Signs and symptoms of an overactive thyroid (hyperthyroidism)

An overactive thyroid, also known as hyperthyroidism or thyrotoxicosis, affects between one in 200 to one in 50 people in the UK. It is around 10 times more common in women and usually develops between 20–40 years of age. You are at greater risk of hyperthyroidism if you have type 1 diabetes, pernicious anaemia or primary adrenal insufficiency.

To understand how hyperthyroidism can affect your body, it first helps to know about the function of your thyroid. 

What does your thyroid do?

Your thyroid is a butterfly-shaped gland that produces the thyroid hormones, thyroxine and triiodothyronine. It is located in front of your windpipe in your neck and the hormones it produces affect your metabolism ie your ability to turn food into energy. 

Consequently, your thyroid has a role to play in the health of your brain, bones, digestive system, heart and muscles. 

An overactive thyroid produces excessive amounts of thyroid hormones, in contrast to an underactive thyroid (hypothyroidism) where not enough thyroid hormones are produced. 

What causes hyperthyroidism?

The most common cause of hyperthyroidism, accounting for three-quarters of all cases, is Graves’ disease, which usually develops in women between 20–40 years of age. This is an autoimmune condition, which means your immune system mistakenly attacks your healthy cells.

In the case of Graves’ disease, your immune system attacks thyroid cells. It does this by creating antibodies that stick to thyroid cells and mimic the effects of thyroid stimulating hormone (TSH).

TSH is produced by your pituitary gland, which sits at the base of your brain. This gland detects levels of thyroid hormone in your blood. When levels are too low, it produces TSH to direct your thyroid to produce more thyroid hormones. Grave’s disease mimics this effect irrespective of the levels of thyroid hormones in your blood.

Hyperthyroidism can also be caused by thyroid nodules. These lumps can sometimes include extra thyroid tissue, causing the excess production of thyroid hormones. Thyroid nodules usually occur in those aged over 60 years.

The antiarrhythmic medication, amiodarone, can also cause hyperthyroidism as it contains iodine, which can stimulate thyroid hormone production. Amiodarone can, conversely, also cause hypothyroidism.

Other causes of hyperthyroidism include thyroid cancer, swelling of your thyroid (thyroiditis), a noncancerous tumour in your pituitary gland (pituitary adenoma) and, in some pregnant women, high levels of the hormone human chorionic gonadotropin.

Symptoms of hyperthyroidism

As thyroid hormones affect your whole body, hyperthyroidism can affect many different organs and body systems. 

Hyperthyroidism can make you hyperactive, lose interest in sex, experience mood swings, and feel anxious, irritable and nervous. 

You may also experience persistent thirst, fatigue, an irregular heartbeat (arrhythmia), fast heart rate (palpitations), difficulty sleeping and muscle weakness. Your body may become more sensitive to heat, with excessive sweating, red palms and warm skin. Your skin may develop a raised itchy rash (hives), your hair may thin and your nails may become loose. 

Hyperthyroidism can also affect your eyes, causing them to become red and dry, which can affect your vision. Your toileting habits may also change, with frequent diarrhoea and more frequent urination. 

You may tremble or twitch uncontrollably, unintentionally lose weight despite an increase in your appetite, and develop a swollen thyroid (goitre), which can make swallowing difficult (dysphagia). 

In women, hyperthyroidism can cause irregular or lighter periods, while in men, it can lead to a reduced sperm count and consequently reduced fertility.

If you notice any of these symptoms, it is important to see your GP to get a diagnosis and treatment, as left untreated, hyperthyroidism can cause complications. 

Complications of hyperthyroidism

Hyperthyroidism can lead to arrhythmia, which is why if left untreated, it increases your risk of cardiovascular problems including blood clots, heart failure and stroke. 

It can also lead to thyroid eye disease, where your eyes become red, swollen and bulging, and your eyelids retract. This can cause vision problems, such as double vision, and, in one in every 20–30 cases, results in vision loss. 

Untreated hyperthyroidism is linked to brittle bone disease (osteoporosis) where your bones become less dense and more likely to fracture.

In pregnancy, untreated or uncontrolled hyperthyroidism increases your chances of miscarriage, premature birth, pre-eclampsia (high blood pressure during and after pregnancy) and low birth weight of your baby. 

In very rare cases, untreated or uncontrolled hyperthyroidism can cause a thyroid storm. Symptoms include a rapid heartbeat, agitation, confusion, diarrhoea, fever, loss of consciousness, vomiting and yellowing of your skin and eyes (jaundice). This needs emergency medical treatment. 

Getting a diagnosis

Hyperthyroidism 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, and for thyroxine, one of the two thyroid hormones produced by your thyroid. 

In hyperthyroidism, TSH levels are low. This is because your pituitary gland detects that levels of thyroid hormones in your blood are high and consequently, produces less TSH in an effort to prevent your thyroid gland from producing more thyroid hormones. 

Your thyroxine levels will be high if you have hyperthyroidism. 

If your results show that you have hyperthyroidism, you may need further blood tests to determine the underlying cause of your overactive thyroid. This may include testing for antibodies that attack thyroid cells, which indicates Graves’ disease, or a blood test called erythrocyte sedimentation rate to check for inflammation, which indicates thyroiditis. 

In rare cases, depending on your physical symptoms, your doctor may recommend a thyroid ultrasound scan to check for thyroid nodules. 

Hyperthyroidism treatments

If you’re diagnosed with hyperthyroidism, you will be referred to a doctor who specialises in treating hormonal conditions (an endocrinologist). Depending on the cause of your overactive thyroid, there are three main treatments: medication, radioactive iodine treatment and surgery.

As hyperthyroidism medication and radioactive iodine treatment can take a while to work, you may be given beta blockers, a drug that will reduce some of your symptoms, in the meantime.

Medication

The two main drugs that reduce thyroid hormone production are carbimazole and propylthiouracil. It takes one to two months for these drugs to take effect. In some cases, medication must be taken for a lifetime but in other cases, it can be gradually reduced and stopped.

Radioactive iodine treatment

This is usually a one-off treatment, which involves taking a capsule or drink that contains iodine and a low dose of radiation. As your thyroid is the only tissue in your body that can absorb iodine, radioactive iodine will accumulate in your thyroid. Consequently, over several weeks or months, your thyroid cells will be destroyed, reducing the production of thyroid hormones.

In some cases, this can cause hypothyroidism but this can be effectively treated with hormone replacement treatment in the form of a daily tablet called levothyroxine.

Thyroid surgery

If other treatments aren’t successful or suitable, or if you have severe goitre, severe eye problems relating to your hyperthyroidism, or thyroid cancer, part or all of your thyroid may be removed. After surgery, you will need to take levothyroxine, a daily tablet used to treat hypothyroidism, for the rest of your life.