Just as the name suggests, antithyroid drugs work in opposition to the thyroid. These drugs work by blocking the production of thyroid hormone by making it more difficult for the thyroid to use iodine. These drugs, which are called thionamides, have been around since the 1940s. Many patients are prescribed the drugs as the sole therapy for hyperthyroidism.
It can sometimes take several months for the effects of these medications to kick in. That's because the drugs do not wipe out the excess hormone already produced before you started taking the medication.
Most people who take medications for hyperthyroidism rely on PTU, which is usually taken three times a day. In addition to blocking the production of thyroid hormone, PTU blocks the conversion of T4 to the more active form of T3. The other option is methimazole, which is sold under the brand name Tapazole. Methimazole is usually taken daily or twice a day. For people living in Europe, carbimazole is another type of antithyroid medication that's available. In the body, carbimazole is rapidly converted into methimazole, where it blocks the production of thyroid hormone.
Antithyroid medications are usually given in the four to six weeks leading up to surgery for hyperthyroidism. They are also given in preparation for RAI in patients with severe hyperthyroidism. In addition, these drugs are used when someone has thyroid storm, a condition characterized by severe symptoms of hyperthyroidism and considered a bona fide medical emergency.
People who use antithyroid drugs usually stay on them for about two years. During that time, the dosage is adjusted so that your thyroid function gradually becomes normal. Approximately 20 percent of patients will have to discontinue the medication due to side effects such as a skin rash or nausea.
But for others, the antithyroid drugs can offer complete relief. After two years, approximately 50 percent of patients will be effectively cured, can stop the antithyroid drugs, and have normal thyroid tests. For the remaining 30 percent of patients who remained on the drugs, normal thyroid function is not achieved. Sometimes, the hyperthyroidism returns when the drugs are stopped. For these people, RAI or surgery becomes necessary.
Antithyroid Drugs Before RAI
Sometimes, doctors prescribe antithyroid medications to prepare patients for RAI. The drugs help bring the levels of thyroid hormone back to normal and reduce the odds of a sudden surge in hormone levels after RAI treatment. However, antithyroid drugs interfere with the uptake of iodine, so they're generally stopped two weeks before RAI.
Antithyroid drugs also interfere with RAIU and scans. That's why Dr. Friedman recommends performing RAIU and scans to determine the type of hyperthyroidism before starting antithyroid drugs. In the meantime, beta-blockers can be started immediately and can be used to tame symptoms of hyperthyroidism regardless of the cause.
Antithyroid Drugs Plus Thyroid Hormone
Some physicians may use a combination of an antithyroid treatment with thyroid hormone replacement. The antithyroid drug is given first to suppress the TSH and shut down the thyroid gland completely. It is then followed up with thyroid hormone replacement to gradually restore normal thyroid function. Some experts believe that the combination of the two drugs is more likely to induce a patient with Graves' disease into remission — a return to normal thyroid levels — than is antithyroid medication alone.
But not everyone is a good candidate for this combination therapy. Patients who are sensitive to either medication may be even more vulnerable to side effects when the drugs are given together. In addition, the combination is not suitable for patients who are sensitive to antithyroid drugs, since higher dosages are needed to completely suppress the thyroid gland.
As with any medication, antithyroid drugs can cause side effects. Some people may develop a low-grade fever at first that eventually subsides. Other side effects include a skin rash, itching, hives, hair loss, stomach upset, altered taste, tingling, joint pain and swelling, and nausea and vomiting. People who experience side effects may consider switching to the other drug but may experience similar problems with the second drug as well.
In rare cases, the antithyroid medications can produce major side effects. The most serious of these side effects is a decrease in the production of white blood cells, a condition known as agranulocytosis. This condition affects less than 1 percent of patients on antithyroid drugs and may be more likely to occur in older adults who take PTU or in people who take extremely high doses of methimazole.
Antithyroid medications aren't the only drugs that can cause agranulocytosis. Anticonvulsants, used to control seizures, and certain anti-psychotics, such as clozapine, can also cause a significant reduction in white blood cell production.
Agranulocytosis tends to occur in the first three months after starting treatment and is more common in patients aged forty and up. Because it is a possible side effect, it is important to tell your doctor about any sign or symptom that signals an infection such as unexplained fever or sore throat, or unusual bleeding or bruising. Your doctor will probably order a white blood cell count. Although the condition usually disappears once you discontinue the antithyroid drug, it is potentially deadly.
Other serious side effects of using antithyroid medications are liver damage from hepatitis, aplastic anemia, and vasculitis. In aplasticanemia, the bone marrow stops producing blood cells. With vasculitis, there is inflammation of the blood vessels. Fortunately, these side effects are exceedingly rare and tend to disappear fully once the drug is discontinued. But you should alert your doctor to any unusual signs or symptoms, including abdominal pain, yellowing of the skin or eyes, and loss of appetite.