Hypothyroidism (sometimes referred to as an "underactive thyroid") is any state in which the production of thyroid hormones is insufficient to meet the needs of the body, resulting in a range of symptoms. The condition can be due to a deficiency of iodine, a problem with the functioning of the pituitary gland or hypothalamus, or a problem with the thyroid gland itself.
Hypothyroidism can be accurately diagnosed with thyroid function tests and successfully treated with synthetic thyroid hormone.
The thyroid gland
The thyroid gland is part of the endocrine system - a network of 10 hormone-producing glands that help to regulate most functions in the body. It is located below the Adam’s apple at the front of the neck. The thyroid's job is to make thyroid hormones, which are secreted into the blood and then carried to tissue around the body. Thyroid hormone helps the body use energy, stay warm and keeps the brain, heart, muscles, and other organs working as they should. A well-functioning thyroid gland is essential for normal health and wellbeing.
The functioning of the thyroid gland is regulated by the pituitary gland - a small endocrine gland attached to the base of the brain. The pituitary gland produces thyroid stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroid hormones in response to the body's needs. The pituitary gland is in turn regulated by the hypothalamus - a small area of the brain just above the pituitary gland. The hypothalamus produces thyrotropin releasing hormone (TRH), which stimulates the pituitary gland to release TSH.
Signs and symptoms
Hypothyroidism is sometimes referred to as a “silent” disease because the early symptoms develop slowly over many years and may be so mild that they go unnoticed. The symptoms can also closely resemble those produced by other medical conditions, especially in the elderly and those with depression. Adults who develop hypothyroidism will probably have more than one of the following symptoms:
- Weakness, lethargy, fatigue
- Weight gain or difficulty losing weight
- Lack of appetite
- Thinning or coarse, dry hair
- Brittle nails
- Dry, coarse skin
- Intolerance to cold
- Muscle cramps
- Depression or irritability
- Altered menstrual cycles
- Poor memory
- Loss of sex drive
- Bradycardia (slow heartbeat)
- An enlarged thyroid gland (goitre).
These symptoms will vary in severity depending on the extent and duration of the hypothyroidism.
Although hypothyroidism mainly occurs in adults, it can also occur in infants, children, and teenagers. Infants with hypothyroidism may have few signs and symptoms. If symptoms are present they may include:
- Jaundice (yellow skin and whites of the eyes)
- Frequent choking
- A large, protruding tongue
- A puffy face
- Difficulty feeding
- Failure to thrive and develop normally
- Poor muscle tone
- Excessive sleepiness.
Children and teenagers who develop hypothyroidism generally experience the same signs and symptoms as adults but may also have the following:
- Slow growth rate, resulting in below average height
- Delayed start of puberty
- Delayed development of permanent teeth
- Impaired mental development.
Hypothyroidism is twice as common in women as it is in men. It is most common in middle age or older, affecting approximately 10% of women over the age of 60 years. A family history of thyroid problems makes an individual more susceptible to hypothyroidism, as do obesity and raised blood cholesterol levels.
A significant proportion of people who develop hypothyroidism have no known risk factors. However, people with autoimmune diseases including systemic lupus erythematosus (SLE), rheumatoid arthritis, type 1 diabetes, Addison’s disease and pernicious anaemia are at a higher risk of developing the condition. It is also more common in people whose hair goes prematurely grey.
There are two main types of hypothyroidism - primary and secondary.
When a disease process acts directly on the thyroid gland reducing the hormone output, it is called primary hypothyroidism. Thyroiditis (inflammation of the thyroid gland) is the most common cause of primary hypothyroidism; and the most common type of thyroiditis is Hashimoto’s thyroiditis. This is an inherited autoimmune disorder - where the immune system mistakenly attacks thyroid tissue causing inflammation, which eventually destroys the gland. Hashimoto’s thyroiditis often affects women following childbirth.
Other causes of primary hypothyroidism include:
- Surgical removal of the gland because of cancer or another disorder
- Radioactive iodine used to treat an overactive thyroid (hyperthyroidism) can damage the thyroid gland
- Congenital hypothyroidism (also known as cretinism), affecting one in 4000 babies who are without a properly functioning thyroid gland
- Viral or bacterial infections that temporarily depress or stop hormone production in the thyroid. Sometimes this can become permanent
- Some medications that induce temporary hypothyroidism; for example, lithium, which is used to treat some mental illnesses
- A diet severely deficient in iodine (a mineral essential for the production of thyroid hormones found in seafood, seaweed, plants grown in iodine-rich soil and iodized salt)
- Certain industrial chemicals have been known to cause hypothyroidism.
Secondary hypothyroidism is rare and arises from the pituitary gland not producing enough thyroid stimulating hormone (TSH). This can be caused by abnormalities or disease process affecting the pituitary gland (eg: pituitary tumours).
Sometimes a problem in the hypothalamus, the part of the brain that controls the whole endocrine system, can also result in hypothyroidism.
The diagnosis of hypothyroidism is based on a person’s symptoms and medical history, a physical examination of the thyroid gland, and thyroid function tests.
Initially a blood test called a TSH assay is done. This test is very sensitive and measures the level of TSH in the blood. With primary hypothyroidism, the TSH is abnormally high as the pituitary gland produces more TSH in an effort to stimulate thyroid gland function. It is possible for the TSH to be elevated even before symptoms of thyroid hormone deficiency appear. In cases of secondary hypothyroidism, however, the TSH may be abnormally low, due to decreased production of TSH by the pituitary gland.
Infants are routinely tested in the first few days after birth for the congenital form of thyroid deficiency (cretinism). This is done by a heel prick blood test. If hypothyroidism in infants isn't treated, it can lead to severe physical and mental retardation.
Hypothyroidism cannot be cured, but it can be controlled. Replacement thyroxine, given in tablet form, is an effective treatment for most people with hypothyroidism. A synthetic hormone called levothyroxine is given in a daily dose. Other medications containing iron or antacids should be avoided as they can interfere with absorption of the levothyroxine. The main side effect of this medication is a rapid heartbeat with palpitations.
A few weeks after thyroxine replacement therapy is started, the TSH assay blood test is repeated. The amount of levothyroxine prescribed is adjusted depending on the TSH level. In older people, a lower starting dose is usually given, to avoid symptoms affecting the heart. This is slowly increased every few weeks until the TSH has fallen. If heart symptoms increase, levothyroxine is given at the lowest possible level so that symptoms are minimised.
Women who are undergoing menopausal hormone therapy (formerly known as hormone replacement therapy or HRT) or taking the contraceptive pill may need a higher dose of levothyroxine, and conversely a reduction when MHT is discontinued. This is because increased oestrogen levels decrease the uptake of thyroxine by the body.
Once stabilised, levothyroxine is usually taken for life with monitoring by blood test every 6-12 months. Symptoms gradually disappear and even in those more severely affected a few months of treatment usually relieve dry skin, tiredness, brittle nails and other symptoms.
Those with borderline hypothyroidism are often treated because there is evidence that they have an increased risk of coronary artery disease.
In infants diagnosed with congenital hypothyroidism, treatment with thyroid hormones in the first month of life can prevent problems, such as mental retardation, from occurring. Delayed growth and learning difficulties in older children and adolescents with low thyroid hormone are reversible with treatment.
Hypothyroidism that is allowed to progress untreated may develop into a condition called myxoedema. This is a rare and potentially fatal form of the disease where the symptoms can include enlarged tongue, swollen face, hoarseness and extreme lethargy, which may progress to a coma. Myxoedema requires immediate hospitalisation and treatment with thyroid hormones given directly into a vein.
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