The parathyroid glands are four small glands found at the back of the thyroid gland in the neck. The…
What is hyperthyroidism?
Hyperthyroidism, also called thyrotoxicosis, is a condition resulting from an overactive thyroid gland. It leads to an oversupply of thyroid hormones that help to control metabolism in the body. Hyperthyroidism is 4-5 times more likely to occur in women than men, affecting up to 2% of women at some point in their lives. 
The thyroid gland plays a key role in the endocrine system, which produces hormones that control bodily functions such as metabolism. If hormone levels are not within a normal range, this can affect the whole body, causing a wide range of symptoms and health problems.
The thyroid gland is located at the front of the throat, just above the hollow between the upper ends of the collarbones and below the Adam's apple (the larynx). It has two lobes that sit on either side of the windpipe. It is controlled by the pituitary gland, which sits at the base of the brain and secretes thyroid-stimulating hormone (TSH).
The thyroid produces two hormones:
- Tri-iodothyronine (T3), and;
- Thyroxine (T4).
The thyroid hormones circulate throughout the bloodstream, regulating metabolism. The higher the levels of T3 and T4 are, the faster metabolism will be.
The nutrient iodine is very important in the production of T3 and T4. It is found in a range of foods including iodised salt, dairy products, seafood and bread made with iodised salt. Not having enough iodine in the diet can lead to an underactive thyroid as well as other health problems. Consuming too much iodine can lead to an overactive thyroid.
There are many causes of hyperthyroidism. Some of the more common ones include:
- A condition of the immune system that leads to antibodies overstimulating the thyroid to produce excessive thyroid hormones (Graves' disease);
- Abnormal growths of the thyroid gland (such as multinodular goitre and toxic adenoma);
- Inflammation of the thyroid gland that causes the release of thyroid hormones (thyroiditis);
- Consuming too much iodine via foods or undergoing medical imaging tests that use dyes containing iodine, and;
- Taking the medication amiodarone, which contains a large amount of iodine.
More rare causes of hyperthyroidism include:
- Consuming large amounts of thyroid hormone (thyrotoxicosis factitia);
- Some tumours of the ovaries, which can produce thyroid hormones;
- High blood levels of human chorionic gonadotropin, a hormone that can stimulate the thyroid to produce extra hormone;
- Oversecretion of thyroid-stimulating hormone due to a tumour in the pituitary gland, and;
- Thyroid cancer.
Graves' disease is the most common cause of hyperthyroidism. It is an autoimmune disorder in which antibodies produced by the immune system mimic the action of TSH and stimulate the thyroid gland to produce an excessive amount of thyroid hormones.
It is 5-10 times more common in women than men.  Smoking and stress can increase the risk of developing Graves' disease and the risk is also increased for women following a pregnancy. Graves' disease tends to run in families. Consuming excessive levels of iodine or taking the drug amiodarone can also increase the risk of developing this condition.
People with Graves' disease may develop symptoms affecting their eyes (see potential complications).
Benign growths of the thyroid
Growths of the thyroid such as toxic multinodular goitre and thyroid adenomas can produce excess hormones. These growths are usually not cancerous.
A toxic multinodular goitre is a growth on the thyroid that grows out of a simple goitre and overproduces thyroid hormones. When there is one growth it is called a toxic nodule, when there is more than one growth it is a multinodular goitre. It is most common in older people, particularly women over 60 years of age.
Inflammation of the thyroid (thyroiditis) can cause the release of hormones stored in the gland. This causes a temporary form of hyperthyroidism. Because of the damage done to the thyroid, some people may go on to develop an underactive thyroid (hypothyroidism), for which they may require treatment.
Thyroiditis can be triggered by:
- Viral infections;
- Pregnancy (symptoms usually occur after the pregnancy), and;
- Some medications including lithium and medications used in chemotherapy.
The recommended daily intake of iodine for adults who are not pregnant is 150 micrograms. Consistently consuming more than this can lead to hyperthyroidism.
The most common causes of taking in too much iodine include:
- Dyes containing iodine used for medical imaging tests;
- Overconsumption of foods very high in iodine such as kelp and nori, and;
- Medications high in iodine such as amiodarone (commonly used to treat heart arrhythmias).
Human chorionic gonadotropin-related hyperthyroidism
Human chorionic gonadotropin (hCG) is a hormone that is produced in high amounts in the first four months of pregnancy. Like the antibodies in Graves' disease, hCG can mimic the action of thyroid-stimulating hormone (TSH) and stimulate the thyroid gland to produce too much T3 and T4 hormones.
Some men can develop hCG-related hyperthyroidism due to hCG being produced by testicular cancer.
Signs and symptoms
There are many symptoms that can occur with hyperthyroidism and they can vary a lot from person to person.
Symptoms can include:
- Rapid pulse;
- Rapid or irregular heartbeat (palpitations);
- Shaking of the hands;
- Finding the heat extremely uncomfortable (heat intolerance) and increased sweating;
- Problems sleeping and fatigue;
- Nervousness, restlessness, agitation and anxiety;
- Frequent bowel movements and diarrhoea;
- Reduced or irregular menstruation in women;
- An enlarged thyroid gland, sometimes called a goitre (see below);
- Increased appetite;
- Weight loss despite eating more (weight gain in rare cases);
- Protruding, staring eyes;
- Flushed skin;
- Hair loss or fine hair, and;
A goitre is an enlargement of the thyroid gland and can appear as a lump or swelling on the front of the throat.
Methods for diagnosis
There can be many reasons why thyroid hormone levels are elevated and not all are due to hyperthyroidism. The doctor will consider your symptoms and health history.
A physical examination can help identify signs of hyperthyroidism. The exam can include:
- Measuring heart rate;
- Checking the hands for trembling;
- Checking the neck for signs of an enlarged thyroid gland;
- Measuring blood pressure, and;
- Examining the eyes for signs of swelling and inflammation.
A blood test can measure the levels of TSH, T3 and T4 hormones. In hyperthyroidism the TSH levels are usually low and either or both of the T3 and T4 hormones levels are high. A blood test can also detect thyroid antibodies that can cause Graves' disease and thyroiditis.
Nuclear thyroid scan
During a nuclear thyroid scan, iodine is marked with a radioactive label called a tracer and absorbed by the thyroid from the bloodstream. Measuring the amount of iodine the thyroid takes up can help to determine what is causing the hyperthyroidism. It is particularly useful for detecting toxic nodules and areas of the gland that are overactive.
A picture of the thyroid is obtained using ultrasound and can show enlargement of the gland and indicate some changes in the gland tissue.
Types of treatment
Treatment of hyperthyroidism will vary depending on the cause, the symptoms and other aspects of general health, for example, whether or not a woman is pregnant.
Beta-blocker drugs such as propranol can be prescribed to help relieve symptoms such as shaking, a racing heartbeat, sweating and anxiety until antithyroid medications can take effect.
Antithyroid drugs such as carbimazole and propylthiouracil work by reducing the amount of hormone the thyroid produces. They are usually taken for around 12-18 months.
Radioactive iodine treatment
Taken by mouth, radioactive iodine is taken up by cells in the thyroid, where the radiation destroys the cells over the following weeks or months. This reduces the amount of hormone the thyroid can produce. Some people may require more than one dose.
Thyroid surgery (thyroidectomy)
Thyroid surgery usually involves the removal of all or part of the thyroid gland. This operation is performed under a general anaesthetic in a hospital. It will often require you to stay in hospital for 1-2 days.
Complications with hyperthyroidism can depend on what type of the condition it is and the treatment.
They can include:
People with Graves' disease may develop symptoms affecting their eyes. In this condition inflammatory cells accumulate in the soft tissues around the eyes, causing swelling, particularly of the eyelids and the muscles that move the eyes.
- Push the eyes forward, making it difficult to shut the eyelids completely;
- Restrict eye movements, which in severe cases can cause double vision, and;
- Cause damage to the optic nerves because of pressure created by the swelling.
Treatment for Graves' ophthalmopathy includes corticosteroids to help reduce the inflammation, tear supplements to help keep eyes moist and comfortable and surgery if necessary. Smoking increases the risk of developing symptoms.
Also called a thyroid storm, this is an extreme form of hyperthyroidism that is a medical emergency and requires intensive treatment. It can be triggered by thyroid surgery or radioactive iodine treatment.
Prolonged hyperthyroidism can promote bone loss leading to osteoporosis.
Treatments, including antithyroid medications, radioactive iodine and thyroid surgery can lead to an underactive thyroid, which is also known as hypothyroidism. This is treated by taking replacement thyroid hormone daily. This medication will usually need to be taken permanently.
Side effects of antithyroid medications
Antithyroid medications can cause a range of side effects including nausea, vomiting and rash.
Agranulocytosis, which a severe reduction in white blood cells, is a rare but serious complication (0.2-0.5% of cases  ) that can be life-threatening. Symptoms include fever, mouth ulcers and sore throat.
Complications due to surgery
Complications that can occur with surgery include:
- Scarring of the neck;
- Speech problems due to damage to the laryngeal nerve, and;
- Damage to the parathyroid glands attached to the thyroid that control calcium levels in the blood.
Prognosis depends on the type of hyperthyroidism, the severity of the symptoms and treatment. It is not uncommon for people to develop hypothyroidism after treatment, although it may take years to develop.
Most forms of hyperthyroidism cannot be prevented. Avoiding excess iodine intake may prevent iodine-related forms of hyperthyroidism, however, it is important to consume adequate amounts of iodine to prevent hypothyroidism, particularly during pregnancy.