Breastfeeding and antithyroid drugs: a view from within

The aim of this communication is to provide information regarding the use of antithyroid drugs (ATD) during lactation. Three ATD are used today: propylthiouracil (PTU), methimazole (MMI) and carbimazole (CMZ). The latter is a prodrug which is bioactivated to MMI. PTU is transferred in small amounts (0.025%) into milk. These amounts were considered nonsignificant for inducing adverse effects for the suckling infant. The amount of MMI excreted in milk is equal to MMI levels in serum. Due to its lower concentrations in milk, PTU was used for decades as the treatment of choice during breastfeeding. Recent studies have demonstrated that physical development, intelligence scores and thyroid status of children whose mothers had received MMI while breastfeeding were similar to those of healthy children. These new data offered clinicians an alternative drug approach. Several hepatic dysfunction studies have been published so far. Clinical manifestations varied from mild to severe hepatic failure, liver transplantation or death. Most PTU cases were more severe, idiosyncratic and not dose related. We recommend that PTU should not be prescribed for thyrotoxicosis during lactation. MMI should be used instead, in doses up to 30 mg/day, while PTU should be used in special cases for a restricted time period.