Breastfeeding and antithyroid drugs: a view from within

Area of Science:

• Endocrinology
• Pharmacology
• Neonatal Care

Background:

• Antithyroid drugs (ATD) are used to manage hyperthyroidism in lactating mothers.
• Propylthiouracil (PTU) was historically preferred due to lower milk transfer, but concerns exist.
• Methimazole (MMI) and carbimazole (CMZ) are other available ATDs.

Purpose of the Study:

• To review the safety and efficacy of ATDs during lactation.
• To provide updated recommendations for ATD use in breastfeeding mothers.

Main Methods:

• Literature review of studies on ATD transfer into breast milk.
• Analysis of infant outcomes in infants exposed to PTU and MMI via lactation.
• Review of reported adverse events, particularly hepatic dysfunction.

Main Results:

• PTU transfers in small amounts (0.025%) into milk, historically considered safe.
• MMI levels in milk correlate with maternal serum levels.
• Infants exposed to MMI during breastfeeding showed normal development and thyroid status.
• PTU is associated with severe, idiosyncratic hepatic dysfunction, often unrelated to dosage.
• MMI appears to be a safer alternative with comparable infant outcomes.

Conclusions:

• PTU is not recommended for treating thyrotoxicosis during lactation due to severe hepatic risks.
• MMI is recommended as the preferred ATD for breastfeeding mothers, up to 30 mg/day.
• PTU may be considered only in specific cases for limited durations.