Hyperthyroidism

Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Hyperthyroidism affects about 1% of the U.S. population and can occur at any age but is more common in women during menopause and after childbirth.

Causes
Hyperthyroidism has several causes, including Graves' disease, thyroiditis, inflammation from toxic substances or radiation exposure, toxic thyroid nodules, and overstimulation due to an overactive pituitary gland. Graves' disease, the most common cause of hyperthyroidism, is an autoimmune disorder caused by an abnormal protein (antibody) in the blood that stimulates the thyroid to produce and secrete excess thyroid hormones into the blood. This cause of hyperthyroidism is often hereditary, especially in women, and almost always leads to a diffusely enlarged thyroid. Graves' disease may go into spontaneous remission, and therapy is required only during the hyperthyroid phase.
Thyroiditis is inflammation of the thyroid gland. In subacute painful thyroiditis, subacute painless thyroiditis, and, much less often, Hashimoto's thyroiditis, as stored hormone is released from the inflamed gland, hyperthyroidism occurs. Hypothyroidism usually follows because the levels of stored hormones are depleted. Finally, the gland usually returns to normal function.
Inflammation from toxic substances or radiation exposure, like the three main types of thyroiditis, can also cause hyperthyroidism. A toxic thyroid nodule (adenoma) is an area of abnormal local tissue growth within the thyroid gland. This abnormal tissue produces thyroid hormone even without stimulation by thyroid-stimulating hormone. Thus, a nodule escapes the mechanisms that normally control the thyroid gland and produces thyroid hormone in large quantities. Toxic multinodular goiter (Plummer's disease), in which there are many nodules, is uncommon in adolescents and young adults and tends to become more common with age.
An overactive pituitary gland can produce too much thyroid-stimulating hormone, which in turn leads to overproduction of thyroid hormone. However, this is an extremely rare cause of hyperthyroidism.

Symptoms
Most people with hyperthyroidism have an enlarged thyroid gland (goiter). The entire gland may be enlarged, or nodules may develop within certain areas. The gland may be tender and painful. Symptoms of hyperthyroidism, regardless of the cause, reflect the speeding up of body functions: increased heart rate and blood pressure, abnormal heart rhythms (arrhythmias) , excessive sweating, hand tremors (shakiness), nervousness and anxiety, difficulty sleeping (insomnia), weight loss despite increased appetite, increased activity level despite fatigue and weakness, and frequent bowel movements, occasionally with diarrhea. Older people with hyperthyroidism may not develop these characteristic symptoms but have what is sometimes called apathetic or masked hyperthyroidism, in which they become weak, sleepy, confused, withdrawn, and depressed. Hyperthyroidism can cause changes in the eyes. A person with hyperthyroidism may appear to be staring.
If the cause of hyperthyroidism is Graves' disease, eye symptoms include puffiness around the eyes, increased tear formation, irritation, and unusual sensitivity to light. Two distinctive additional symptoms may occur: bulging eyes (exophthalmos) and double vision (diplopia). The eyes bulge outward because of a substance that builds up in the orbits behind the eyes. The muscles that move the eyes become inflamed and unable to function properly, making it difficult or impossible to move the eyes normally or to coordinate eye movements, resulting in double vision. The eyelids may not close completely, exposing the eyes to injury from foreign particles and dryness.
These eye changes may begin before any other symptoms of hyperthyroidism, providing an early clue to Graves' disease, but most often occur when other symptoms of hyperthyroidism are noticed. Eye symptoms may even appear or worsen after the excessive thyroid hormone secretion has been treated and controlled.
When Graves' disease affects the eyes, a substance similar to the one deposited behind the eyes is also occasionally deposited in the skin, usually over the shins. The thickened area may be itchy and red and feels hard when pressed with a finger. As with deposits behind the eyes, this problem may begin before or after other symptoms of hyperthyroidism are noticed.

Diagnosis
A doctor usually suspects hyperthyroidism on the basis of the symptoms. Blood tests are used to confirm the diagnosis. Often, testing begins with measurement of thyroid-stimulating hormone (TSH). If the thyroid gland is overactive, the level of TSH is low. However, in rare cases in which the pituitary gland is overactive, the level of TSH is normal or high. If the level of TSH in the serum is low, doctors measure the levels of the thyroid hormones in the blood. If there is a question of whether Graves' disease is the cause, doctors check a sample of blood for the presence of antithyroid antibodies. More specific antibodies can be measured, but such a test is rarely needed.
If a toxic thyroid nodule is suspected as the cause, a thyroid scan will show whether the nodule is overactive, that is, whether it is producing excess hormone. Such a scan may also help doctors in their evaluation of Graves' disease: In a person with Graves' disease, the scan shows the entire gland to be overactive, not just one area. In thyroiditis, the scan shows low activity.

Prognosis and Treatment
Treatment of hyperthyroidism depends on the cause. In most cases, the problem causing hyperthyroidism can be cured, or the symptoms can be eliminated or greatly reduced. If left untreated, however, hyperthyroidism places undue stress on the heart and many other organs.
Beta-blockers such as propranolol help control many of the symptoms of hyperthyroidism. These drugs can slow a fast heart rate, reduce tremors, and control anxiety. Doctors therefore find beta-blockers particularly useful for people with extreme hyperthyroidism and for people with bothersome or dangerous symptoms that have not responded to other treatments. However, beta-blockers do not control abnormal thyroid function. Therefore, they are given until other treatments bring hormone production to normal levels. Propylthiouracil or methimazole are the drugs most commonly used to treat hyperthyroidism; they work by decreasing the gland's production of thyroid hormone. Each drug is taken by mouth, beginning with high doses that are later adjusted according to blood test results. These drugs can usually control thyroid function in 6 to 12 weeks. Larger doses of these drugs may work more quickly but increase the risk of side effects. Pregnant women who take propylthiouracil or methimazole are closely monitored, because these drugs cross the placenta and can induce goiter or hypothyroidism in the fetus. Carbimazole, a drug that is widely used in Europe, is converted into methimazole in the body.
Iodine, given by mouth, is sometimes used to treat hyperthyroidism. It is reserved for those in whom rapid treatment is needed. It may also be used to control hyperthyroidism until the person can have surgery to remove the thyroid. It is not used long-term.
Radioactive iodine may be given by mouth to destroy part of the thyroid gland. Very little radioactivity is introduced to the body as a whole but a great deal is delivered to the thyroid gland because the thyroid gland takes up the iodine and concentrates it. Hospitalization is rarely necessary. After treatment, the person should probably not be near infants and young children for 2 to 4 days. No special precautions are needed in the workplace. There are no precautions needed for sleeping with a partner.
Pregnancy should be avoided for about 6 months. Some doctors try to adjust the dose of radioactive iodine to destroy only enough of the thyroid gland to bring its hormone production back to normal, without reducing thyroid function too much; others use a larger dose to completely destroy the thyroid. Most of the time, people who undergo this treatment must take thyroid hormone replacement therapy for the rest of their lives. Concern that radioactive iodine may cause cancer has never been confirmed. Radioactive iodine is not given to pregnant or nursing women, because it crosses the placenta and enters the milk and may destroy the
fetus's or breastfed infant's thyroid gland.
Surgical removal of the thyroid gland, called thyroidectomy, is a treatment option for young people with hyperthyroidism. Surgery is also an option for people who have a very large goiter as well as for those who are allergic to or who develop severe side effects from the drugs used to treat hyperthyroidism. Hyperthyroidism is permanently controlled in more than 90% of those who choose this option. Hypothyroidism often occurs after surgery, and people then have to take replacement thyroid hormone for the rest of their lives. Rare complications of surgery include paralysis of the vocal cords and damage to the parathyroid glands (the tiny glands behind the thyroid gland that control calcium levels in the blood).
In Graves' disease, additional treatment may be needed for the eye and skin symptoms. Eye symptoms may be helped by elevating the head of the bed, by applying eye drops, by sleeping with the eyelids taped shut, and, occasionally, by taking diuretics (drugs that hasten fluid excretion).
Double vision may be helped by using eyeglass prisms. Finally, corticosteroids taken by mouth, x-ray treatment to the orbits, or eye surgery may be needed if the eyes are severely affected. Corticosteroid creams or ointments can help relieve the itching and hardness of the abnormal skin. Often the problem disappears without treatment months or years later.

Thyroid Storm
Thyroid storm, which is sudden extreme overactivity of the thyroid gland, is a life-threatening emergency. All of the body functions are accelerated to dangerously high levels. Severe strain on the heart can lead to a lifethreatening irregular heartbeat (arrhythmia) , extremely fast pulse, and shock. Thyroid storm may also cause fever, extreme weakness and loss of muscle, restlessness, mood swings, confusion, altered consciousness (even coma), and an enlarged liver with mild jaundice (a yellowish discoloration of the skin and the whites of the eyes).
Thyroid storm is generally caused by untreated or inadequately treated hyperthyroidism and can be triggered by infection, injury, surgery, poorly controlled diabetes, pregnancy or labor, discontinuance of thyroid drugs, or other stresses. It is rare in children.
[Merck]