Is Methimazole Safe in Breastfeeding

I am a breastfeeding mother and i want to know if it is safe to use Methimazole? Is Methimazole safe for nursing mother and child? Does Methimazole extracts into breast milk? Does Methimazole has any long term or short term side effects on infants? Can Methimazole influence milk supply or can Methimazole decrease milk supply in lactating mothers?

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)

Maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Taking methimazole right after nursing and waiting for 3 to 4 hours before nursing again should minimize the infant dosage. No cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended.[1] Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant's complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.

Five mothers taking methimazole 5 to 15 mg daily at 6 pm breastfed their infants during the day using expressed milk or formula at night. Thyroid Five mothers taking methimazole 5 to 15 mg daily at 6 pm breastfed their infants during the day using expressed milk or formula at night. Thyroid function test remained normal during 90 days of study and none of the infants showed any clinical signs of hypothyroidism and methimazole was undetectable (<10 mcg/L) in the infants' serum.[4] Twelve mothers taking methimazole 5 mg daily breastfed their infants from the time of delivery. Another 17 women were given methimazole 5 mg twice daily beginning 2 to 8 months postpartum and allowed to breastfeed their infants exclusively with supplementation given to those over 6 months of age. A third group of 6 mothers took methimazole 10 mg twice daily. All infants were normal clinically and had normal thyroid function tests when measured 2 to 4 weeks after the start of therapy.[7] (Note: this paper is apparently the full publication of a previous report in abstract form.[8]) The breastfed infants of 16 mothers who became hypothyroid during methimazole therapy were studied 5 times between birth and 12 months of age in comparison to a control group of 18 breastfed infants whose mothers did not become hypothyroid during methimazole therapy and a group of 24 infants of normal mothers. There were no differences in thyroid function tests of the infants from the 3 groups.[6] Fifty-one infants of mothers who took methimazole during pregnancy and continued during lactation with a dose of 5 mg daily and 88 infants of mothers who took the drug starting 2 to 8 months postpartum were studied. All infants had normal thyroid function tests for up to a year of maternal therapy with doses of 5 to 20 mg daily of methimazole. Fourteen children who had been breastfed as infants were compared to a control group of children whose mothers did not take methimazole. Performance on IQ tests did not differ between the two groups.[9] Forty-two of these children were restudied after 4 years of age and found to have IQ scores equal to children under the age of 6 in a matched control group.[10]