Thyroid dysfunction as a health risk: underactive and overactive thyroid explained

The thyroid is a small butterfly-shaped gland in the neck, just in front of the windpipe. The NHS explains that the hormones it produces affect things such as heart rate and body temperature. Because those hormones act on almost every tissue, a gland that runs too slowly or too fast can influence the heart, bones, mood, weight, periods and fertility.

Thyroid dysfunction is therefore treated in medicine both as a condition in its own right and as a risk factor: a state that raises the chance of other health problems if it is left unrecognised or poorly controlled. The good news, as the NHS stresses, is that an underactive thyroid is not usually serious when it is properly treated, and an overactive thyroid is usually treatable.

This page looks at the risk side of the picture. For the full condition guides, see the related pages at the end of this article.

The NHS says symptoms of an underactive thyroid may be mild and may not be easy to notice, and that they usually develop slowly and get worse over time. Many of them are common and can have other causes, which is why a blood test is needed rather than guessing.

• Feeling extremely tired and feeling cold more than usual.

• Putting on weight and constipation.

• Difficulty concentrating or thinking clearly, and low mood or depression.

• Dry skin, dry hair or hair loss, and a croaky voice.

• Irregular or heavy periods.

According to the NHS, if an underactive thyroid is not treated, or you are on the wrong dose of medicine, you may have an increased risk of problems with your heart, including coronary heart disease and heart failure, and problems with your bones, including osteoporosis. In very rare cases a severe underactive thyroid may lead to myxoedema coma, a life-threatening condition that is more common in older people and is an emergency needing treatment in hospital.

The NHS also says that an untreated underactive thyroid during pregnancy can increase the chance of serious complications, including high blood pressure (pre-eclampsia), birth defects, premature birth or miscarriage, and that treatment usually helps prevent these complications.

An overactive thyroid, also called hyperthyroidism or thyrotoxicosis, can affect anyone. The NHS says it is about 10 times more common in women than men and typically happens between the ages of 20 and 40. Symptoms listed by the NHS include nervousness, anxiety and irritability, mood swings, difficulty sleeping, persistent tiredness and weakness, sensitivity to heat, swelling in the neck from an enlarged gland (goitre), palpitations, twitching or trembling, and weight loss.

The NHS lists eye problems, pregnancy complications and thyroid storm as the main further problems. Eye problems, known as thyroid eye disease, affect around 1 in 4 people whose overactive thyroid is caused by Graves' disease, and can include dry gritty eyes, double vision and bulging eyes; severe disease can threaten vision. An overactive thyroid can also increase the chance of atrial fibrillation, osteoporosis and heart failure.

Thyroid storm is rare. It is a sudden flare-up of symptoms in an undiagnosed or poorly controlled overactive thyroid and can be triggered by infection, pregnancy, not taking medicine correctly or damage to the gland. The NHS says symptoms include a rapid heartbeat, high temperature, high blood pressure, jaundice, severe agitation and confusion and loss of consciousness, and that it is a medical emergency: call 999.

In the UK, the main cause of an underactive thyroid is an autoimmune condition called Hashimoto's disease, in which the immune system attacks the thyroid. The NHS says it usually starts between 30 and 50, although anyone can be affected. Other causes include thyroiditis, surgery or radiotherapy around the neck, treatment for an overactive thyroid, certain medicines and, rarely in the UK, not getting enough iodine. You are also more likely to have an underactive thyroid if others in your family have had a thyroid condition.

For an overactive thyroid, the NHS names Graves' disease as the commonest cause, accounting for about 4 in every 5 cases. Lumps (nodules) on the thyroid can produce hormone, and some medicines, such as amiodarone used for irregular heartbeats, can also be responsible.

A GP will ask about your symptoms and about whether anyone in your family has had a thyroid or autoimmune condition or whether you have recently been pregnant, and may feel your neck to check for swelling. The NHS says blood tests then check thyroid-stimulating hormone (TSH) and thyroxine (T4).

In an overactive thyroid, the NHS says a low TSH and high T3 and/or T4 usually means the gland is overactive. What counts as normal varies with age and with the laboratory technique. If hormone levels are high you may be referred for tests to find the cause, such as an anti-thyroid antibody blood test, an ESR test for inflammation, or a thyroid scan to look for nodules.

An underactive thyroid is usually treated with levothyroxine tablets to replace the missing hormone, normally for life. The NHS says you should be able to live a normal life, and that regular blood tests check you are on the right dose, because the wrong dose can cause side effects such as feeling or being sick, diarrhoea, headaches or problems sleeping.

An overactive thyroid is usually treatable with medicine that stops the gland producing too much hormone, radioiodine treatment, or surgery to remove some or all of the thyroid. The NHS says each has benefits and drawbacks and you will usually see an endocrinologist to decide. Radioiodine often leaves the thyroid underactive afterwards, sometimes permanently, which then needs levothyroxine.

The NHS stresses that carbimazole and radioactive iodine treatment within the last 6 months can harm an unborn baby, so reliable contraception is important, and that you should tell your doctor immediately if you plan a pregnancy or become pregnant.

Most people with thyroid dysfunction are looked after by their GP, with referral to an endocrinologist for an overactive thyroid, thyroid eye disease, or when the picture is complicated. Keeping up with regular blood tests is the single most useful thing you can do to keep the long-term risks low. Charities such as the British Thyroid Foundation and Thyroid UK offer information and support, as the NHS notes.

Some people also choose to see a private endocrinologist, usually for quicker appointments or a second opinion. The tests and treatments are the same ones used in the NHS, so the practical difference is mostly speed and choice. Whichever route you use, make sure one clinician is responsible for monitoring your hormone levels over time.