What do antithyroid drugs do?

What do antithyroid drugs do?

Antithyroid drugs block pathways leading to thyroid hormone production.

Antithyroid drugs used in this country are Propylthiouracil (PTU) and Tapazole®. Some physicians will recommend antithyroid medication as a first line of treatment to see if the patient is one of the lucky 30% of patients who go into a remission after taking antithyroid medication for one to two years. (Patients are said to be in remission if their hyperthyroidism does not recur after discontinuing the antithyroid drugs.) If antithyroid drugs do not work for the patient, then physicians usually recommend radioactive iodine.

Antithyroid drugs are also used to treat very young children, older patients with heart conditions, and pregnant women. For severe or complicated cases of hyperthyroidism, especially in older patients, PTU or Tapazole® can be given for four to six weeks to bring the hyperthyroidism under better control prior to administering radioactive iodine treatment.

In cases when women are diagnosed with Graves’ disease while they are pregnant, PTU is prescribed. The smallest dose possible is given because the medication does cross over to the fetus. The mother should be checked every three to four weeks during the pregnancy so that the lowest possible dose can be given. Too much PTU can cause fetal goiter, hypothyroidism, and mental retardation.

® Tapozole is a registered trademark of Jones Medical Industries.

Are there any side effects?

Antithyroid drugs cause side effects in about 10% of patients. Reactions can include:

  • skin rash
  • swollen, stiff, painful joints
  • sore throat and fever
  • low white blood count, which can lead to serious infections
  • jaundice (yellow coloring of the skin) and, rarely, liver failure.

Most side effects clear up once the drugs are stopped. If you think you are having a reaction to antithyroid drugs, call your doctor immediately.

What can be expected with antithyroid drug treatment?

  • Several pills are taken from one to four times a day, every day for six to 24 months.
  • Some patients complain that the pills have an unpleasant smell and taste.
  • There is usually some symptom relief within one to two weeks. In some cases, it can take several months to relieve symptoms.
  • Antithyroid drugs have a relatively low success rate. While PTU or Tapazole® may correct the problem temporarily or for a few years, the chances of a permanent remission are about 30% once the drugs are stopped.
  • The likelihood of achieving a permanent remission is increased if the patient takes the medication for one to two years.
  • There are side effects in 10% of the people treated with Tapazole® or PTU. These are:
    • skin rash over most of the body swollen, stiff, painful joints
    • sore throat and fever — if this happens, the antithyroid drugs should be stopped immediately and the physician contacted
    • jaundice
    • liver damage, which is fatal in rare cases
  • Because antithyroid drugs pass into breast milk, only PTU in a dosage less than 200 mg a day is advised if the baby is not weaned.
  • Within 15 years, the thyroid gland may burn out, resulting in hypothyroidism, and the patient will need thyroid hormone replacement.