that seeks to help you on your thyroid journey. Whether or not you have a diagnosis, they offer plenty of support and information.
  • British Thyroid Foundation – BTF is a charity that helps people who have been diagnosed and are living with a thyroid disorder. And it offers support based on patient experience.



  1. Thyroid function tests (no date) British Thyroid Foundation. Available at: (Accessed: April 19, 2023).
  2. Problems with the assessment of thyroid function and levothyroxine replacement levels in pituitary disease (no date) Problems with the assessment of thyroid function in pituitary disease | The Pituitary Foundation. Available at: (Accessed: April 19, 2023).
  3. Office of dietary supplements - iodine (no date) NIH Office of Dietary Supplements. U.S. Department of Health and Human Services. Available at:,presence%20of%20elevated%20TSH%20levels (Accessed: April 19, 2023).
  4. Dong, B.J. (2000) “How medications affect thyroid function,” Western Journal of Medicine, 172(2), pp. 102–106. Available at:
  5. Pregnancy and Thyroid Disease publication, NIH publication number 12-6234
  6. Gesing, A. (2015) “The thyroid gland and the process of aging,” Thyroid Research, 8(Suppl 1). Available at:

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