Excretion into breast milk is insignificant (Low 1979, Teller 1980).No adverse effects reported in breastfed infants (Rylance 1987). Plasma levels in those infants were shown to be low or undetectable (Azizi 2002-2003). Maternal daily dose as high as 20 - 30 mg has shown to be safe for the intant at short and long term with normal serum levels of T3, T4, TSH and psychomotor development that make routine blood test controls unnecessary (Lamberg 1984, Cooper 1987, Rylance 1987, Azizi 1996, 2000, 2002, 2003 y 2006, Mandel 2001, Bartalena 2005, Marx 2008, Inoue 2009, Glatstein 2009, Stagnaro 2011, Serrano 2014, Alexander 2017). It would be advisable to take the medication immediately after the breast feed to minimize infant exposition (Hudzik 2016). Given the risk of hepatic toxicity of propylthiouracil, methimazole is considered preferential treatment for hyperthyroidism, especially during breastfeeding (Karras 2009, 2010 and 2012, Azizi 2011, Serrano 2014, Hudzik 2016). American Academy of Pediatrics: Maternal Medication Usually Compatible With Breastfeeding (AAP 2001)