Synthroid (Levothyroxine) During Breastfeeding
Synthroid is safe to use during breastfeeding and should be continued without interruption. 1
Key Recommendation
Breastfeeding women with hypothyroidism should continue their levothyroxine therapy at their prescribed dose, as adequate treatment is essential for maternal health and may actually normalize milk production in hypothyroid mothers. 1
Evidence Supporting Safety
Levothyroxine is present in human milk but no adverse effects on breastfed infants have been reported. 1 The FDA drug label explicitly states this based on published studies, making this the highest-quality evidence available.
The amount of levothyroxine transferred to breast milk is minimal and physiologic. 1 Since levothyroxine is simply replacing the mother's missing thyroid hormone (which would normally be present), the infant receives only trace amounts that do not affect their thyroid function.
Adequate levothyroxine treatment during lactation may actually improve milk supply in hypothyroid mothers who have low milk production. 1 This represents a therapeutic benefit rather than a risk.
Clinical Management
Continue the same levothyroxine dose used during pregnancy immediately after delivery. 1 Postpartum TSH levels typically return to pre-pregnancy values, so the pre-pregnancy dose should be resumed right after birth.
Monitor maternal thyroid function but do not interrupt breastfeeding. 1 The focus should be on maintaining the mother's euthyroid state, which benefits both mother and infant.
Important Distinction
Do not confuse levothyroxine (thyroid hormone replacement) with antithyroid drugs. 2, 3 Antithyroid medications like propylthiouracil and methimazole, which are used to treat hyperthyroidism, have different safety profiles during breastfeeding. Levothyroxine is fundamentally different—it replaces a deficient hormone rather than suppressing excess hormone production.
Risk-Benefit Analysis
The developmental and health benefits of breastfeeding should be weighed alongside the mother's clinical need for levothyroxine. 1 However, this is not a difficult decision: untreated maternal hypothyroidism poses significant risks including impaired milk production, while levothyroxine treatment poses essentially no risk to the breastfed infant.
Discontinuing levothyroxine would harm the mother and potentially compromise breastfeeding success, with no benefit to the infant. 1