Underactive thyroid (hypothyroidism): symptoms, causes and treatment

The NHS says symptoms may be mild and may not be easy to notice, and that they usually develop slowly and get worse over time. NICE Clinical Knowledge Summaries notes that signs may be especially mild and non-specific in older people, including fatigue, cold intolerance, weight gain, constipation and depression.

• feeling extremely tired (fatigue)

• feeling cold more than usual

• putting on weight

• constipation

• difficulty concentrating or thinking clearly

• low mood or depression

• dry skin, dry hair or hair loss

• a croaky (hoarse) voice

• irregular or heavy periods

The NHS says it is usually treated with medicine to replace the thyroid hormone your body cannot produce, usually levothyroxine tablets. You will usually need to take them for the rest of your life to prevent symptoms returning, and you should be able to live a normal life.

You will have regular blood tests to check you are on the right dose. The NHS says being on the wrong dose can cause side effects such as feeling or being sick, diarrhoea, headaches or problems sleeping, so talk to your GP or pharmacist if you have any. NICE Clinical Knowledge Summaries advises reviewing symptoms and thyroid function tests every 3 months after starting or changing the dose, and checking TSH annually once stable.

The NHS says a GP will ask about symptoms, whether someone else in your family has had a thyroid or autoimmune condition, and whether you have recently been pregnant. They may examine the neck to see if the thyroid is swollen, and arrange blood tests to check how much TSH and T4 is in your blood. They may refer you to a specialist for more tests.

NICE Clinical Knowledge Summaries describes overt hypothyroidism as a TSH above the normal range with free T4 below the normal range, and subclinical hypothyroidism as TSH above the normal range with T3 and T4 within the normal range.

The NHS says that in the UK the main cause is an autoimmune condition called Hashimoto's disease, where the immune system attacks the thyroid gland. It is most common in women and usually starts between the ages of 30 and 50, though anyone can get it.

Other causes listed by the NHS include another thyroid condition such as thyroiditis, surgery or radiotherapy around the neck or thyroid, treatment for an overactive thyroid, not getting enough iodine in the diet (rare in the UK) and treatment with certain medicines. You are also more likely to have it if others in your family have had a thyroid condition.

The NHS says an underactive thyroid is not usually serious if you are having the right treatment. If untreated, or on the wrong dose, you may have an increased risk of heart problems (including coronary heart disease and heart failure) and bone problems such as osteoporosis. In very rare cases a severe underactive thyroid can lead to myxoedema coma, a life-threatening emergency that needs treatment in hospital and is more common in older people.

The NHS says that if you have an underactive thyroid and are pregnant or planning to get pregnant you should tell your GP. You will need further tests, and your levothyroxine dose is likely to need to change during pregnancy. If untreated it can increase the chance of serious complications in pregnancy, including pre-eclampsia, birth defects, premature birth or miscarriage, but treatment usually helps prevent these.

NICE Clinical Knowledge Summaries advises referral to an endocrinology specialist for women with overt or subclinical hypothyroidism who are planning a pregnancy, are pregnant (urgent thyroid tests are needed), or are postpartum and were treated in pregnancy.

Putting on weight is a common symptom, according to the NHS. The NHS lists not getting enough iodine in the diet as a rare cause in the UK. The NHS and NICE pages read for this guide do not describe a specific diet for hypothyroidism, so if you are worried about your weight or diet, ask your GP or a dietitian, and make sure your levothyroxine dose is being checked.

The sources read for this guide describe hypothyroidism mainly in adults. NICE Clinical Knowledge Summaries lists screening and assessment pathways in its guidance. If you are concerned that a child might have an underactive thyroid, for example because of slow growth, tiredness or constipation, speak to a GP.

The NHS says to see a GP if you think you might have an underactive thyroid, because some symptoms are common and can be caused by other conditions, so it is important to get them checked.

On the NHS, a GP can diagnose and treat hypothyroidism, with blood tests and long-term monitoring of levothyroxine. NICE Clinical Knowledge Summaries lists endocrinology referral for suspected secondary hypothyroidism, when specialist management is needed, or when planning pregnancy. Private blood tests and endocrinology consultations are available to people who choose to pay or have insurance, but ongoing monitoring is usually shared with the GP, so ask how it will be arranged.