Last update: Oct. 4, 2020
Minimal risk for breastfeeding and infant.
An antithyroid used in the treatment of hyperthyroidism.
Oral administration in one to three daily doses.
It is excreted in breastmilk in clinically insignificant amounts (Teller 1980, Low 1979) and no problems have been observed in infants whose mothers were taking it (Rylance 1987). The plasma levels of these infants were undetectable or very low (Azizi 2003-2002).
Maternal doses of up to 20-30 mg per day have been shown to be safe for the infant in the short and long term, with normal T3, T4 and TSH determinations and psychomotor development (Kobaly 2019, Alexander 2017, Serrano 2014, Stagnaro 2011, Inoue 2009, Glatstein 2009, Marx 2008, Bartalena 2005, Mandel 2001, Aziz 2006, 2003, 2002, 2000 and 1996, Cooper 1987, Rylance 1987, Lamberg 1984).
It is not necessary to monitor thyroid function in infants, it is sufficient to monitor adequate physical and psychomotor development (Kobaly 2019, Alexander 2017, Inoue 2009, Marx 2008, Mandel 2001).
To minimize infant exposure, the dose of methimazole should be taken immediately after feeding (Hudzik 2016).
Given the risk of liver toxicity from propylthiouracil, methimazole is considered the preferred treatment for hyperthyroidism, especially during breastfeeding (Karras 2009, 2010 and 2012, Azizi 2011, Serrano 2014, Hudzik 2016).
If doses greater than 30 mg of methimazole are required, switching to propylthiouracil to control hyperthyroidism can be tried (doses up to 450 mg/day while breastfeeding have shown to be safe), or continuing with methimazole but with clinical and analytical monitoring (T4 and TSH) of the infant every 2 weeks at first.
American Academy of Pediatrics: Maternal Medication Usually Compatible With Breastfeeding (AAP 2001)
We do not have alternatives for Methimazole since it is relatively safe.
Suggestions made at e-lactancia are done by APILAM´s pediatricians and pharmacists, and are based on updated scientific publications.
It is not intended to replace the relationship you have with your doctor but to compound it.
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Thank you for helping to protect and promote breastfeeding.
e-lactancia is a resource recommended by Asociación Española de Bancos de Leche Humana from Spain
Would you like to recommend the use of e-lactancia? Write to us at corporate mail of APILAM