Hyperthyroidism
What is hyperthyroidism?
Hyperthyroidism is a condition in which there is an excessive amount of thyroid hormones circulating in the blood because of an overactive thyroid gland.
("Hyper" means "over" in Greek). Thyrotoxicosis is a term that refers to 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 and inflammation of 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 article.
The thyroid gland incorporates iodine (mostly available from the diet in foods such as seafood, bread, and salt) and uses it to produce thyroid hormones. The two most important thyroid hormones are
thyroxine (T4) and triiodothyronine (T3) representing 99.9% and 0.1% of thyroid hormones respectively. The hormone with the most biological activity is actually T3. Once released from the thyroid gland
into the blood, a large amount of T4 is converted to T3 - the active hormone that affects the metabolism of cells.
Thyroid- T4 and T3
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 more thyroid hormone production. In contrast, when there is an excessive amount of circulating thyroid hormone, TSH levels fall
as the pituitary attempts to decrease the production of thyroid hormone. In persons with hyperthyroidism, there is a continuously elevated level of circulating thyroid hormones.
{ What causes hyperthyroidism?
Some common causes of hyperthyroidism are listed below followed by a discussion of each condition: - Graves' Disease
- Functioning
Adenoma ("hot nodule") & Toxic Multinodular Goiter (TMNG)
- Excessive Intake of Thyroid Hormones
- Abnormal Secretion of TSH
- Thyroiditis (inflammation of the thyroid
gland)
- Excessive Iodine Intake
Graves' Disease
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. Graves' disease is hereditary and is up to 5 times more common in women than in men.
Graves' disease is thought to be an autoimmune disease and antibodies that are characteristic of the illness may be found in the blood. These antibodies include thyroid stimulating immunoglobulin (TSI
antibodies), thyroid peroxidase antibodies (TPO), and TSH receptor antibodies. The triggers for this disease include stress, smoking, radiation to the neck, medications, and infectious organisms such
as viruses. Graves' disease can be diagnosed by a standard thyroid scan which shows diffusely increased uptake of a radioactive-labeled iodine tracer. In addition, a blood test may reveal elevated TSI
levels.
Graves disease may be associated with eye disease (Graves' ophthalmopathy) and skin lesions (dermopathy). Ophthalmopathy can happen before, after, or at the same time as the
hyperthyroidism. Early on, it may cause sensitivity to light and a feeling of " sand in the eyes." The eyes may protrude and double vision can occur. The degree of ophthalmopathy is worsened in those
who smoke. The course of the eye disease is often independent of the thyroid disease and steroid therapy may be necessary to control the inflammation. In addition, surgical intervention may be
required. The skin condition (dermopathy) is rare and causes a painless, red , lumpy skin rash that appears on the front of the legs.
Functioning Adenoma &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 and produces thyroid hormones
independently. This becomes more likely if the nodule is larger that 3 cm. When there is a single nodule that is independently producing thyroid hormones, it is called a functioning nodule. If there
are more than one functioning nodules, the term toxic multinodular goiter is used. These lesions 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 the medicine. Others 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
radioiodine on a thyroid scan.
Abnormal secretion of TSH
A tumor in the pituitary gland may cause an increased secretion of TSH (the thyroid stimulating hormone). This leads to excessive signaling to the thyroid to
produce 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)
An 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. Inflammation of the gland with 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 bloodstream is increased. Lymphocytic thyroiditis is most
common after a pregnancy and can actually occur in up to 8% of women after they deliver. 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 medication, 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 a number of signs and symptoms. Patients with mild disease usually experience no symptoms. In patients
older than 70 years, the classical manifestations may also be absent. In general, the symptoms become more obvious as the condition worsens. The symptoms are usually related to an increase in body
metabolism. Common symptoms are listed below: - Excessive sweating
- Heat intolerance
- Increased bowel movements
- Tremor
- Nervousness; agitation
- Rapid
heart rate
- Weight loss
- Fatigue
- Decreased concentration
- Irregular and scant menstrual flow
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.
{ How is hyperthyroidism diagnosed?
Hyperthyroidism can be suspected in patients with tremors, excessive sweating, smooth, velvety skin, fine hair, a 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 blood levels of thyroid hormones can be measured directly and are
usually elevated with this disease. However, the main tool for detection of hyperthyroidism is measurement of the blood TSH level. As mentioned earlier, TSH is secreted by the pituitary gland. If an
excess amount of thyroid hormone is present, TSH is " down-regulated" and the level of TSH falls in an attempt to control thyroid hormone production. Thus, the measurement of TSH should result in low
or undetectable levels in cases of hyperthyroidism. However, there is one exception. If the excessive amount of thyroid hormone is actually due to a TSH secreting pituitary tumor, then the levels are
abnormally high. This uncommon disease is known as "secondary hyperthyroidism."
Although the blood tests mentioned above can confirm the presence of excessive thyroid hormone, they do not
point to a specific cause. If there is obvious eye involvement, a diagnosis of Graves' disease is almost certain. A combination of antibody screening (as mentioned above for Graves' disease) and a
thyroid scan can help diagnose the underlying thyroid cause. These investigations are chosen on a case by case basis.
{ How is hyperthyroidism treated?
The options for treating hyperthyroidism include: - Treating Symptoms
- Antithyroid Drugs
- Radioactive Iodine Therapy
- Surgery
Treating symptoms
There 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 called beta-blockers (Inderal, Tenormin, Lopressor). These medications counteract the increased metabolic effect of the thyroid hormones, but do not alter the
actual hormone levels. A doctor determines a treatment protocol based on a number of variables including the underlying cause of hyperthyroidism, the age of the patient, the size of the thyroid, and
the presence of coexisting conditions.
Antithyroid Drugs
There are 2 main antithyroid drugs available for use in the United States, { propylthiouracil ( PTU). These drugs accumulate in the thyroid tissue and inhibit
hormone production. PTU also blocks the conversion of T4 hormone to the more metabolically active T3 hormone. The major risk of these medications is possible suppression of white cell production by the
bone marrow (agranulocytosis). (White cells are needed to fight infection.) It is impossible to tell if and when this side effect is going to occur, so regular blood tests on these medications are not
useful. It is important for patients to know that if they develop a fever, a sore throat, or any signs of infection on these medications, they should see a doctor right away. While a concern, the
actual risk of developing agranulocytosis is less than 1%. In general, patients should be followed at monthly intervals while on antithyroid medication. The dose is adjusted to maintain the patient in
as close to a normal thyroid state as possible (euthyroid). Once the dosing is stable, patients can be seen at 3 month intervals if long-term therapy is planned.
Usually, long-term antithyroid
therapy is only used for patients with Graves' disease, since this disease may actually go into remission over time under treatment without requiring radiation treatment or thyroid surgery. If treated
from 1 to 2 years, the data shows remission rates of 40 to 70%. When the disease is in remission, the gland is no longer over active and antithyroid medication is not needed. Recent studies have also
shown that adding a pill of thyroid hormone to the regimen actually results in higher remission rates. The rationale for this may be that by providing an external source for thyroid hormone, higher
doses of antithyroid medications can be given, which may suppress the overactive immune system in persons with Graves' disease. This type of therapy remains controversial. When long-term therapy is
withdrawn, patients should continue to be seen by the doctor every 3 months for the first year, since a relapse of Graves' disease is most likely in this time period. If a patient does relapse,
antithyroid drug therapy can be restarted, or radioactive iodine or surgery may be considered.
Radioactive Iodine
Radioactive iodine is given orally (either by pill or liquid) on a one-time basis. 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 consequences to this therapy. Radioactive iodine has been safely used for over 50
years, and the only major contraindications to its use are pregnancy and breast-feeding. This form of therapy is the treatment of choice for recurring Graves' disease, patients with severe cardiac
involvement, those with multinodular goiter or toxic adenomas, and patients who cannot tolerate antithyroid drugs. Radioactive iodine must be used with caution in patients with Graves' related eye
disease since recent studies have shown that the eye disease may worsen with therapy.
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 6
months after radioactive iodine, if it persists longer than 6 months, thyroid replacement therapy (Synthroid, Levoxyl) is usually begun.
Surgery
Surgery 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
(Synthroid, Levoxyl) 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 (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. Surgery is indicated in pregnant patients and children who have major adverse reactions to antithyroid
medications. It is also indicated in patients with large goiters and in those who have compressive symptoms, such as difficulty swallowing, hoarseness, and shortness of breath, that are due to an
enlarged thyroid gland.
Hyperthyroidism At A Glance - Hyperthyroidism is a condition in which there is an excessive amount of thyroid hormones.
- Thyroid hormones regulate the metabolism of the cells.
- Normally, the rate of thyroid hormone production is controlled by the brain at the pituitary gland.
- There are many possible causes of hyperthyroidism.
- Common symptoms of hyperthyroidism
include restlessness, tremors, weight loss despite an increased appetite, sweating, rapid heart rate, intolerance to heat, and frequent bowel movements.
- Treatments for hyperthyroidism include
medications and surgery.
Based on WordNet (r) 2.0 (August 2003)
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