Benefit should outweigh risk

AU TGA pregnancy category: C
US FDA pregnancy category: D

Comments:
-If a patient is, or becomes pregnant during therapy, she should be advised of the potential risk of liver toxicity; due to risk of maternal hepatotoxicity, it may be preferable to switch to methimazole for the second and third trimesters of pregnancy.
-To ensure minimum risk to the fetus, close clinical monitoring is needed to ensure the lowest effective dose is used.

Untreated or inadequately treated Graves' disease during pregnancy has been shown to increase the risk of maternal heart failure, spontaneous abortion, preterm birth, stillbirth, and fetal or neonatal hyperthyroidism. Due to risk of fetal abnormalities with first trimester exposure to methimazole (carbimazole, thiamazole), this drug is suggested for use prior to conception and in the first trimester. Because this drug crosses the placenta and can induce goiter and cretinism in the developing fetus, excessive doses should not be given during pregnancy. In many women, thyroid dysfunction diminishes as the pregnancy proceeds and a reduction in dose may be possible. Since this drug may cause maternal hepatotoxicity, it may be preferable to switch to methimazole for the second and third trimesters of pregnancy. There are no controlled data in human pregnancy.

AU TGA pregnancy category C: Drugs which, owing to their pharmacological effects, have caused or may be suspected of causing, harmful effects on the human fetus or neonate without causing malformations. These effects may be reversible. Accompanying texts should be consulted for further details.

US FDA pregnancy category D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

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From a study in 9 lactating women, it is estimated that 0.025% of the administered dose is excreted into breastmilk during the 4 hours following administration. Until recently, this drug had been considered the antithyroid drug of choice during lactation; however, recent findings of liver injury are questioning this choice. No cases of PTU-induced liver damage have been reported in breastfed infants and it is unknown if the small amounts excreted in breastmilk can cause liver damage. Rare idiosyncratic reactions may occur, such as agranulocytosis. Monitor for signs of infection and obtain CBC with differential if there is a suspicion of a drug-induced blood dyscrasia. Some experts recommend limiting the dose of this drug to 450 mg/day during breastfeeding.

Benefit should outweigh risk

Excreted into human milk: Yes

Comments: It is unknown if the small amounts of drug in breastmilk can cause liver damage; discontinue breastfeeding if liver toxicity is suspected.

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