Underactive thyroid (hypothyroidism) is when your body doesn’t produce enough thyroid hormones to meet its needs, resulting in a range of symptoms.
The condition can be due to a problem with the thyroid gland, problems with other glands that work with the thyroid, or a deficiency of iodine in the body.
A well-functioning thyroid gland is essential for normal health and wellbeing. Its job is to make thyroid hormones, which are carried in the bloodstream to tissues around the body. Thyroid hormone helps the body use energy and stay warm. It also keeps the brain, heart, muscles, and other organs working as they should.
Underactive thyroid 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 an underactive thyroid 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 thyroid condition.
Although underactive thyroid mainly occurs in adults, it can also occur in infants, children, and teenagers. Infants 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 an underactive thyroid 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.
A number of factors are associated with developing underactive thyroid:
- Being a woman (it is twice as common in women as it is in men)
- Age (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
- Raised cholesterol levels
- Existing autoimmune conditions (including systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, and Addison’s disease - a type of hormone disorder)
- It is also more common in people whose hair goes prematurely grey.
A significant proportion of people who develop underactive thyroid have no known risk factors.
There are two main types of underactive thyroid – primary hypothyroidism and secondary hypothyroidism.
Primary hypothyroidism is the result of problems with the thyroid gland itself. 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.
Hashimoto’s thyroiditis 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 abnormalities or disease processes affecting with the pituitary gland, the job of which is to regulate how much thyroid hormone is produced.
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 measures the level of thyroid stimulating hormone (TSH) in the blood. Abnormally high levels of TSH can indicate primary hypothyroidism while abnormally low levels of TSH can indicate secondary hypothyroidism.
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 intellectual disability.
Hypothyroidism cannot be cured, but it can be controlled. Replacement thyroxine given in tablet form is an effective treatment for most people with hypothyroidism, although it can have side effects including rapid heartbeat and palpitations.
A synthetic hormone called levothyroxine is given in a daily dose. A few weeks after therapy is started, the TSH assay blood test is repeated and the amount of levothyroxine prescribed is adjusted. 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 reached a more normal level. If heart symptoms increase, levothyroxine is given at the lowest possible level so that symptoms are minimised.
Other medications containing iron or antacids should be avoided as they can interfere with absorption of the levothyroxine.
Women who are undergoing menopausal hormone therapy (MHT; formerly known as hormone replacement therapy or HRT) or taking the contraceptive pill may need a higher dose of levothyroxine (and a reduction when MHT is discontinued).
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 relieves 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 intellectual disability from occurring. Delayed growth and learning difficulties in older children and adolescents with low thyroid hormone are reversible with treatment.
Underactive thyroid problems, if 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.
Other possible complications of an underactive thyroid include goitre (swelling of the thyroid gland), heart disease, mental health issues (including depression), peripheral neuropathy (damage to nerves in the body’s extremities), and infertility.
American Thyroid Association (2017). Hypothyroidism (FAQs) [Pamphlet PDF]. Falls Church, VA: American Thyroid Association. https://www.thyroid.org/wp-content/uploads/patients/brochures/Hypo_brochure.pdf
Haggerty, M. (2006). Hypothyroidism. The Gale Encyclopedia of Medicine. Third Edition. Jacqueline L. Longe, Editor. Farmington Mills, MI: Thompson Gale.
O’Toole, M.T. (Ed.) (2017). Hypothyroidism. Mosby’s Dictionary of Medicine, Nursing & Health Professions (10th ed.). St Louis, MI: Elsevier.
Orlander, P.R. (2019). Hypothyroidism (Web page). Medscape Drugs and Diseases. New York, NY: WebMD LLC. http://emedicine.medscape.com/article/122393-overview#a1 [Accessed: 17/01/20].
Last Reviewed – February 2020