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There are two types of drugs used to treat Graves' disease (GD):

• Antithyroid drugs (also called thionamides) - which act by inhibiting and blocking the production of thyroid hormone.

• Adjuvant drugs - these drugs are used to prevent the excitatory effect of thyroid hormone on the body, thereby alleviating the symptoms.

Antithyroid Drugs

There are three antithyroid medications available:

• Methimazole (MMI) - once daily dosing

  This is often the drug of choice since it may be given once a day rather than multiple times during the day, works faster than other drugs, and has fewer side effects at low doses. Normal T4 values are achieved usually after approximately 5-6 weeks.

• Propylthiouracil (PTU) - two to three times daily dosing

  Normal T4 levels are usually achieved after two months, depending on the severity of the hyperthyroidism. PTU is taken several times a day. It is a good alternative for people who cannot tolerate MMI and is also preferred for women are pregnant or nursing as it is less likely to cross the placenta or enter into breast milk.

• Carbimazole - decreases the amount of thyroid hormones produced by the thyroid gland by preventing the conversion of iodine to its usable form. Carbimazole is available in Europe but not in the US.

MMI and PTU can be given either:

• Short term - to prepare the patient for another treatment such as surgery or radioiodine treatment
• Long-term for definitive treatment of Graves' disease

Usually, antithyroid drugs must be taken for at least three weeks before there is a reduction of thyroid hormone levels since they block the production of new hormones and do not have any effect on those already circulating in the blood. Antithyroid medications are usually taken for 1-2 years with regular monitoring of the thyroid functioning until a state of being euthyroid is achieved.

Some researchers estimate that approximately 30-40% of Graves' disease patients given antithyroid drugs as a long-term treatment (12-18 months) experience remission of Graves' disease (normal levels of thyroid hormone). They may remain euthyroid up to 10 years or longer. If hyperthyroidism recurs, however, estimates are that less than 10% of patients achieve remission and for those that do, the remission is not likely to be permanent or long-term. Recurrence can occur as soon as 10 days after cessation of medication or up to several years later.

There is a lack of consensus regarding the optimum dosage of antithyroid drugs to achieve euthyroidism and minimize the risk of hypothyroidism. Some studies indicate that an initial high dose may be followed by a lower relapse rate but further studies are required to verify this observation.

If goiter and hyperthyroidism are severe, antithyroid drugs may initially be prescribed at higher doses and then tapered. Any gastrointestinal side effects can often be offset by spreading the dose out over the course of the day.

Approximately 60% of patients fail treatment following 2 years of antithyroid medication, regardless of the drug or dose used. At that point, the doctor may consider using other treatment modalities to treat the Graves' disease.