What Is Hyperthyroidism? Hyperthyroidism is a condition in which an overactive thyroid gland is producing an excessive amount of thyroid hormones that circulate in the blood. Thyrotoxicosis is a toxic condition that is caused by an excess of thyroid hormones from any cause. Thyrotoxicosis can be caused by an excessive intake of thyroid hormone or by overproduction of thyroid hormones by the thyroid gland. Because both physicians and patients often use these words interchangeably, we will take some liberty by using the term "hyperthyroidism" throughout this slideshow.
What Are Thyroid Hormones? Thyroid hormones stimulate the metabolism of cells. They are produced by the thyroid gland. The thyroid gland is located in the lower part of the neck, below the Adam's apple. The gland wraps around the windpipe (trachea) and has a shape that is similar to a butterfly formed by two wings (lobes) and attached by a middle part (isthmus). The thyroid gland removes iodine from the blood (which comes mostly from a diet of foods such as seafood, bread, and salt) and uses it to produce thyroid hormones.
Thyroid Hormone Regulation – The Chain of Command
The thyroid itself is regulated by another gland located in the brain, called the pituitary. In turn, the pituitary is regulated in part by thyroid hormone that is circulating in the blood (a "feedback" effect of thyroid hormone on the pituitary gland) and in part by another gland called the hypothalamus, also a part of the brain. The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a signal to the pituitary to release thyroid stimulating hormone (TSH). In turn, TSH sends a signal to the thyroid to release thyroid hormones. If overactivity of any of these three glands occurs, an excessive amount of thyroid hormones can be produced, thereby resulting in hyperthyroidism. The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary in an attempt to stimulate the thyroid to produce more thyroid hormone. In contrast, when there is an excessive amount of circulating thyroid hormone, the release of TSH is reduced as the pituitary attempts to decrease the production of thyroid hormone.
What Causes Hyperthyroidism?
Some common causes of hyperthyroidism include:
- Graves' Disease
- Functioning adenoma ("hot nodule") and Toxic Multinodular Goiter (TMNG)
- Excessive intake of thyroid hormones
- Abnormal secretion of TSH
- Thyroiditis (inflammation of the thyroid gland)
- Excessive iodine intake
We'll take a look at the causes of each of these conditions on the following slides.
Graves' disease, which is caused by a generalized overactivity of the thyroid gland, is the most common cause of hyperthyroidism. In this condition, the thyroid gland usually is renegade, which means it has lost the ability to respond to the normal control by the pituitary gland via TSH. Graves' disease is hereditary and is up to five times more common among women than men. The triggers for Graves' disease include stress, smoking, radiation to the neck, medications, and infectious organisms such as viruses. Graves' disease can be diagnosed by a nuclear medicine thyroid scan and blood test. Graves' disease may be associated with eye disease (Graves' ophthalmopathy) and skin lesions (dermopathy)
Functioning Adenoma and Toxic Multinodular Goiter
The thyroid gland (like many other areas of the body) becomes lumpier as we get older. In the majority of cases, these lumps do not produce thyroid hormones and require no treatment. Occasionally, a nodule may become "autonomous," which means that it does not respond to pituitary regulation via TSH and produces thyroid hormones independently. This becomes more likely if the nodule is larger than 3 cm. When there is a single nodule that is independently producing thyroid hormones, it is called a functioning nodule. If there is more than one functioning nodule, the term toxic, multinodular goiter is used. Functioning nodules may be readily detected with a thyroid scan.
Excessive Intake of Thyroid Hormones
Taking too much thyroid hormone medication is actually quite common. Excessive doses of thyroid hormones frequently go undetected due to the lack of follow-up of patients taking thyroid medicine. Other persons may be abusing the drug in an attempt to achieve other goals such as weight loss. These patients can be identified by having a low uptake of radioactively-labeled iodine (radioiodine) on a thyroid scan.
Abnormal Secretion of TSH
A tumor in the pituitary gland may produce an abnormally high secretion of TSH (the thyroid stimulating hormone). This leads to excessive signaling to the thyroid gland to produce thyroid hormones. This condition is very rare and can be associated with other abnormalities of the pituitary gland. To identify this disorder, an endocrinologist performs elaborate tests to assess the release of TSH.
Thyroiditis (inflammation of the thyroid)
Inflammation of the thyroid gland may occur after a viral illness (subacute thyroiditis). This condition is association with a fever and a sore throat that is often painful on swallowing. The thyroid gland is also tender to touch. There may be generalized neck aches and pains. Inflammation of the gland with an accumulation of white blood cells known as lymphocytes (lymphocytic thyroiditis) may also occur. In both of these conditions, the inflammation leaves the thyroid gland "leaky," so that the amount of thyroid hormone entering the blood is increased. Lymphocytic thyroiditis is most common after a pregnancy and can actually occur in up to 8% of women after delivery. In these cases, the hyperthyroid phase can last from 4 to 12 weeks and is often followed by a hypothyroid (low thyroid output) phase that can last for up to 6 months. The majority of affected women return to a state of normal thyroid function. Thyroiditis can be diagnosed by a thyroid scan
Excessive Iodine Intake
The thyroid gland uses iodine to make thyroid hormones. An excess of iodine may cause hyperthyroidism. Iodine-induced hyperthyroidism is usually seen in patients who already have an underlying abnormal thyroid gland. Certain medications, such as amiodarone (Cordarone), which is used in the treatment of heart problems, contain a large amount of iodine and may be associated with thyroid function abnormalities.
What are the symptoms of hyperthyroidism?
Hyperthyroidism is suggested by several signs and symptoms; however, patients with mild disease usually experience no symptoms. In patients older than 70 years, the typical signs and symptoms also may be absent. In general, the symptoms become more obvious as the degree of hyperthyroidism increases. The symptoms usually are related to an increase in the metabolic rate of the body. In older patients, irregular heart rhythms and heart failure can occur. In its most severe form, untreated hyperthyroidism may result in "thyroid storm," a condition involving high blood pressure, fever, and heart failure. Mental changes, such as confusion and delirium, may also occur
How is hyperthyroidism diagnosed?
Hyperthyroidism can be suspected in patients with: tremors, excessive sweating, smooth velvety skin, fine hair, rapid heart rate, and an enlarged thyroid gland. There may be puffiness around the eyes and a characteristic stare due to the elevation of the upper eyelids. Advanced symptoms are easily detected, but early symptoms, especially in the elderly, may be quite inconspicuous. In all cases, a blood test is needed to confirm the diagnosis. The main tool for detection is measurement of the blood TSH level. As mentioned earlier, TSH is secreted by the pituitary gland. Although the blood tests mentioned previously can confirm the presence of excessive thyroid hormone, they do not point to a specific cause. If there is obvious involvement of the eyes, a diagnosis of Graves' disease is almost certain. A combination of antibody screening (for Graves' disease) and a thyroid scan using radioactively-labelled iodine (which concentrates in the thyroid gland) can help diagnose the underlying thyroid disease. These investigations are chosen on a case-by-case basis.
How is hyperthyroidism treated?
The options for treating hyperthyroidism include:
- Treating the symptoms
- Antithyroid drugs
- Radioactive iodine
Treating SymptomsThere are medications available to immediately treat the symptoms caused by excessive thyroid hormones, such as a rapid heart rate. One of the main classes of drugs used to treat these symptoms is a beta-blocker [for example, propranolol (Inderal), atenolol (Tenormin), metoprolol (Lopressor)]. These medications counteract the effect of thyroid hormone to increase metabolism, but they do not alter the levels of thyroid hormones in the blood. A doctor determines which patients to treat based on a number of variables including the underlying cause of hyperthyroidism, the age of the patient, the size of the thyroid gland, and the presence of coexisting medical illnesses
Antithyroid DrugsThere are two main antithyroid drugs available for use in the United States, methimazole (Tapazole) and propylthiouracil ( PTU). These drugs accumulate in the thyroid tissue and block production of thyroid hormones. PTU also blocks the conversion of T4 hormone to the more metabolically active T3 hormone. The major risk of these medications is occasional suppression of production of white blood cells (needed to fight infection) by the bone marrow (agranulocytosis). If patients develop a fever, a sore throat, or any signs of infection while taking methimazole or propylthiouracil, they should see a doctor immediately. While a concern, the actual risk of developing agranulocytosis is less than 1%. The dose of antithyroid medication is adjusted to maintain the patient in as close to a normal thyroid state as possible (euthyroid). Usually, long-term antithyroid therapy is only used for patients with Graves' disease.
Radioactive IodineRadioactive iodine is given orally (either by pill or liquid) on a one-time basis to ablate a hyperactive gland. The iodine given for ablative treatment is different from the iodine used in a scan. Radioactive iodine is given after a routine iodine scan, and uptake of the iodine is determined to confirm hyperthyroidism. The radioactive iodine is picked up by the active cells in the thyroid and destroys them. Since iodine is only picked up by thyroid cells, the destruction is local, and there are no widespread side effects with this therapy. Radioactive iodine ablation has been safely used for over 50 years, and the only major reasons for not using it are pregnancy and breast-feeding. In general, more than 80% of patients are cured with a single dose of radioactive iodine. It takes between 8 to 12 weeks for the thyroid to become normal after therapy.Permanent hypothyroidism is the major complication of this form of treatment. While a temporary hypothyroid state may be seen up to six months after treatment with radioactive iodine, if it persists longer than six months, thyroid replacement therapy (with T4 or T3) usually is begun.
SurgerySurgery to partially remove the thyroid gland (partial thyroidectomy) was once a common form of treatment for hyperthyroidism. The goal is to remove the thyroid tissue that was producing the excessive thyroid hormone. However, if too much tissue is removed, an inadequate production of thyroid hormone (hypothyroidism) may result. In this case, thyroid replacement therapy is begun. The major complication of surgery is disruption of the surrounding tissue, including the nerves supplying the vocal cords and the four tiny glands in the neck that regulate calcium levels in the body (the parathyroid glands). Accidental removal of these glands may result in low calcium levels and require calcium replacement therapy. With the introduction of radioactive iodine therapy and antithyroid drugs, surgery for hyperthyroidism is not as common as it used to be.
What's Best for You?If you are concerned that you may have an excess amount of thyroid hormone, you should mention your symptoms to your doctor. A simple blood test is the first step in the diagnosis. From there, both you and your doctor can decide what the next step should be. If treatment is warranted, it is important for you to let your doctor know of any concerns or questions you have about the options available. Remember that thyroid disease is very common, and in good hands, the diseases that cause an excess of thyroid hormones can be easily diagnosed and treated.