Small amount of Propylthiouracil can reach the breast milk but it doesn't cause harm to the thyroid function (Low 1979, Kampmann 1980, Cooper 1987). The American Thyroid Association among others (Mandel 2001, Stagnaro 2011, Alexander 2017) stated that a daily dose as high as 450 mg is safe while breastfeeding. However, some studies have shown no harmful effect at a daily dose as high as 750 mg (Kampmann 1980, McDougall 1986, Cooper 1987, Momotani 1989 y 2000, Bartalena 2005, Azizi 2006, Marx 2008, Inoue 2009, Glatstein 2009). Methimazole or Carbimazole is preferred since liver toxicity has been found among patients who were treated with Propylthiouracil (Karras 2009, 2010 y 2012, Azizi 2011, Serrano 2014, Hudzik 2016). The American Academy of Pediatrics has rated it (2001) as compatible with breastfeeding. Eleventh WHO Model List of Essential Drugs 2002: Compatible with breastfeeding.