Hyperthyroidism in pregnancy

Why it is treated at all

Overt hyperthyroidism during pregnancy is associated with adverse effects to the mother and fetus, so treatment is required.

The question is therefore not whether to treat but with what. The two antithyroid medications carry different risks, and which one is preferred changes with the trimester.

The drug of choice by trimester

• First trimester: propylthiouracil is preferred, since methimazole is associated with birth defects when used in the first trimester
• After the first trimester: methimazole should be considered, because the risk of congenital anomalies is less than the risk of liver failure associated with propylthiouracil

So neither drug is preferred at every point of the pregnancy. The reason the choice changes is that the two risks are not equal at the same time: birth defects with methimazole early, liver failure with propylthiouracil later.