Drug Levels and Effects:

Summary of Use during Lactation:

Levothyroxine (T4) is a normal component of human milk. Limited data on exogenous replacement doses of levothyroxine during breastfeeding indicate no adverse effects in infants. If levothyroxine is required by the mother, it is not a reason to discontinue breastfeeding. Levothyroxine is recommended treatment for postpartum thyroiditis and tapering of the dose should be avoided when a woman is breastfeeding.[1] Levothyroxine dosage requirement may be increased in the postpartum period compared to prepregnancy requirements patients with Hashimoto's thyroiditis.[2]

Drug Levels:

Milk levels of thyroid hormones have not been measured after exogenous administration in humans. Levothyroxine is a normal component of human milk. Although somewhat controversial, it appears that levothyroxine passes into milk poorly.[3][4][5][6][7]

Maternal Levels.

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

Infant Levels.

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

Effects in Breastfed Infants:

Effects of exogenous thyroid hormone administration to mothers on their infant have not been reported. One case of apparent mitigation of cretinism in hypothyroid infants by breastfeeding has been reported, but the amounts of thyroid hormones in milk are not optimal,[8] and this result has been disputed.[9] The thyroid hormone content of human milk from the mothers of very preterm infants appears not to be sufficient to affect the infants' thyroid status.[10] The amounts of thyroid hormones in milk are apparently not sufficient to interfere with diagnosis of hypothyroidism.[11]

In a telephone follow-up study, 5 nursing mothers reported taking levothyroxine (dosage unspecified). The mothers reported no adverse reactions in their infants.[12]

One mother with who had undergone a thyroidectomy was taking levothyroxine 100 mcg daily as well ascalciumcarbonate and calcitriol. Her breastfed infant was reportedly "thriving" at 3 months of age.[13]

Possible Effects on Lactation:

Adequate thyroid hormone serum levels are required for normal lactation. Replacing deficient thyroid levels should improve milk production caused by hypothyroidism. Supraphysiologic doses would not be expected to further improve lactation.


1. Stagnaro-Green A, Abalovich M, Alexander E et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011;21:1081-125. PMID:21787128
2. Galofre JC, Haber RS, Mitchell AA, Pessah R, Davies TF. Increased postpartum thyroxine replacement in Hashimoto's thyroiditis. Thyroid. 2010;20:901-8. PMID:20615129
3. Sato T, Suzuki Y. Presence of triiodothyronine, no detectable thyroxine and reverse triiodothyronine in human milk. Endocrinol Jpn. 1979;26:507-13. PMID:499092
4. Varma SK, Collins M, Row A et al. Thyroxine, tri-iodothyronine, and reverse tri-iodothyronine concentrations in human milk. J Pediatr. 1978;93:803-6. PMID:712487
5. Mallol J, Obregon MJ, Morreale de Escobar GM. Analytical artifacts in radioimmunoassay of L-thyroxin in human milk. Clin Chem. 1982;28:1277-82. PMID:7074933
6. Oberkotter LV, Tenore A. Separation and radioimmunoassay of T3 and T4 in human breast milk. Horm Res. 1983;17:11-8. PMID:6551313
7. Koldovsky O. Hormones in milk. Vitam Horm. 1995;50:77-149. PMID:7709605
8. Bode HH, Vanjonack WJ, Crawford JD. Mitigation of cretinism by breast-feeding. Pediatrics. 1978;62:13-6. PMID:683777
9. Letarte J, Guyda H, Dussault JH et al. Lack of protective effect of breast-feeding in congenital hypothyroidism: report of 12 cases. Pediatrics. 1980;65:703-5. PMID:7367075
10. van Wassenaer AG, Stulp MR, Valianpour F et al. The quantity of thyroid hormone in human milk is too low to influence plasma thyroid hormone levels in the very preterm infant. Clin Endocrinol. 2002;56:621-7. PMID:12030913
11. Abbassi V, Steinour A. Successful diagnosis of congenital hypothroidism in four breast- fed neonates. J Pediatr. 1980;97:259-61. PMID:7400893
12. Ito S, Blajchman A, Stephenson M et al. Prospective follow-up of adverse reactions in breast-fed infants exposed to maternal medication. Am J Obstet Gynecol. 1993;168:1393-9. PMID:8498418
13. Caplan RH, Wickus GG. Reduced calcitriol requirements for treating hypoparathyroidism during lactation. A case report. J Reprod Med. 1993;38:914-8. PMID:8277494

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Thyroid Hormones

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

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