The Basics
What is Levothyroxine?
Used to treat hypothyroidism; also used with surgery and radioactive iodine therapy to treat thyroid cancer.
Brand names for Levothyroxine
Levothyroxine Sodium
How Levothyroxine is classified
Thyroid Hormones
Levothyroxine During Pregnancy
Levothyroxine pregnancy category
Category ANote that the FDA has deprecated the use of pregnancy categories, so for some medications, this information isn’t available. We still think it’s useful to list historical info, however, given what a common proxy this has been in the past.
What we know about taking Levothyroxine while pregnant
There are no reported cases of Levothyroxine Sodium for Injection used to treat myxedema coma in patients who were pregnant or lactating. Studies in pregnant women treated with oral levothyroxine to maintain a euthyroid state have not shown an increased risk of fetal abnormalities. Therefore, pregnant patients who develop myxedema should be treated with Levothyroxine Sodium for Injection as the risk of nontreatment is associated with a high probability of significant morbidity or mortality to the maternal patient and the fetus.
Taking Levothyroxine While Breastfeeding
What are recommendations for lactation if you're taking Levothyroxine?
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. The American Thyroid Association recommends that subclinical and overt hypothyroidism should be treated with levothyroxine in lactating women seeking to breastfeed.[1] Levothyroxine dosage requirement may be increased in the postpartum period compared to prepregnancy requirements patients with Hashimoto’s thyroiditis.[2]
Maternal / infant drug levels
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. The American Thyroid Association recommends that subclinical and overt hypothyroidism should be treated with levothyroxine in lactating women seeking to breastfeed.[1] Levothyroxine dosage requirement may be increased in the postpartum period compared to prepregnancy requirements patients with Hashimoto’s thyroiditis.[2]
Possible effects of Levothyroxine on milk supply
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.
Possible alternatives to Levothyroxine
None listed
List of References
Lactation sources: Drugs and Lactation Database (LactMed) [Internet]. Bethesda (MD): National Library of Medicine (US); 2006-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK501922/1. Alexander EK, Pearce EN, Brent GA et al. 2016 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease during Pregnancy and the Postpartum. Thyroid. 2017;27:315-89. PMID: 28056690
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. Mallya M, Ogilvy-Stuart AL. Thyrotropic hormones. Best Pract Res Clin Endocrinol Metab. 2018;32:17-25. PMID: 29549956
9. Zhang Q, Lian XL, Chai XF et al. [Relationship between maternal milk and serum thyroid hormones in patients with thyroid related diseases.]. Zhongguo Yi Xue Ke Xue Yuan Xue Bao. 2013;35:427-31. PMID: 23987491
10. Bode HH, Vanjonack WJ, Crawford JD. Mitigation of cretinism by breast-feeding. Pediatrics. 1978;62:13-6. PMID: 683777
11. 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
12. 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
13. Abbassi V, Steinour A. Successful diagnosis of congenital hypothroidism in four breast- fed neonates. J Pediatr. 1980;97:259-61. PMID: 7400893
14. 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
15. Caplan RH, Wickus GG. Reduced calcitriol requirements for treating hypoparathyroidism during lactation. A case report. J Reprod Med. 1993;38:914-8. PMID: 8277494
16. Mungan NO, Kor D, Buyukkurt S et al. Propionic acidemia: A Turkish case report of a successful pregnancy, labor and lactation. J Pediatr Endocrinol Metab. 2016;29:863-6. PMID: 27089410
Disclaimer: This material is provided for educational purposes only and is not intended for medical advice, diagnosis, or treatment. Consult your healthcare provider with any questions.
