Hypothyroidism: underactive thyroid causes and symptoms explained

An underactive thyroid, also known as hypothyroidism, affects around two in every 100 people in the UK and is more common in women and those aged over 60. Here we’ll look at its causes, symptoms and treatment but first, it helps to understand what your thyroid does. 

The role of your thyroid

Your thyroid is a butterfly-shaped gland that sits in front of your windpipe in your neck. It produces the hormones, thyroxine and triiodothyronine, which control how your body turns food into energy (your metabolism). Consequently, your thyroid affects many different parts of your body, including your brain, bones, digestive system, heart and muscles.

The amount of hormones produced by your thyroid is regulated by your pituitary gland, which is a pea-sized gland at the base of your brain. Your pituitary gland detects levels of thyroid hormones in your blood and if the levels are too low, it produces a hormone called thyroid-stimulating hormone (TSH). TSH triggers your thyroid to produce more thyroid hormones.

If thyroid hormone levels in your blood are too high, your pituitary gland produces less TSH, which consequently decreases hormone production by your thyroid.

What is an underactive thyroid?

An underactive thyroid doesn’t produce enough thyroid hormones, namely thyroxine and triiodothyronine. This affects multiple organs and systems in your body, causing symptoms of hypothyroidism.

An underactive thyroid is the opposite of an overactive thyroid, where the gland produces excessively high amounts of thyroid hormones and causes hyperthyroidism.

Causes of hypothyroidism

Hypothyroidism can be caused by surgery to remove part of the thyroid or radioactive iodine treatment to destroy cells in the thyroid. Both of these treatments are used to treat thyroid cancer and hyperthyroidism. However, the most common cause of hypothyroidism is an autoimmune condition called Hashimoto’s disease.

Your immune system and hypothyroidism

A healthy immune system should only attack harmful substances or organisms (eg bacteria, viruses and fungi), or dead or damaged cells. However, autoimmune conditions cause your immune system to mistakenly attack your own healthy cells. This is what occurs in Hashimoto’s disease, where your immune system attacks your thyroid. Over time, this reduces its ability to produce thyroid hormones.

Hashimoto’s disease is four to 10 times more common in women and usually develops between the ages of 30 to 50 years. You are more likely to develop Hashimoto’s disease if you have a family history of the disease and/or if you have other autoimmune conditions, such as type 1 diabetes, rheumatoid arthritis and coeliac disease. 

Rare causes of hypothyroidism

In the UK, iodine deficiency is rare but worldwide, iodine deficiency is one of the most common causes of hypothyroidism. This is because iodine is needed to make the hormone thyroxine, which is in turn needed to make the hormone triiodothyronine, both of which are produced by your thyroid.

Other rare causes of hypothyroidism in the UK include damage to the pituitary gland and certain medications (ie the antidepressant lithium, the antiarrhythmic drug amiodarone and interferons, which can be used to treat cancer and hepatitis C).

In the UK, around one in every 3,500 babies is born with congenital hypothyroidism, where the thyroid doesn’t develop properly.

Signs of an underactive thyroid

Symptoms of hypothyroidism often develop gradually as the ability of your thyroid gland to produce hormones slowly decreases. As the symptoms also overlap with other conditions, you may not immediately recognise that you have a problem with your thyroid. 

Mental symptoms include feeling depressed, slow thinking (or brain fog), forgetfulness and a loss of interest in sex. Physical symptoms include: 

• Constipation 
• Dry, scaly skin, and/or brittle hair and nails
• Fatigue and slow movements
• Sensitivity to the cold
• Unintentional weight gain
• Weak, achy and cramping muscles

You may also develop carpal tunnel syndrome — signs include pain, numbness and tingling in your hands and fingers. 

If you are a woman, your periods may become irregular or heavy. 

In children, growth and development may slow down, and in teenagers, puberty may start sooner. 

If you notice any of these symptoms, see your GP. Based on your symptoms and medical history, they may recommend you get tested for hypothyroidism. 

Getting a diagnosis

Hypothyroidism is diagnosed using a blood test called a thyroid function test. 

A sample of your blood will be collected and sent to a lab to test for TSH, the hormone that tells your thyroid to produce thyroid hormones. 

In most cases of hypothyroidism, TSH levels are raised. This is because your pituitary gland detects that levels of thyroid hormones in your blood are low and consequently, produces more TSH to stimulate your thyroid to increase its production of thyroid hormones.

In children, if high TSH levels are detected, levels of thyroxine may also be measured as this hormone is important for brain development. 

If your test results indicate that you have hypothyroidism, your doctor may recommend hormone replacement treatment. It is important not to delay treatment as untreated hypothyroidism can lead to other health conditions and complications. 

Complications of untreated hypothyroidism

Left untreated, hypothyroidism can lead to high cholesterol and consequently, cardiovascular disease. You may also develop goitre, where your thyroid gland swells, which can cause tightness around your neck and difficulty swallowing (dysphagia). 

In extreme cases, where your thyroid hormone levels become very low, you can become confused and drowsy, and develop hypothermia. This is called myxoedema coma and needs emergency medical treatment. 

Untreated hypothyroidism during pregnancy poses risks to both the mother and baby. 

Hypothyroidism in pregnancy

During pregnancy, if left untreated, hypothyroidism can lead to premature birth, stillbirth or miscarriage.

In the mother, untreated hypothyroidism can also cause: 

• Anaemia
• Excessive bleeding after birth 
• Pre-eclampsia ie high blood pressure during and after pregnancy — this can cause you to retain fluid, which may affect the growth of your baby

Untreated hypothyroidism in the mother can cause mental and physical problems in the baby's development, including birth defects, an underactive thyroid and low birth weight. 

If you have hypothyroidism or suspect you may have hypothyroidism and are pregnant or trying to get pregnant, it is important to tell your doctor so you can receive appropriate treatment. 

Hypothyroidism treatment

Hypothyroidism is treated using hormone replacement in the form of daily levothyroxine tablets. Levothyroxine replaces thyroxine — this is one of the two hormones produced by your thyroid. As your thyroid makes triiodothyronine from thyroxine, once your thyroxine levels are restored to normal levels, your triiodothyronine levels will also return to normal.

Your doctor will first prescribe a low dose of levothyroxine. You will have regular blood tests and your symptoms will be monitored to check if this dose is effective. If not enough improvement is seen in your symptoms and hormone levels, you may need to take a higher dose.

Once the appropriate levothyroxine dose is identified, you will continue to have annual blood tests to monitor your hormone levels.