Drug Levels and Effects:

Summary of Use during Lactation:

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. TheAmericanThyroid Association[1] and others[2] state that it is safe for nursing mothers to use methimazole in dosages up to 20 to 30 mg daily;[1] however, doses greater than 20 mg daily have an insufficient amount of data to support safe use during breastfeeding.[3] With doses over 20 mg, taking the drug just before the infant's longest sleep period and waiting several hours before nursing should reduce the infant's exposure to the drug. 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. Some authors have recommended routine monitoring of the breastfed infant's thyroxine (T4) and thyrotropin (TSH) during maternal propylthiouracil use;[1][2][4] however, no case of thyroid function alteration has been reported among infants exposed to methimazole via breastmilk.

Drug Levels:

Maternal Levels.

One mother (time postpartum not stated) taking 2.5 mg of methimazole every 12 hours orally had a peak milk level of 67 mcg/L at about 2 hours after nursing.[5]

Four lactating women who were 3 to 6 months postpartum received a single dose of methimazole 40 mg orally. Milk levels paralleled serum levels closely at about the same concentration over the next 8 hours during which time an average of 70 mcg of methimazole was excreted into breastmilk. The average peak level of 720 mcg/L occurred at 1 hour after the dose.[6]

Six lactating women were given a single oral dose of methimazole 15 mg. Peak milk levels of 320 mcg/L occurred 2 hours after the dose. At 12 hours after the dose, milk levels were 30 mcg/L. The half-life of the drug in milk averaged 4.2 hours.[7]

Infant Levels.

Ten women taking methimazole nursed their infants 2 hours after taking a 10 mg dose of methimazole orally (approximately the time of the peak milk level). Infant serum levels were measured 2 hours after nursing. Seven of the 10 infants had undetectable (<30 mcg/L) serum levels of methimazole, In the other 3, serum levels were 30, 34 and 35 mcg/L.[8][9]

Effects in Breastfed Infants:

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.[7]

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.[10] (Note: this paper is apparently the full publication of a previous report in abstract form.[11])

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.[9]

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.[12] 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.[13]

Possible Effects on Lactation:

Relevant published information was not found as of the revision date.

Alternate Drugs to Consider:



1. Stagnaro-Green A, Abalovich M, Alexander E et al. Guidelines of theAmericanThyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011;21:1081-125. PMID:21787128
2. Fumarola A, Di Fiore A, Dainelli M et al. Therapy of hyperthyroidism in pregnancy and breastfeeding. Obstet Gynecol Surv. 2011;66:378-85. PMID:21851752
3. Mandel SJ, Cooper DS. The use of antithyroid drugs in pregnancy and lactation. J Clin Endocrinol Metab. 2001;86:2354-9. PMID:11397822
4. Glatstein MM, Garcia-Bournissen F, Giglio N et al. Pharmacologic treatment of hyperthyroidism during lactation. Can Fam Physician. 2009;55:797-8. PMID:19675263
5. Tegler L, Lindstrom B. Antithyroid drugs in milk. Lancet. 1980;2:591. Letter. PMID:6158636
6. Cooper DS, Bode HH, Nath B et al. Methimazole pharmacology in man: studies using a newly developed radioimmunoassay for methimazole. J Clin Endocrinol Metab. 1984;58:473-9. PMID:6546390
7. Abe Y, Sato H, Sakai H et al. Antithyroid treatment of maternal hyperthyroidism during lactation. Thyroid. 1995;5(suppl 1):S108. Abstract.
8. Azizi F, Heydayati M. Thyroid function in breast-fed infants whose mothers take high doses of methimazole. J Endocrinol Invest. 2002;25:493-6. PMID:12109618
9. 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;26:301-4. PMID:12841536
10. Azizi F. Effect of methimazole treatment of maternal thyrotoxicosis on thyroid function in breast-feeding infants. J Pediatr. 1996;128:855-8. PMID:8648549
11. Azizi F. Methimazole treatment of maternal hyperthyroidism during lactation. Thyroid. 1995;5(suppl 1):S108. Abstract.
12. Azizi F, Khoshniat M, Bahrainian M et al. Thyroid function and intellectual development of infants nursed by mothers taking methimazole. J Clin Endocrinol Metab. 2000;85:3233-8. PMID:10999814
13. Azizi F, Bahrainian M, Khamseh ME et al. Intellectual development and thyroid function in children who were breast-fed by thyrotoxic mothers taking methimazole. J Pediatr Endocrinol Metab. 2003;16:1239-43. PMID:14714745

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Antithyroid Agents

  • Thionamides

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

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