Underactive thyroid; myxoedema
Hypothyroidism is a disease that develops when the thyroid gland is unable to produce sufficient thyroid hormones. ‘Subclinical or mild hypothyroidism’ refers to a condition when the thyroid hormone levels in the blood are within the normal range but the thyroid gland has to work much harder than usual to maintain these levels. People with subclinical hypothyroidism usually do not have symptoms, but a significant proportion will develop hypothyroidism in the future.
In most cases, the cause is autoimmune. This means the body’s immune system mistakenly recognises its own thyroid gland as a foreign body, and therefore attacks and damages it, in some cases destroying it. . Interestingly, in people with autoimmune hypothyroidism the body’s immune system otherwise functions normally, including still being able to fight infection.
Other less common causes of hypothyroidism are due to iodine deficiency (iodine is an essential component of thyroid hormones), excessive dose of medications used to treat hyperthyroidism (an overactive thyroid gland), destruction of the thyroid gland following treatment with radioactive iodine, and surgical removal of the thyroid gland.
Rarely, hypothyroidism can be caused by certain medications (for example lithium and amiodarone) or diseases of the pituitary gland.
The symptoms of hypothyroidism vary greatly from one person to another. Some people may have very few symptoms, while others are severely affected. To some extent these differences can be explained by how severe the hypothyroidism is. For example, in some cases the thyroid is completely unable to produce thyroid hormones while in others it can produce some, but not enough for the body’s needs.
Another reason for the symptoms being so different is the speed at which the hypothyroidism develops. This can be from a matter of days to years. The age of the person is also important. For instance, hypothyroidism in a child may cause poor growth.
The symptoms of hypothyroidism are not specific. Surveys of normal populations have shown that up to a third of all people experience symptoms similar to that of hypothyroidism. This causes confusion and sometimes a wrong belief that a person with otherwise unexplained symptoms must have hypothyroidism. The symptoms of hypothyroidism can include tiredness, weight gain, feeling the cold, slowing down physically and mentally, dry skin, loss of hair, muscle cramps and muscle weakness, puffiness around the eyes, gruff voice, depression, hearing loss and constipation. Women can develop heavy periods or they may lose them altogether. Some people with hypothyroidism develop an enlarged thyroid gland also known as goitre. The dual presence of autoimmune hypothyroidism and goitre is typical of Hashimoto’s thyroiditis, where the goitre is caused by the infiltration of immune cells within the thyroid gland.
Hypothyroidism is common. It affects women more frequently than men and usually at around middle-age. Hypothyroidism affects approximately 1 in 1,000 men and 18 in 1,000 women. Subclinical hypothyroidism is much more common and affects approximately 28 in 1,000 men and 75 in 1,000 women.
People who have a relative with thyroid disease (either hyperthyroidism or hypothyroidism) are at a higher risk of developing hypothyroidism compared with the rest of the population. The genetics of hypothyroidism however are complex and it is not possible to predict if someone will develop hypothyroidism by a genetic test.
People who have other autoimmune conditions, such as type 1 diabetes, rheumatoid arthritis or multiple sclerosis, also have a higher risk of developing hypothyroidism compared with the rest of the population.
Hypothyroidism can also very rarely be seen at birth (known as congenital hypothyroidism). All newborn babies in the UK are screened for congenital hypothyroidism by a heel prick test a few days after they are born. Some causes of congenital hypothyroidism are inherited.
Hypothyroidism should be diagnosed by a doctor. To make an accurate diagnosis, a full medical history is first taken, followed by a physical examination and blood tests. The blood levels of the thyroid hormone thyroxine and thyroid stimulating hormone are measured. If hypothyroidism is confirmed, the levels of antibodies (special proteins produced by the immune system) specific for certain parts of the thyroid gland may also be measured. These tests are carried out on an outpatient basis, most often in a doctor’s surgery.
Hypothyroidism is treated with levothyroxine tablets – a synthetic version of thyroxine given to replace the sub-optimal level of thyroid hormones. The treatment is given on an outpatient basis. Once the levothyroxine is absorbed in the bloodstream, it is converted to triiodothyronine, which is the active thyroid hormone that the tissues and cells require to function. Combination treatment of levothyroxine and triiodothyronine should not be used unless further research published in peer-reviewed journals demonstrates a greater benefit.
Side-effects to levothyroxine treatment are exceptionally rare as levothyroxine is identical to the thyroid hormone naturally produced by the thyroid gland. Hyperthyroidism may occur if the dose is excessive or hypothyroidism may remain if the dose is inadequate.
People who are treated with levothyroxine have a normal life expectancy and lead a normal life. One of the commonest long-term problems is that it is easy to forget to take tablets regularly. Special pill boxes and counting tablets are sometimes helpful strategies. Once patients are on a stable dose of levothyroxine, an annual thyroid blood test is recommended.
The requirements of levothyroxine often increase in the early stages of pregnancy. Women with hypothyroidism who plan to get pregnant or who have just had a positive pregnancy test should have thyroid blood tests and regular monitoring of their thyroid hormones during pregnancy as the dose of levothyroxine needs to be increased usually by 30-50%. Untreated hypothyroidism during pregnancy may have severe consequences on the pregnancy outcome itself (including miscarriage, stillbirth, premature delivery) and cause a permanent impaired neurological development of the baby. Older people may require a reduction in the dose of levothyroxine as the requirements decline with age.
People with subclinical hypothyroidism may be given the option of taking levothyroxine, mainly depending on the presence of symptoms. If not, they require an annual thyroid blood test to determine if they go on to develop hypothyroidism.
Last reviewed: Jan 2019