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Signs and causes of an underactive thyroid

Laura Harcourt

Written byLaura Harcourt

Dr Shree Datta

Reviewed byDr Shree Datta

2 people standing on beach, happy calm expressions
Hypothyroidism or an underactive thyroid is a common condition that affects your quality of life. Find out about the signs and causes of an underactive thyroid.
Good health and wellbeing rely on your body working exactly as it should. This means all body parts – including organs or glands you might not have thought much about before, like your thyroid.

Some people have an underactive thyroid. This means that it’s not working as hard as it should be to do its job – which is to regulate certain hormones in your body.1

What is the thyroid?

Although you may have heard of your thyroid before, just how much do you really know about it?

This small, butterfly-shaped gland is located at the base of your neck.2 It forms part of your endocrine system, which means that it plays a key role in balancing your hormones for biological processes, such as growth and development.1

Principally, it’s concerned with producing the hormones that control your metabolism – the process that your body uses to transform food into energy.2 To do this, your thyroid makes two hormones – thyroxine (T4) and triiodothyronine (T3).1,2

What is an underactive thyroid (hypothyroidism)?

In an ideal world, your thyroid would be firing on all cylinders and creating exactly the right balance of T4 and T3 to keep your metabolism working well.3

The reality is that sometimes, that little gland can start to operate at a slower pace and not produce enough of these hormones. When this happens, it’s officially known as hypothyroidism.3 However, it’s better known simply as an underactive thyroid.3

Hypothyroidism is fairly common, with current estimates suggesting that around 2% of people in the UK have the condition.4 It’s more likely to affect people over the age of 60, and women are five to ten times more likely to be affected than men.4

While an underactive thyroid may not seem particularly dangerous, if it goes untreated, there’s a risk that you could suffer complications, including heart disease.4,5

Symptoms of an underactive thyroid

Many people who have an underactive thyroid don’t actually realise that they have hypothyroidism.

Let’s explore some of the most common symptoms in more detail so that you know what to look out for:
  • fatigue: feeling more tired than usual? Fatigue is one of the most common symptoms of hypothyroidism
  • sensitivity to cold: particularly when everyone else around you is warm3
  • constipation: if you’re opening your bowels less frequently, it could be a sign that your digestive system has slowed down7,8
  • dry skin and thinning hair: this can be a sign that your body isn’t getting the nutrients that it needs9
  • weight gain: your metabolism plays a crucial role in keeping your weight stable.10 If your metabolism slows down, as is the case with hypothyroidism, then you could gain weight3,11
  • muscle cramps and aches: if you’re feeling unusually beaten up, stiff and, sore, it could be a sign that you’ve got an underactive thyroid12
  • menstrual cycles that are heavier than usual or irregular: period changes can be caused by many factors, with some studies relating to an underactive thyroid13
  • depression and mood disorders: unsurprisingly, rollercoaster hormones also result in mood swings and even depression. Since your thyroid and hormones are irrevocably linked, it makes sense that hypothyroidism can result in changes to your mood14
If you can identify with any of these symptoms, particularly if you’re experiencing a combination, speak to your GP about the possibility of having an underactive thyroid.

Causes of an underactive thyroid

For the majority of people with an underactive thyroid, their condition may be caused by an autoimmune disorder.4,15

These are disorders that occur when your own immune system, which usually protects you and helps your body repair from illness and injury, can’t tell the difference between your own cells and unhealthy or foreign ones, causing it to attack your healthy cells instead.16

Other potential causes of hypothyroidism include:17
  • previous thyroid treatment, such as surgery or radioactive
  • iodine therapy, which can be used to treat thyroid cancer iodine deficiency (although this is uncommon in the UK) 
  • problems with your pituitary gland, which regulates the thyroid
  • some medications, including those to treat depression/bipolar disorder, an irregular heartbeat, hepatitis C, and some types of cancer
It’s also worth noting that pregnancy causes significant changes in the hormone levels produced by your thyroid, which can lead to temporary hypothyroidism.18,19

How is hypothyroidism diagnosed?

Like many other health conditions, the main way to diagnose an underactive thyroid is by using a blood test.20

This blood test, known as a thyroid function test, will check something called your thyroid-stimulating hormone (TSH) level.20

The results of this, along with the information about your symptoms provided at your consultation appointment, should be enough to confirm or deny a diagnosis of hypothyroidism.20

Treatment for hypothyroidism

Firstly, not everyone who has hypothyroidism will need to take medication. If your condition is very mild, or you have certain pre-existing conditions, the normal medicines prescribed for an underactive thyroid may not be suitable.

Fortunately, if you do need treatment for an underactive thyroid, it’s fairly straightforward.

Most people with the condition are recommended to start taking a daily hormone replacement medication called levothyroxine.21 This drug replaces the hormone that your thyroid isn’t making enough of (i.e.thyroxine), helping to stabilise your levels and control your symptoms.21

There’s no one-size-fits-all dose, and you may need to have yours adjusted a few times to find the one that works best for you. Once settled on the correct dose of levothyroxine, you’ll be monitored using annual blood tests to keep track of your hormone levels.

It's important to note that levothyroxine is only available on prescription. It also needs to be taken indefinitely, as without it, your symptoms will return. If you get pregnant, you’ll need to be more closely monitored, so it’s crucial that you speak to your doctor or midwife right away.21

It’s also useful to know that despite what some resources say, there are currently no specific foods or dietary supplements that are proven to be helpful in treating hypothyroidism. Nevertheless, eating a balanced diet that contains a comprehensive mix of nutrients can only benefit your overall health and wellbeing. Making sure you get enough sleep can also be helpful.

The final say

Living with an underactive thyroid can be quite debilitating, not to mention the risk of long-term complications.4,5

If you’re concerned that you may have an underactive thyroid, such as because you’re experiencing some of the symptoms listed here, it’s always best to schedule an appointment with your doctor. They can arrange the necessary tests and help you to take back control of your health.

The advice in this article is for information only and should not replace medical care. Please check with your GP or healthcare professional if you think you have this condition.

Sources

  1. United States Environmental Protection Agency. Overview of the Endocrine System [Internet]. [cited 2024 Feb 21] 2015. Available from: https://www.epa.gov/endocrine-disruption/overview-endocrine-system/
  2. Shahid, Muhammad A., et al. Physiology, Thyroid Hormone. StatPearls, StatPearls Publishing. [cited 2024 Feb 21] 2023. Available from: http://www.ncbi.nlm.nih.gov/books/NBK500006/. 
  3. NHS. Overview: Underactive Thyroid (Hypothyroidism) [Internet]. [cited 2024 Feb 21]. 2021. Available from: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/
  4. NICE National Institute for Health and Care Excellence. Thyroid Disease: Assessment and Management [Internet]. [cited 2024 Feb 21]. 2019. Available from: https://www.nice.org.uk/guidance/ng145/chapter/Context
  5. Rodondi, N., Bauer, D. C., Cappola, A. R., et al. Subclinical Thyroid Dysfunction, Cardiac Function, and the Risk of Heart Failure: The Cardiovascular Health Study. Journal of the American College of Cardiology. 2008;52(14), 1152-1159. Available from: https://doi.org/10.1016/j.jacc.2008.07.009
  6. Ruíz-Pacheco, María Guadalupe, et al. Severity of Fatigue and Its Relationship with TSH before and after Levothyroxine Replacement Therapy in Patients with Primary Hypothyroidism. Biomedicines. Biomedicines. 2023;11(3):811. Available from: https://doi.org/10.3390/biomedicines11030811.
  7. Yaylali, O., Kirac, S., et al. Does Hypothyroidism Affect Gastrointestinal Motility? Gastroenterology Research and Practice. 2009. Available from: https://doi.org/10.1155/2009/529802
  8. Daher, Ronald, et al. Consequences of Dysthyroidism on the Digestive Tract and Viscera. World Journal of Gastroenterology. 2009;15(23):2834. Available from: https://doi.org/10.3748/wjg.15.2834.
  9. Thyroid UK. Signs and Symptoms of Hypothyroidism [Internet]. [cited 2024 Feb 21]. Available from: https://thyroiduk.org/signs-and-symptoms/hypothyroid-signs-and-symptoms/
  10. Farhana, Aisha, and Anis Rehman. Metabolic Consequences of Weight Reduction. StatPearls, StatPearls Publishing, 2024. Available from: http://www.ncbi.nlm.nih.gov/books/NBK572145/.
  11. Sanyal, D., Raychaudhuri, M. Hypothyroidism and obesity: An intriguing link. Indian Journal of Endocrinology and Metabolism. 2016;20(4), 554-557. Available from: https://doi.org/10.4103/2230-8210.183454
  12. Fariduddin, Maria M., and Nidhi Bansal. Hypothyroid Myopathy. StatPearls, StatPearls Publishing, 2023. Available from: http://www.ncbi.nlm.nih.gov/books/NBK519513/.
  13. Jacobson, Melanie H., et al. Thyroid Hormones and Menstrual Cycle Function in a Longitudinal Cohort of Premenopausal Women. Paediatric and Perinatal Epidemiology. 2018;32(3):225–34. Available from: https://doi.org/10.1111/ppe.12462.
  14. Nuguru, Surya P., et al. Hypothyroidism and Depression: A Narrative Review. Cureus. 2022;14(8):e28201. Available from: https://doi.org/10.7759/cureus.28201.
  15. John Hopkins Medicine. What are Common Symptoms of Autoimmune Disease [Internet]. [cited 2024 Feb 21]. 2022. Available from: https://www.hopkinsmedicine.org/health/wellness-and-prevention/what-are-common-symptoms-of-autoimmune-disease/
  16. NIH, National Institute of Environmental Health Sciences. Autoimmune Disease. [Internet]. [cited 2024 Feb 21]. Available from: https://www.niehs.nih.gov/health/topics/conditions/autoimmune
  17. NHS. Causes: Underactive Thyroid (Hypothyroidism) [Internet]. NHS. [cited 2024 Feb 21]. 2019. Available from: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/causes
  18. Sahay, Rakesh Kumar, and V. Sri Nagesh. Hypothyroidism in Pregnancy. Indian Journal of Endocrinology and Metabolism. 2012;16(3):364. Available from: https://doi.org/10.4103/2230-8210.95667.
  19. Alemu, A., Terefe, B., Abebe, M., & Biadgo, B. Thyroid hormone dysfunction during pregnancy: A review. International Journal of Reproductive Biomedicine. 2016;14(11), 677-686. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5153572/
  20. NHS. Diagnosis: Underactive Thyroid (Hypothyroidism) [Internet]. NHS. 2019. [Cited 2024 March 19]. Available from: https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/diagnosis/.
  21. NHS. Levothyroxine [Internet]. NHS. 2021. [Cited 2024 March 19]. Available from: https://www.nhs.uk/medicines/levothyroxine/
 

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