Is thyroxine (levothyroxine) 100 mcg safe for a breastfeeding mother with hypothyroidism?

Levothyroxine 100 mcg is Safe and Should Be Continued During Breastfeeding

Levothyroxine 100 mcg is safe for breastfeeding mothers with hypothyroidism and should be continued without interruption to maintain maternal health and milk production. 1

Safety Profile During Lactation

• Levothyroxine is present in human milk following oral administration, but no adverse effects on breastfed infants have been reported in published studies 1
• The amount of levothyroxine transferred into breast milk is minimal and does not affect infant thyroid function 1
• Adequate levothyroxine treatment during lactation may actually normalize milk production in hypothyroid mothers with low milk supply 1

Critical Importance of Continued Treatment

• Levothyroxine should never be discontinued during the postpartum period, as untreated maternal hypothyroidism can lead to decreased milk production and maternal complications 1
• Postpartum TSH levels typically return to pre-pregnancy values, so the levothyroxine dose should return to the pre-pregnancy dose immediately after delivery 1
• Maintaining euthyroid status is essential for maternal well-being and successful breastfeeding 2

Monitoring Requirements

• TSH levels should be monitored after delivery to ensure appropriate dosing, as pregnancy-related dose increases are no longer needed 1, 3
• The dose adjustment should occur immediately postpartum, returning to pre-conception levels 1
• If the mother was hypothyroid before pregnancy and required dose increases during gestation, the pre-pregnancy dose should be resumed right after delivery 3

Key Distinction from Antithyroid Medications

• This guidance applies specifically to levothyroxine (thyroid hormone replacement), not antithyroid drugs used for hyperthyroidism 4, 5
• Levothyroxine for hypothyroidism has an entirely different safety profile than antithyroid medications like methimazole or propylthiouracil 4
• The FDA explicitly states that breastfeeding should be continued while taking levothyroxine, with the developmental and health benefits of breastfeeding considered alongside the mother's clinical need for treatment 1

Common Pitfalls to Avoid

• Never discontinue levothyroxine due to breastfeeding concerns, as this creates unnecessary risk to maternal health without benefit to the infant 1
• Do not confuse levothyroxine (safe in breastfeeding) with antithyroid drugs (which require more careful consideration) 4, 5
• Failing to adjust the dose back to pre-pregnancy levels after delivery can result in overtreatment, as pregnancy-related increases are no longer needed 1, 3