The thyroid is a gland located in your neck, just beneath the Adam's apple. It releases hormones – thyroxine (T4) and triiodothyronine (T3) – that increase the body's metabolic rate.
A metabolic rate is the rate of chemical processes occurring within the body that are necessary to maintain life. In metabolism, some substances are broken down to provide energy for vital processes while other substances are made.
The thyroid gland is self-regulated by thyroid-stimulating hormone (TSH), made by the pituitary gland (sometimes called the master gland) in the brain. TSH stimulates the thyroid gland to produce thyroid hormones (T4 and T3). When thyroid hormone levels in the body are high, TSH production will "switch off," stopping the thyroid from making more T4 and T3.
Hyperthyroidism occurs when the thyroid gland becomes overactive, making an excessive amount of thyroid hormones. It is much less common than hypothyroidism (underactive thyroid gland) and has a variety of causes.
Hyperthyroidism is 5 to 10 times more likely to occur in women than men. Risk factors for developing hyperthyroidism include having another autoimmune disease (e.g., type 1 diabetes, pernicious anemia), a family history, eating large amounts of iodine, being a woman, and previous goiter.
Thyroid overactivity sometimes results from inappropriately elevated TSH levels, or may be due to problems in the thyroid gland itself.
Graves' disease occurs when the immune system produces an abnormal antibody that acts like TSH to stimulate the entire thyroid gland. This leads to an overactive thyroid and higher levels of thyroid hormones. Graves' disease is the most common cause of hyperthyroidism, accounting for up to 80% of all hyperthyroid cases. The abnormal antibody may also stimulate inflammation in the eyes, and less commonly the skin.
Another form of hyperthyroidism is called toxic nodular goiter. Abnormal tissue or "nodules" in the thyroid (called adenomas) become autonomous; that is, they stop working under the control of TSH and are constantly "on" in terms of producing thyroid hormones.
When the thyroid gland becomes inflamed, you can develop a condition called thyroiditis. Inflammation causes the thyroid to release stored thyroid hormone for a brief period (usually no more than a few weeks). Long-term damage from inflammation will usually lead to hypothyroidism.
Hyperthyroidism can also be caused by taking too much thyroid hormone, or by medications such as amiodarone* that contain high amounts of iodine.
In rare cases, secondary hyperthyroidism is caused by a tumour of the pituitary gland making too much TSH, leading to constant stimulation of the thyroid gland. More rarely, the pituitary gland becomes insensitive to thyroid hormones, no longer knowing when levels are high and that it should "switch off" the production of TSH.
Symptoms and Complications
Hyperthyroidism results in high levels of T4 and T3 circulating in the blood. These hormones will speed up metabolism. When this happens, the following symptoms may occur:
- increased heart rate
- abnormal heartbeat rhythm (palpitations or "pounding")
- increased body heat, causing one to feel warm even in a cool room
- moist skin because of perspiration
- increased activity of nerves, leading to nervousness or slight trembling of hands
- increased activity level despite feeling tired and weak
- increased appetite (but usually accompanied by weight loss)
- interrupted sleep
- frequent bowel movements, sometimes with diarrhea
- development of puffiness around eyes, increased tears, an intense stare, or a sensitivity to light
- increased risk of bone loss (osteoporosis)
- development of a bulge in the neck (goiter) because of the enlargement of the thyroid gland
- cessation of menstrual periods (women)
It is important to note that these symptoms tend to be more subtle in seniors. Seniors may experience symptoms of depression or dementia, heart rhythm irregularities, fainting, weakness, or heart failure.
Graves' disease, in addition to the common symptoms of hyperthyroidism, causes the eyes to become inflamed and to bulge out. In severe cases you can develop double vision and have significant damage to the eyes. Sometimes, the skin over the shins becomes thicker.
If hyperthyroidism isn't adequately treated, a life-threatening complication called thyroid storm may occur. Thyroid storm is a medical emergency caused by extreme thyroid overactivity. Symptoms include rapid heartbeat, shaking, sweating, fever, restlessness, diarrhea, confusion, and loss of consciousness. It can be triggered by trauma, infection, surgery, uncontrolled diabetes, pregnancy, labour, or failing to take thyroid medication.
Making the Diagnosis
If your doctor suspects thyroid disease, several tests may be performed to check thyroid gland function. Levels of TSH can be measured – low levels indicate the thyroid is overactive and is stopping the production of TSH by the pituitary. Levels of T4 and T3 in the blood are also measured. Thyroid stimulating immunoglobulins, the antibodies involved in the development of Graves' disease, are highly specific to Graves' disease and can be measured with a blood test.
Ultrasounds and thyroid scans are also used to examine the thyroid gland or nodules. A radioactive iodine uptake test (RAIU) may be done to determine how much iodine your thyroid collects from your bloodstream.
Treatment and Prevention
The thyroid gland requires a small amount of iodine to function properly, but too much iodine will cause it to stop releasing excess hormones. Iodine treatment is therefore an option, but is only appropriate for short-term use or to bring hyperthyroidism under control quickly (for instance, to bring thyroid storm under control).
Low levels of radioactive iodine can be used to destroy some of the thyroid gland, enough to bring hyperthyroidism under control. However, if too much of the thyroid is destroyed, the result is hypothyroidism. If this occurs, it is likely that you will need to take a thyroid hormone replacement regularly in order to treat the hypothyroidism.
Radioactive iodine is used at low enough levels so that no damage is caused to the rest of the body. It isn't given to pregnant women, as the radioactive iodine can destroy the fetus's thyroid gland.
Oral medication (propylthiouracil or methimazole) can usually bring hyperthyroidism under control within 6 weeks to 3 months. Larger doses will work more quickly, but may increase the risk of side effects such as skin rashes, nausea, loss of taste, and very rarely, suppression of blood cell production in the bone marrow and liver injury. These side effects usually go away when you stop taking medication.
Symptoms of hyperthyroidism such as shakiness, increased heart rate, anxiety, and nervousness can be controlled by beta-blocker medications such as propranolol. However, these medications do not directly affect thyroid function.
Surgical removal of the thyroid (called thyroidectomy) may be necessary in rare cases. This usually leads to hypothyroidism, which must then be treated with thyroid hormone replacement for the rest of a person's life.
*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.
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