Antithyroid Drugs

Antithyroid drugs are a class of mediations that blocks the binding of iodine. Typically they are called the Thioamide class.

This class is described as a hormone antagonist and it directly affects thyroid hormones. Essentially this group of medications affects medical conditions that cause an overactive thyroid.

Antithyroid drugs decrease the levels of the two hormones produced by the thyroid:

1.)  Thyroxine (T4)
2.)  Triiodothyronine (T3)


  –  Methimazole
  –  Propylthiouracil  (PTU)
  –  Carbimazole  –  [found in the United Kingdom]


1.)  Hyperthyroidism

2.)  Grave’s Disease

Side Effects:

–  Agranulocytosis  [a decrease of white blood cells] –  seen more often with PTU but can also be seen in methimazole
–  Aplastic anemia
–  Sore throat
–  Fever
–  Rash

Use During Pregnancy:

1.)  Propylthiouracil (PTU) used to be the drug of choice during pregnancy

2.)  This was because it was believed to cause less severe birth defects than methimazole.

3.)  Current recommendations for propylthiouracil is that it be used during the first trimester only.

4.)  Rare liver damage has been seen in some people.

5.)  After the first trimester, women should switch to methimazole for the rest of the pregnancy.