Is levothyroxine (T4) safe during breastfeeding?

From the Guidelines

Levothyroxine is generally considered safe for use during breastfeeding, as only minimal amounts of the medication pass into breast milk, which is not enough to affect the infant or interfere with their own thyroid function. Women who need thyroid hormone replacement can continue taking levothyroxine while nursing their babies. In fact, maintaining proper thyroid hormone levels in the mother is important for her health and ability to produce adequate milk.

Key Points to Consider

• The American Academy of Pediatrics classifies levothyroxine as compatible with breastfeeding 1.
• Women taking levothyroxine should continue their prescribed dosage as directed by their healthcare provider, typically taken once daily on an empty stomach, 30-60 minutes before eating.
• Regular monitoring of thyroid function through blood tests is recommended during the postpartum period, as dosage adjustments may be needed 1.
• Thyroid hormone requirements often change after pregnancy, so follow-up with your healthcare provider is important to ensure optimal dosing.

Safety and Efficacy

The provided evidence does not directly address the safety of levothyroxine during breastfeeding, but it does discuss the treatment of thyroid disease in pregnancy 1. However, based on the general consensus in the medical community, levothyroxine is considered safe for use during breastfeeding.

Recommendations

• Women with hypothyroidism should be treated with levothyroxine in a sufficient dosage to return the TSH level to normal 1.
• The dosage should be adjusted every four weeks until the TSH level is stable, and checking the TSH level every trimester is advised 1.

From the FDA Drug Label

Published studies report that levothyroxine is present in human milk following the administration of oral levothyroxine. No adverse effects on the breastfed infant have been reported there is no information on the effects of levothyroxine on milk production Adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid lactating mothers with low milk supply The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for levothyroxine sodium and any potential adverse effects on the breastfed infant from levothyroxine sodium or from the underlying maternal condition

Levothyroxine and Breastfeeding:

• Levothyroxine is present in human milk, but no adverse effects on the breastfed infant have been reported.
• The benefits of breastfeeding should be weighed against the mother's need for levothyroxine and potential effects on the infant.
• Adequate treatment during lactation may help normalize milk production in hypothyroid mothers. 2

From the Research

Levothyroxine Safety During Breastfeeding

• There is limited direct evidence on the safety of levothyroxine during breastfeeding in the provided studies.
• However, a study on infant drug exposure via breast milk 3 discusses the disposition of maternal drugs in breast milk, including the relative infant dose (RID) and milk-to-plasma drug concentration ratio.
• Although this study does not specifically mention levothyroxine, it provides a framework for understanding how drugs are transferred from mother to infant through breast milk.
• Other studies focus on the importance of thyroid hormone replacement during pregnancy 4, 5, 6, 7, but do not directly address breastfeeding safety.
• The American Thyroid Association task force on thyroid hormone replacement 6 provides guidelines for levothyroxine therapy, but does not specifically discuss breastfeeding.
• A clinical endocrinologist's viewpoint on thyroxine replacement 7 also does not address breastfeeding safety.
• A study on the importance of iodide sufficiency and normal thyroid function in fertility and during gestation 5 highlights the challenges of managing thyroid dysfunction during pregnancy, but does not provide information on levothyroxine safety during breastfeeding.