Underactive thyroid: Symptoms and causes

March 8, 2022
5 minute read

The thyroid gland is a hormone-producing gland, shaped like a butterfly that sits in your neck. Thyroid hormones help control your metabolism, processing the food you eat and turning it into energy.

Sometimes the thyroid gland can produce too much thyroid hormones or not enough. In this blog, we will be discussing the symptoms, causes and diagnosis of an underactive thyroid.

An underactive thyroid means your thyroid gland is not producing enough thyroid hormones to function sufficiently, also known as hypothyroidism.

Who does it affect?

An underactive thyroid can affect anybody but is more common in women than men. It can also develop in children. Some babies are born with it and all babies born in the UK are screened for it at five days old.


The symptoms of an underactive thyroid are similar to symptoms of other conditions which can make it confusing. In addition, symptoms usually develop slowly over time so you may not realize there is a medical issue for several years.

Common symptoms include:

  • tiredness
  • being sensitive to cold
  • weight gain
  • constipation
  • depression
  • slow movements and thoughts
  • muscle aches and weakness
  • muscle cramps
  • dry and scaly skin
  • brittle hair and nails
  • loss of libido (sex drive)
  • pain, numbness and a tingling sensation in the hand and fingers (carpal tunnel syndrome)
  • irregular periods or heavy periods

It is best to see a GP to be tested for thyroid function when you have any of these symptoms.

The thyroid gland sits at the front of the throat, below the Adam’s apple (larynx)


Immune System

Most of the time, an underactive thyroid is caused by a person's immune system attacking their own thyroid gland. This causes damage to the thyroid gland and prevents it from producing enough of the hormone, thyroxine, and therefore, the thyroid does not function normally.  

In some cases, this damage can be significant enough to cause auto-immunity in the body. This type of auto-immune condition of the thyroid is called Hashimoto’s disease. It is common in people with other immune system disorders such as type 1 diabetes and vitiligo, and it also runs in the family.

Previous Treatment

Sometimes, an underactive thyroid can be caused by previous thyroid treatments such as surgery or thyroid cancer treatments such as radioactive iodine therapy.

Nutritional Deficiency

A less common cause of an underactive thyroid can be due to insufficiency of iodine, a mineral we obtain from our diet that is required for thyroid function. However, in the UK it is an uncommon cause of underactive thyroid.

It is best to speak to your GP if you’re concerned about any treatments or medication you are taking that could affect your thyroid.


If you are concerned about your thyroid, the best way to test for your thyroid levels is by getting a thyroid function blood test from your GP. If you repeatedly have the symptoms as above, it is important you get this as soon as possible. If it is left undiagnosed, it may lead to more serious conditions such as heart attacks or angina.

The thyroid blood test typically looks at the two main thyroid hormones called thyroid stimulating hormone (TSH) and thyroxine (T4).  

If your test results show a high level of TSH and low T4, you may have an underactive thyroid.

If your test results show a high level of TSH and normal levels of T4, you may be at risk of developing an underactive thyroid in the future.

To check for autoimmunity, to either diagnose or rule out Hashimoto’s, your GP will test for antibodies. They will only test for antibodies if they suspect you have autoimmunity.

After your blood test, your GP will discuss the best treatment options for you or they may be referred to an endocrinologist (someone who specialises in hormone disorders).

For further information on hypothyroidism, visit the NHS website here and the British Thyroid Foundation.

You can also contact our friendly pharmacy team for help and advice.  

Please get in touch with us today if you would like to speak with one of our pharmacists.


Content last reviewed on:
March 8, 2022
Next review date:
March 8, 2024
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