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What is an underactive thyroid?

Understand what an underactive thyroid is and how it can be treated.

What is an underactive thyroid?

An underactive thyroid gland, also referred to as hypothyroidism, is a condition in which your thyroid does not produce enough hormones (1).

The thyroid is a small gland positioned in front of your windpipe. It creates hormones thatregulate your heartbeat, digestive system and many other bodily functions. The hormones are called triiodothyronine (T3) and thyroxine (T4), and without these, your body begins to slow down.

What causes an underactive thyroid?

An underactive thyroid can occur in both men and women at any age but is more common in women over 60. 1 in 20 women of this age have hypothyroidism (2). Some babies can also be born with hypothyroidism, so the UK uses blood spot tests to screen babies at five days old.

Hypothyroidism could have many causes, including:

  • Immune system – autoimmune thyroid disease is a process in which your immune system attacks your thyroid gland, reducing its ability to produce hormones. The common name for this is Hashimoto’s thyroiditis.
  • Response to treatments – some cancer treatments that target the neck and head can damage your thyroid glands. Or, if an overactive thyroid gland is removed, you may need replacement hormones for life.
  • Certain medicines – Medicines and supplements with large amounts of iodine, lithium used to treat mental health, and amiodarone used to treat heart problems could all cause hypothyroidism (3).

What are the symptoms?

Low thyroid hormones result in your mental and physical processes slowing down. The symptoms of anunderactive thyroid are often similar to other conditions and develop slowly, so may go unnoticed for years. Common symptoms include:

  • Fatigue/tiredness
  • Weight gain
  • Sensitivity to the cold
  • Dry hair and skin
  • Muscle aches
  • Depression
  • Slow movements and thoughts
  • Low libido
  • Constipation

The intensity in which signs and symptoms appear indicates the severity of the condition. However, many early symptoms are common with ageing, which often leads to women and doctors mistaking hypothyroidism with menopause (4).

How is hypothyroidism diagnosed?

Typically, this condition is diagnosed with a consultation, physical examination and blood tests.

Your doctor will evaluate your medical history and check for physical signs such as dry skin, slow reflexes, swelling, and a slow heart rate (5). If you have any thyroid or autoimmune conditions in your family, it is important to tell your doctor.

A blood test is a very accurate method of diagnosing an underactive thyroid. This condition causes high levels of a thyroid-stimulating hormone (TSH) and low levels of thyroxine (T4) (6). However, there is a spectrum in thyroid disorders, so other tests may be necessary.

Check out Medichecks’ range of thyroid tests or read our thyroid buying guide if you need help choosing which test is most suitable for you.

How can it be treated?

An underactive thyroid is a lifelong condition, but the symptoms can be treated with daily hormone medication. Lifestyle changes may also help you to control your symptoms.

Levothyroxine is hormone replacement medication which raises your thyroxine (T4) levels to be normal. It has minimal side-effects when taken in the right dose. Dosages are dependant on your weight and blood results. Your doctor will take regular blood tests to measure the correct dose and then monitor your hormone levels annually. Once you have begun treatment and your hormones are restored, your symptoms should improve within several weeks.

A balanced diet can support your thyroid health. Your thyroid needs iodine to function, which isfound infish, shellfish, dairy, eggs and fortified plant milks.Take care with iodine quantities, as having too much can be as harmful to the thyroid as can too little.

What are the related health risks?

An underactive thyroid can cause problems if it is not treated appropriately or early enough. The condition causes the heart to slow down, and with time, can weaken it or cause permanent damage (8).

Hypothyroidism also commonly occurs with other autoimmune diseases such as

  • Coeliac disease
  • Diabetes
  • Rheumatoid arthritis
  • Lupus (SLE)
  • Adrenal gland disorders

Women with the condition can have healthy pregnancies; however, if they are not undergoing treatment, they may face some complications. Uncontrolled hypothyroidism in pregnancy can cause a range of problems including miscarriage, pre-eclampsia and developmental problems in the newborn child. If you are planning a pregnancy and have an underactive thyroid, it is important to check that your hormone levels are normal. As soon as you are pregnant, you should visit your GP to discuss arranging antenatal care. It is likely that your levothyroxine dose will increase and your target TSH level will be lower than before (9).

What next?

If you have any concerns or are experiencing any symptoms related to an underactive thyroid, visit your doctor. Medichecks also provides a range of thyroid tests and guides for further support and information.


1. NHS (2018). Underactive thyroid (hypothyroidism). [online] Available at: [Accessed: 02/03/20].

2. NICE (2017) Thyroid disease: assessment and management. [online] Available at: [Accessed: 13/03/20].

3. British Thyroid Foundation (2018). Hypothyroidism. [online] Available at: [Accessed 02/03/20].

4. Koumourou, R (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway. P. 213.

5. Holland, K (2017). Everything You Need to Know About Hypothyroidism. [online] Healthline. Available at: [Accessed 02/ 03/20].

6. British Thyroid Foundation (2018). Hypothyroidism. [online] Available at: [Accessed 02/03/20]. 7. Otun, J. et al. (2019) Systematic Review and Meta-Analysis on the Effect of Soy on Thyroid Function. [online] Scientific Reports 9, no. 1. 3964. Available at: [Accessed: 13/03/20].

8. Koumourou, R (2004) Running on Empty: Hypothyroidism, Introduction to an Underactive Thyroid Gland. Melbourne: GN + EJ Ridgway. P. 213.

9. British Thyroid Foundation (2018). Hypothyroidism. [online] Available at: [Accessed 02/03/20].