The thyroid gland is a part of the body’s endocrine (hormone) system – which is made up of the glands that help regulate many of the body’s different functions. If the thyroid gland is overactive, it produces too much of the thyroid hormone thyroxine. An overactive thyroid (medical term: hyperthyroidism) can be caused by various things. The most common cause is a condition known as Graves’ disease.
The higher levels of thyroid hormones unnecessarily speed up a lot of functions in the body. This can lead to noticeable symptoms such as weight loss, sweating, nervousness or a rapid heartbeat. An overactive thyroid increases the risk of cardiovascular disease (problems affecting the heart and/or blood vessels).
Hyperthyroidism often causes the thyroid gland, which is located at the front of the neck, to become noticeably larger. A swollen thyroid is called a goiter. The whole gland isn’t always swollen. A lump (nodule) may be felt or seen instead. But some people have an overactive thyroid without a goiter or thyroid nodule – and those who have a goiter or nodules don’t always have an overactive thyroid.
- a pounding heart (palpitations) and rapid heartbeat, and sometimes an irregular heartbeat too
- high blood pressure
- weight loss despite an increased appetite
- trembling, nervous restlessness, irritability, sleep problems
- weakness, tiredness
People who have an overactive thyroid often sweat a lot and feel uncomfortable when they are warm. Their skin is often warm and moist to the touch, and they may have thinning hair.
The most noticeable symptom of Graves’ disease affects the eyes: they bulge out visibly, and people may have vision problems (double vision). In rare cases, Graves’ disease also leads to swelling in the lower legs, or in the tips of the fingers or toes.
The most common cause of an overactive thyroid gland is Graves’ disease. This condition arises from a disorder of the body's immune system. The immune system then produces antibodies that make the thyroid gland grow and release too many hormones.
Hyperthyroidism may also arise if thyroid gland cells start producing hormones “autonomously” – in other words, without being "told" to. The activity of the thyroid gland is usually regulated by the pituitary gland. Sometimes, though, the thyroid gland cells no longer react to signals from the pituitary gland. As a result, the thyroid gland may produce far more hormones than are actually needed. These autonomous cells are sometimes found in the whole thyroid gland, and sometimes they are only found in one or several benign (non-cancerous) growths called nodules. The medical term for nodules like this is thyroid adenomas. High thyroid hormone levels are only very rarely caused by malignant (cancerous) thyroid tumors.
Sometimes there are too many thyroid hormones in the blood even though the thyroid gland isn’t overactive – for instance, if the thyroid gland is inflamed (thyroiditis). The inflammation can destroy thyroid tissue. As a result, thyroid hormones that are stored in the tissue are suddenly released, temporarily leading to symptoms similar to those caused by an overactive thyroid.
About 1 out of 100 people have an overactive thyroid, making it quite a common condition. Graves’ disease is the main cause, affecting about 2 out of 1,000 men and 20 out of 1,000 women. It typically starts between the ages of 20 and 50.
If people who have an overactive thyroid don’t get proper treatment, they may develop cardiovascular diseases such as atrial fibrillation (an irregular heartbeat). Because their old bone tissue is broken down and new bone tissue is formed at a faster rate, osteoporosis may develop over the long term too.
In rare cases, an overactive thyroid can lead to a potentially life-threatening situation known as a thyrotoxic crisis or a “thyroid storm.” This can be triggered by iodine-containing medication or x-ray contrast agents, for example. It causes the typical symptoms of an overactive thyroid, as well as symptoms such as a high fever, vomiting, restlessness and anxiety, an irregular heartbeat, confusion and drowsiness, and sometimes even loss of consciousness or cardiovascular collapse (where not enough blood flows around the body). A thyroid storm is a medical emergency, requiring immediate hospital treatment in an intensive care unit.
If it is thought that someone may have an overactive thyroid, the first diagnostic step is usually an in-depth talk with their doctor. The talk is typically followed by a physical examination to palpate (feel) the thyroid gland. This allows the doctor to feel whether the gland is bigger than usual or has any nodules in it. A thorough examination of the rest of the body can detect signs of certain causes. For instance, people with Graves' disease sometimes have bulging, swollen eyes.
Blood tests can provide other important information, including whether the thyroid gland is making too many hormones and whether the production of these hormones is being properly regulated by the pituitary gland. Graves’ disease also causes certain antibodies to be produced, and these can be detected in blood tests.
Ultrasound examinations allow doctors to take a closer look at the thyroid gland tissue and any nodules in the gland. An examination known as a thyroid scan (or thyroid scintigraphy) can be done to see whether the thyroid cells in the nodule are producing more hormones than other thyroid cells. If they are, they are considered to be “hot” nodules.
Mild hyperthyroidism sometimes gets better on its own, without treatment. But the overproduction of thyroid hormones is usually first stopped using tablets called anti-thyroid drugs. In Graves’ disease, long-term treatment with this medication can make the thyroid function return to normal. If that doesn’t happen, the thyroid gland is surgically removed or treated with radioactive iodine. Known as radioiodine therapy, this involves swallowing radioactive iodine which is then absorbed by your thyroid gland, where it destroys overactive cells.
If the overactive thyroid is caused by an enlarged thyroid gland with “autonomous” cells, the thyroid gland that has been “slowed down” with anti-thyroid drugs is usually surgically removed. If surgery isn’t possible or the thyroid gland isn’t greatly enlarged, radioiodine therapy tends to be more suitable.
When people are ill or need medical advice, they usually go to see their family doctor first. Read about how to find the right doctor, how to prepare for the appointment and what to remember.
De Leo S, Lee SY, Braverman LE. Hyperthyroidism. Lancet 2016; 388(10047): 906-918.
Jameson JL, Mandel SJ, Weetmann AP. Disorders of the Thyreoid Gland. In: D. L. Kaspers, S. L. Hauser, J. L. Jameson, Fauci AS, D. L. Longo, J. Loscalzo (Ed). Harrison's Principles of Internal Medicine. McGraw-Hill Education; 2015.
Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid 2016; 26(10): 1343-1421.
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