Levothyroxine Safety During Lactation
Levothyroxine is safe to use during breastfeeding and poses no significant risk to the nursing infant. According to the FDA drug label, levothyroxine is present in human milk following oral administration, but no adverse effects on breastfed infants have been reported 1.
Evidence Supporting Safety
The FDA drug label clearly states that adequate levothyroxine treatment during lactation may actually normalize milk production in hypothyroid mothers with low milk supply 1. This indicates that not only is levothyroxine safe during breastfeeding, but it may be beneficial for maintaining adequate milk production in women with hypothyroidism.
This safety profile is consistent with the fact that levothyroxine is a replacement for the natural thyroid hormone thyroxine, which is essential for normal physiological function 2. Since it replaces a hormone naturally present in the body, when dosed appropriately, it presents minimal risk to the nursing infant.
Medication Transfer in Breast Milk
When evaluating medication safety during lactation, the relative infant dose (infant dose mg/kg/day divided by maternal dose mg/kg/day) is an important consideration. Medications with a value of <10% are generally considered safe 3. Levothyroxine falls within this safe range, which supports its use during breastfeeding.
Clinical Guidelines
The American College of Rheumatology's 2020 guidelines for reproductive health suggest that disease control should be maintained with lactation-compatible medications during breastfeeding 3. While these guidelines don't specifically mention levothyroxine, they emphasize the importance of maintaining maternal health with appropriate medications during lactation.
Additionally, the 2022 AHA/ACC/HFSA guideline for heart failure management notes that for women who choose to breastfeed, medication review should be conducted with neonatology and pediatrics teams for neonatal safety during lactation 3. This multidisciplinary approach is recommended for all medications during lactation, including levothyroxine.
Benefits of Treatment During Lactation
Maintaining euthyroid status in breastfeeding mothers is important for:
- Maternal health and well-being
- Adequate milk production
- Normal postpartum recovery
Untreated hypothyroidism could potentially affect milk production and overall maternal health, which would indirectly impact the infant's nutrition and development.
Practical Recommendations
For breastfeeding mothers requiring levothyroxine:
- Continue prescribed levothyroxine dosage during lactation
- No special timing of medication in relation to breastfeeding is necessary
- Regular monitoring of thyroid function (TSH, free T4) should continue as normally indicated
- No special monitoring of the infant is required based on maternal levothyroxine use
Comparison to Other Thyroid Medications
It's worth noting that while levothyroxine is considered safe during lactation, antithyroid drugs used for hyperthyroidism (such as methimazole, carbimazole, and propylthiouracil) require more caution during breastfeeding 4, 5. This further emphasizes the favorable safety profile of levothyroxine compared to other thyroid medications.
Conclusion
Based on the FDA drug label and available evidence, levothyroxine is safe during lactation and should be continued as prescribed to maintain maternal euthyroid status. The benefits of treating maternal hypothyroidism clearly outweigh any theoretical risks to the breastfed infant.