Drugs that are used to treat overactivity of the thyroid gland
Hyperthyroidism is a disorder in which an overactive thyroid gland secretes excessive amounts of thyroid hormones. The symptoms of hyperthyroidism include weight loss, persistent tremor, and rapid, sometimes irregular heartbeat. Drugs for hyperthyroidism are given to reduce the activity of the thyroid gland.
What are the types?
An overactive thyroid gland may be treated using antithyroid drugs, radioiodine, or surgery. The drugs take several weeks to become effective, and a beta-blocker drug may be prescribed in the interim to control symptoms such as rapid heartbeat.
These drugs are used for long-term treatment of hyperthyroidism. They may also be used in preparation for surgery to remove all or part of an overactive thyroid gland. The drugs decrease the production of thyroid hormones (see How antithyroid drugs work).
Carbimazole is the antithyroid drug that is most commonly used. Antithyroid drugs are taken on a daily basis. Levels of hormones are usually reduced to normal, and symptoms of hyperthyroidism should begin to improve over a period of 1–2 months. In the interim, beta-blockers may be given to alleviate symptoms. Treatment with antithyroid drugs usually continues for 12–18 months, in which time the underlying disorder that is responsible for the hyperthyroidism may have cleared up.
During the treatment of hyperthyroidism, blood tests are performed on a regular basis to monitor levels of thyroid hormones in the blood so that the dose of antithyroid drugs can be adjusted if necessary. These tests are necessary to ensure that thyroxine levels do not become too high or too low.
The side effects of antithyroid drugs are usually minor and include nausea, headache, rashes, itching, and joint pains. Reduced production of white blood cells is a rare but potentially serious side effect that can occur during treatment with carbimazole. Decreased levels of these cells reduce the body’s ability to fight infection. If you develop symptoms of an infection or a severe sore throat while taking carbimazole, inform your doctor immediately.
If you are taking carbimazole and have symptoms of an infection, or a severe sore throat, you should contact your doctor at once. Do not take any more tablets until your doctor tells you it is safe to do so.
A radioactive form of iodine, called radioiodine, may be used to treat an overactive thyroid gland. Normally, the thyroid gland uses iodine from the diet to produce thyroid hormones. If radioiodine is given, it is taken up by the thyroid gland instead of normal iodine. The radioiodine then destroys part of the thyroid tissue, thereby reducing hormone production.
After treatment for an overactive thyroid gland, levels of thyroid hormone should return to normal within 2–3 months. Sometimes, levels of thyroid hormone increase for a short period after radioactive treatment before decreasing. If the levels are still high after 4 months, you will probably be given a second dose of radioiodine.
Following treatment, you may develop symptoms of hypothyroidism, such as dry, thickened skin, hair thinning, weight gain, and tiredness, because your thyroid gland has become underactive. Your doctor will regularly monitor the levels of thyroid hormones in your blood. If the levels become too low, you will need to take synthetic thyroid hormones to compensate (see Drugs for hypothyroidism); you will need to continue this treatment for the rest of your life. When given in correct doses, the drugs cause no side effects.
Radioiodine treatment is not given during pregnancy because there is a risk of damage to the developing fetus.
Radioactive iodine may also be used after surgery to treat thyroid cancer but at a much higher dose in order to destroy any remaining thyroid tissue that may be cancerous. After treatment, you will need to take synthetic thyroid hormones for life.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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