Antithyroid medications—sometimes written as anti-thyroid medications—are a common treatment for hyperthyroidism, particularly if you have an ongoing form of hyperthyroidism caused by Graves' disease or a goiter. The goal of antithyroid medications is to prevent the thyroid from producing excess amounts of hormone.
In the US, there are two antithyroid medications available—propylthiouracil (PTU) and methimazole (also known as Tapazole). The medications are similar, as they both stop the thyroid from producing T3 and T4 hormones.
Though the medications work essentially the same way, they each have their own unique merits and drawbacks.
One of the advantages of PTU is that it has a lower risk of birth defects and therefore it is the first line treatment for pregnant women.
A disadvantage is that PTU is only available in 50-milligram units. You need to take it in three equal doses, approximately 8 hours apart, each day. According to the American Thyroid Association clinical guidelines, daily dosage varies from 100 to 600 milligrams, depending on the seriousness of your condition and your age. This information is general, as dosing varies from patient to patient. Always follow your doctor's specific dosing instructions.
The main benefit of Tapazole is that it can be taken one, two, or three times a day (depending on your dosage). Pills are available in 5 or 10 milligrams. It also has fewer side effects and often reverses hyperthyroidism quickly.
Tapazole is more concentrated than PTU. According to the American Thyroid Association clinical guidelines, daily dosage varies from 10 to 40 milligrams, depending on the seriousness of your condition and your age. Again, this information is general, as dosing varies from patient to patient. Always follow your doctor's specific dosing instructions.
Side Effects of Antithyroid Medications
Adverse reactions to antithyroid medications are uncommon (affecting only 1-3% of patients), but they do occur. These side effects include rash, itching, abnormal hair loss, and fever. Less common side effects include nausea, swelling, heartburn, muscle and joint aches, numbness, and headache.
In very rare instances, both drugs can cause liver damage. In the most severe of cases, this can result in death. Regular follow-up visits with your doctor will greatly reduce the risk of this severe complication.
Agranulocytosis: A Rare but Serious Complication
Another extremely rare, but serious, complication of antithyroid medications is known as agranulocytosis. Researchers have yet to discover what exactly causes agranulocytosis, but they do know that is a reaction characterized by attacking immune cells, called granulocytes. These cells protect the body from outside "invaders," such as bacteria. Without granulocytes, the body is defenseless against bacteria and infection.
Agranulocytosis occurs more in patients taking PTU than Tapazole—twice as often, in fact. Regardless of their dosage, patients taking PTU have the same risk of developing the condition. With Tapazole, on the other hand, the risk is likely dependent on the dosage. The lower the dose, the lower the risk.
If you notice any infection—even if it's as ordinary as a sore throat—seek immediate medical treatment. Again, agranulocytosis is an extremely rare complication of antithyroid medications, but it's important to understand all the risks. You should have regular follow-up with your doctor while you are on these medications.
One of the main drawbacks of antithyroid drugs is that some patients experience a relapse of their symptoms upon decreasing the dose of the antithyroid medicine. Antithyroid medications usually alleviate your hyperthyroid symptoms in six to 12 weeks. While there is no standard for how long you will take the medication, you will most likely continue with it for 12 to 18 months. That time period, combined with a gradual, controlled decrease in your dosage, lessens your chance of developing hyperthyroidism again.
Your doctor may suggest that you not use antithyroid medications as a life-long treatment, as your risk for adverse effects increases the longer you continue with the medication. While some patients may experience a permanent end to their symptoms, these medications do not guarantee life-long prevention of hyperthyroidism.