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)
We do not have alternatives for Methimazole since it is relatively safe.
Very Low Risk
Compatible. Not risky for breastfeeding or infant.
Moderately safe. Mild risk possible. Follow up recommended. Read the Comment.
Poorly safe. Evaluate carefully. Use a safer alternative. Read the Comment.
Very High Risk
Not recommended. Cessation of breastfeeding or alternative.
- ثيامازول (Arabic)
- Тиамазол (Cyrillic)
- 甲巯咪唑 (Chinese)
- チアマゾール (Japanese)
- C4H6N2S (Molecular formula)
Drug trade names
- 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. 2017Abstract Full text (link to original source) Full text (in our servers)
- 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). 2015Abstract
- Karras S, Krassas GE. Breastfeeding and antithyroid drugs: a view from within. Eur Thyroid J. 2012Abstract Full text (link to original source) Full text (in our servers)
- Lazarus JH. Pre-conception counselling in graves' disease. Eur Thyroid J. 2012Abstract Full text (link to original source) Full text (in our servers)
- Azizi F, Amouzegar A. Management of hyperthyroidism during pregnancy and lactation. Eur J Endocrinol. 2011Abstract Full text (link to original source) Full text (in our servers)
- Fumarola A, Di Fiore A, Dainelli M, Grani G, Carbotta G, Calvanese A. Therapy of hyperthyroidism in pregnancy and breastfeeding. Obstet Gynecol Surv. 2011Abstract
- 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. 2011Abstract Full text (link to original source) Full text (in our servers)
- Karras S, Tzotzas T, Kaltsas T, Krassas GE. Pharmacological treatment of hyperthyroidism during lactation: review of the literature and novel data. Pediatr Endocrinol Rev. 2010Abstract
- Inoue M, Arata N, Koren G, Ito S. Hyperthyroidism during pregnancy. Can Fam Physician. 2009Abstract Full text (link to original source) Full text (in our servers)
- Glatstein MM, Garcia-Bournissen F, Giglio N, Finkelstein Y, Koren G. Pharmacologic treatment of hyperthyroidism during lactation. Can Fam Physician. 2009Abstract Full text (link to original source) Full text (in our servers)
- Karras S, Tzotzas T, Krassas GE. Antithyroid drugs used in the treatment of hyperthyroidism during breast feeding. An update and new perspectives. Hormones (Athens). 2009Abstract Full text (link to original source) Full text (in our servers)
- Azizi F. Treatment of post-partum thyrotoxicosis. J Endocrinol Invest. 2006Abstract
- Törnhage CJ, Grankvist K. Acquired neonatal thyroid disease due to TSH receptor antibodies in breast milk. J Pediatr Endocrinol Metab. 2006Abstract
- Koren G, Soldin O. Therapeutic drug monitoring of antithyroid drugs in pregnancy: the knowledge gaps. Ther Drug Monit. 2006Abstract Full text (link to original source) Full text (in our servers)
- 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. 2005Abstract Full text (link to original source) Full text (in our servers)
- Azizi F. Thyroid function in breast-fed infants is not affected by methimazole-induced maternal hypothyroidism: results of a retrospective study. J Endocrinol Invest. 2003Abstract
- 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. 2003Abstract
- Azizi F, Hedayati M. Thyroid function in breast-fed infants whose mothers take high doses of methimazole. J Endocrinol Invest. 2002Abstract
- Mandel SJ, Cooper DS. The use of antithyroid drugs in pregnancy and lactation. J Clin Endocrinol Metab. 2001Abstract Full text (link to original source) Full text (in our servers)
- AAP - American Academy of Pediatrics Committee on Drugs. Transfer of drugs and other chemicals into human milk. Pediatrics. 2001Abstract Full text (link to original source) Full text (in our servers)
- Azizi F, Khoshniat M, Bahrainian M, Hedayati M. Thyroid function and intellectual development of infants nursed by mothers taking methimazole. J Clin Endocrinol Metab. 2000Abstract Full text (link to original source) Full text (in our servers)
- Verd S, Cardo E. Well-being of a baby breast fed by her mother on carbimazol treatment. J Hum Lact. 1998Abstract
- Azizi F. Effect of methimazole treatment of maternal thyrotoxicosis on thyroid function in breast-feeding infants. J Pediatr. 1996Abstract
- Cooper DS. Antithyroid drugs: to breast-feed or not to breast-feed. Am J Obstet Gynecol. 1987Abstract
- Rylance GW, Woods CG, Donnelly MC, Oliver JS, Alexander WD. Carbimazole and breastfeeding. Lancet. 1987Abstract
- Lamberg BA, Ikonen E, Osterlund K, Teramo K, Pekonen F, Peltola J, Välimäki M. Antithyroid treatment of maternal hyperthyroidism during lactation. Clin Endocrinol (Oxf). 1984Abstract
- 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. 1984Abstract
- Johansen K, Andersen AN, Kampmann JP, Mølholm Hansen JM, Mortensen HB. Excretion of methimazole in human milk. Eur J Clin Pharmacol. 1982Abstract
- Kampmann JP, Hansen JM. Clinical pharmacokinetics of antithyroid drugs. Clin Pharmacokinet. 1981Abstract
- Tegler L, Lindström B. Antithyroid drugs in milk. Lancet. 1980Abstract
- Low LC, Lang J, Alexander WD. Excretion of carbimazole and propylthiouracil in breast milk. Lancet. 1979Abstract