Hypothyroidism is any state in which the production of thyroid hormones is insufficient to meet the needs of the body. It may be referred to as an "underactive thyroid". 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 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.
Signs and symptoms
Hypothyroidism is sometimes referred to as a “silent” disease because the early symptoms 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. Someone who has 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.
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
, 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.
Infants are routinely tested in the first few days after birth for a congenital form of thyroid deficiency called cretinism. This is done by a heel prick blood test. Treatment with thyroid hormones in the first month of life can prevent problems, such as mental retardation, from occurring. Older children and adolescents low in thyroid hormone can experience delayed growth and learning difficulties that are reversible with treatment.
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. If the problem lies with the pituitary gland not producing enough thyroid stimulating hormone (TSH), it is called secondary hypothyroidism.
Sometimes a problem in the hypothalamus, the part of the brain that controls the whole endocrine system, can also result in hypothyroidism.
Causes of primary hypothyroidism
Thyroiditis (inflammation of the thyroid gland)
This is the most common cause of primary hypothyroidism. 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.
Surgical removal of the gland because of cancer or another disorder will result in hypothyroidism.
Radioactive iodine used to treat an overactive thyroid (hyperthyroidism), can damage the thyroid gland.
One in 4000 babies is born without a properly functioning thyroid gland.
Viral or bacterial infections
These can temporarily depress or stop hormone production in the thyroid. Sometimes this can become permanent.
Some medications can induce temporary hypothyroidism.
A diet severely deficient in iodine can lead to hypothyroidism.
Certain industrial chemicals have been known to cause hypothyroidism.
Causes of secondary hypothyroidism
Any abnormality or disease process of the pituitary gland (eg: pituitary tumours) can result in insufficient TSH being produced to activate the thyroid. This is a rare cause of hypothyroidism.
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 keep 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.
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.
A specialised thyroid scan called a "radioactive iodine uptake scan" may also be performed. In this test, a small amount of radioactive iodine is given - either by mouth or as an injection into a vein. A special x-ray camera is then used to track its spread in the thyroid gland. This test is often done in addition to an ultrasound scan of the thyroid gland in order to assist with an accurate diagnosis.
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 thyroxine sodium is given in a daily dose, usually in the range of 0.05 - 0.15mg per day. It is usually recommended that it be taken in the morning, thirty minutes before eating. Other medications containing iron or antacids should be avoided as they can interfere with absorption of the thyroxine sodium. 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 thyroxine sodium prescribed is adjusted depending on the TSH level. In older people, a lower starting level of 0.025mg a day 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, thyroxine sodium is given at the lowest possible level so that symptoms are minimised.
Women who are undergoing Hormone Replacement Therapy (HRT) or taking the contraceptive pill may need a higher dose of thyroxine sodium, and conversely a reduction when HRT is discontinued. This is because increased oestrogen levels decrease the uptake of thyroxine by the body.
Once stabilised, thyroxine sodium 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.
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.
American Thyroid Association (2012). Hypothyroidism (Pamphlet).
Anderson, K.N., Anderson, L.E. & Glanze, W.D. (2006) Mosby’s medical, nursing, & allied health dictionary (6th ed.) St. Louis: Mosby-Year Book, Inc.
Haggerty, M. (2006) Hypothyroidism. The Gale Encyclopedia of Medicine. Third Edition. Jacqueline L. Longe, Editor. Farmington Mills, MI. Thompson Gale.
Thyroid disease diagnosis and monitoring. (2000) A handbook for the interpretation of laboratory tests. Edited by Michael Gill. Sonic Healthcare Group.
Last Reviewed – 15 March 2013