Last update: Oct. 4, 2020


Very Low Risk for breastfeeding

Safe. Compatible.
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)

See below the information of these related products:


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.

Jose Maria Paricio, Founder & President of APILAM/e-Lactancia

Your contribution is essential for this service to continue to exist. We need the generosity of people like you who believe in the benefits of breastfeeding.

Thank you for helping to protect and promote breastfeeding.

José María Paricio, founder of e-lactancia.

Other names

Methimazole is also known as

Methimazole in other languages or writings:


Methimazole belongs to this group or family:


Main tradenames from several countries containing Methimazole in its composition:


Variable Value Unit
Oral Bioavail. 95 %
Molecular weight 114 daltons
Protein Binding < 10 %
VD 0,6 l/Kg
Tmax 1 - 2 hours
T1/2 3 - 6 hours
M/P ratio 1,3 -
Theoretical Dose 0,006 mg/Kg/d
Relative Dose 1-8 %
Ped.Relat.Dose 1,5 %


  1. Kobaly K, Mandel SJ. Hyperthyroidism and Pregnancy. Endocrinol Metab Clin North Am. 2019 Sep;48(3):533-545. Abstract
  2. Alexander EK, Pearce EN, Brent GA, Brown RS, Chen H, Dosiou C, Grobman WA, Laurberg P, Lazarus JH, Mandel SJ, Peeters RP, Sullivan S. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017 Abstract Full text (link to original source) Full text (in our servers)
  3. Serrano Aguayo P, García de Quirós Muñoz JM, Bretón Lesmes I, Cózar León MV. Tratamiento de enfermedades endocrinológicas durante la lactancia. [Endocrinologic diseases management during breastfeeding.] Med Clin (Barc). 2015 Jan 20;144(2):73-9. Abstract
  4. Karras S, Krassas GE. Breastfeeding and antithyroid drugs: a view from within. Eur Thyroid J. 2012 Abstract Full text (link to original source) Full text (in our servers)
  5. Lazarus JH. Pre-conception counselling in graves' disease. Eur Thyroid J. 2012 Abstract Full text (link to original source) Full text (in our servers)
  6. Azizi F, Amouzegar A. Management of hyperthyroidism during pregnancy and lactation. Eur J Endocrinol. 2011 Abstract Full text (link to original source) Full text (in our servers)
  7. Fumarola A, Di Fiore A, Dainelli M, Grani G, Carbotta G, Calvanese A. Therapy of hyperthyroidism in pregnancy and breastfeeding. Obstet Gynecol Surv. 2011 Abstract
  8. Stagnaro-Green A, Abalovich M, Alexander E, Azizi F, Mestman J, Negro R, Nixon A, Pearce EN, Soldin OP, Sullivan S, Wiersinga W; American Thyroid Association Taskforce on Thyroid Disease During Pregnancy and Postpartum.. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011 Abstract Full text (link to original source) Full text (in our servers)
  9. Karras S, Tzotzas T, Kaltsas T, Krassas GE. Pharmacological treatment of hyperthyroidism during lactation: review of the literature and novel data. Pediatr Endocrinol Rev. 2010 Abstract
  10. Inoue M, Arata N, Koren G, Ito S. Hyperthyroidism during pregnancy. Can Fam Physician. 2009 Abstract Full text (link to original source) Full text (in our servers)
  11. Glatstein MM, Garcia-Bournissen F, Giglio N, Finkelstein Y, Koren G. Pharmacologic treatment of hyperthyroidism during lactation. Can Fam Physician. 2009 Abstract Full text (link to original source) Full text (in our servers)
  12. Karras S, Tzotzas T, Krassas GE. Antithyroid drugs used in the treatment of hyperthyroidism during breast feeding. An update and new perspectives. Hormones (Athens). 2009 Abstract Full text (link to original source) Full text (in our servers)
  13. Koren G, Soldin O. Therapeutic drug monitoring of antithyroid drugs in pregnancy: the knowledge gaps. Ther Drug Monit. 2006 Abstract Full text (link to original source) Full text (in our servers)
  14. Azizi F. Treatment of post-partum thyrotoxicosis. J Endocrinol Invest. 2006 Abstract
  15. Törnhage CJ, Grankvist K. Acquired neonatal thyroid disease due to TSH receptor antibodies in breast milk. J Pediatr Endocrinol Metab. 2006 Abstract
  16. Bartalena L, Tanda ML, Bogazzi F, Piantanida E, Lai A, Martino E. An update on the pharmacological management of hyperthyroidism due to Graves' disease. Expert Opin Pharmacother. 2005 Abstract Full text (link to original source) Full text (in our servers)
  17. Nice FJ, De Eugenio D, Dimino TA, Freeny IC, Rovnack MB, Gromelski JS. Medications and Breast-Feeding: A Guide for Pharmacists, Pharmacy Technicians, and Other Healthcare Professionals. Part I. J Pharm Technol 2004;20:17-27. doi: 10.1177/875512250402000106.
  18. Azizi F. Thyroid function in breast-fed infants is not affected by methimazole-induced maternal hypothyroidism: results of a retrospective study. J Endocrinol Invest. 2003 Abstract
  19. Azizi F, Bahrainian M, Khamseh ME, Khoshniat M. Intellectual development and thyroid function in children who were breast-fed by thyrotoxic mothers taking methimazole. J Pediatr Endocrinol Metab. 2003 Abstract
  20. Azizi F, Hedayati M. Thyroid function in breast-fed infants whose mothers take high doses of methimazole. J Endocrinol Invest. 2002 Abstract
  21. AAP - American Academy of Pediatrics Committee on Drugs. Transfer of drugs and other chemicals into human milk. Pediatrics. 2001 Sep;108(3):776-89. Abstract Full text (link to original source) Full text (in our servers)
  22. Mandel SJ, Cooper DS. The use of antithyroid drugs in pregnancy and lactation. J Clin Endocrinol Metab. 2001 Abstract Full text (link to original source) Full text (in our servers)
  23. Azizi F, Khoshniat M, Bahrainian M, Hedayati M. Thyroid function and intellectual development of infants nursed by mothers taking methimazole. J Clin Endocrinol Metab. 2000 Abstract Full text (link to original source) Full text (in our servers)
  24. Verd S, Cardo E. Well-being of a baby breast fed by her mother on carbimazol treatment. J Hum Lact. 1998 Abstract
  25. Azizi F. Effect of methimazole treatment of maternal thyrotoxicosis on thyroid function in breast-feeding infants. J Pediatr. 1996 Abstract
  26. Rylance GW, Woods CG, Donnelly MC, Oliver JS, Alexander WD. Carbimazole and breastfeeding. Lancet. 1987 Abstract
  27. Cooper DS. Antithyroid drugs: to breast-feed or not to breast-feed. Am J Obstet Gynecol. 1987 Abstract
  28. Cooper DS, Bode HH, Nath B, Saxe V, Maloof F, Ridgway EC. Methimazole pharmacology in man: studies using a newly developed radioimmunoassay for methimazole. J Clin Endocrinol Metab. 1984 Abstract
  29. Lamberg BA, Ikonen E, Osterlund K, Teramo K, Pekonen F, Peltola J, Välimäki M. Antithyroid treatment of maternal hyperthyroidism during lactation. Clin Endocrinol (Oxf). 1984 Abstract
  30. Johansen K, Andersen AN, Kampmann JP, Mølholm Hansen JM, Mortensen HB. Excretion of methimazole in human milk. Eur J Clin Pharmacol. 1982 Abstract
  31. Kampmann JP, Hansen JM. Clinical pharmacokinetics of antithyroid drugs. Clin Pharmacokinet. 1981 Abstract
  32. Tegler L, Lindström B. Antithyroid drugs in milk. Lancet. 1980 Abstract
  33. Low LC, Lang J, Alexander WD. Excretion of carbimazole and propylthiouracil in breast milk. Lancet. 1979 Abstract

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