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Seaweed use while Breastfeeding

Medically reviewed by Last updated on May 25, 2023.

Seaweed Levels and Effects while Breastfeeding

Summary of Use during Lactation

Kelp (Fucus vesiculosus, other Fucus species, and Ascophyllum nodosum), also known as bladderwrack, consists of the dried whole plant. It contains soluble fiber (e.g., alginic acid), vitamin B12, iron and iodine. It can also contain heavy metals (e.g., arsenic, cadmium, lead). Brown seaweed (Undaria pinnatifida) is a purported galactogogue in some Asian cultures; however, no scientifically valid clinical trials support this use. Galactogogues should never replace evaluation and counseling on modifiable factors that affect milk production.[1,2] No data exist on the excretion of any organic components of seaweed into breastmilk; however, iodine is actively transported into breastmilk and heavy metals are excreted into milk. For further information on iodine in breastmilk and its potential adverse effects, see the iodine record in LactMed. Some sources recommend against using seaweed during breastfeeding because of its high iodine content,[3-5] and potential contamination with heavy metals.

Dietary supplements do not require extensive pre-marketing approval from the U.S. Food and Drug Administration. Manufacturers are responsible to ensure the safety, but do not need to prove the safety and effectiveness of dietary supplements before they are marketed. Dietary supplements may contain multiple ingredients, and differences are often found between labeled and actual ingredients or their amounts. A manufacturer may contract with an independent organization to verify the quality of a product or its ingredients, but that does not certify the safety or effectiveness of a product. Because of the above issues, clinical testing results on one product may not be applicable to other products. More detailed information about dietary supplements is available elsewhere on the LactMed Web site.

Drug Levels

Iodine is a normal component of human milk. Median values of total iodine (including iodide) in breastmilk in the United States have ranged from 35 to 140 mcg/L.[6]

Maternal Levels. Korean mothers are traditionally given large amounts of brown seaweed soup postpartum, up to 4 or 5 times daily in the first few days postpartum. A study of 50 Korean postpartum mothers measured the amount of iodine in their breastmilk. At 2 to 5 days postpartum, iodine content averaged 2170 mcg/L (range 218 to 8671 mcg/L) or 2.7 mg daily. At 4 weeks postpartum when the seaweed intake was less, iodine averaged 892 mcg/L (range 60 to 3838 mg/L) or 1.3 mg daily.[7]

The breastmilk of the mothers of 31 preterm Korean infants was measured for iodine concentration on week 1, 3 and 6 postpartum. Korean mothers often have elevated iodine levels from large amounts of dietary iodine in brown seaweed soup during the first month postpartum. Median breastmilk iodine levels were 2529, 1153 and 822 mcg/L at the 3 times, respectively.[8]

Infant Levels. The preterm infants of Korean mothers with high levels of breastmilk iodine had relatively high urinary iodine levels that were thought to be related to high maternal intake of iodine from seaweed soup.[8]

Effects in Breastfed Infants

A study of 31 preterm infants born at 34 weeks gestational age or less was performed in Korea where mothers typically ingest large amounts of seaweed soup during the first month postpartum. Subclinical hypothyroidism was frequently found in the infants that had high intakes of iodine from breastmilk.[8]

Two mothers originally from Asia (Korea and China) reportedly ate large amounts of soup made from seaweed from their home countries in the postpartum period. Their infants had elevated thyrotropin (TSH) levels when tested at 3 to 4 weeks of age and signs of hypothyroidism. Both were treated with thyroid hormones and regained normal thyroid function.[9]

A 21-day-old breastfed (extent not stated) infant presented with unconjugated hyperbilirubinemia. Neonatal TSH screening was normal, but at 21 days it was 87.3 IU/L (normal 0.27 to 4.2 IU/L). Free T4 was 7.3 pmol/L (normal 12 to 22 pmol/L) and the thyroid was slightly enlarged. The infant's parents were of Korean origin and the mother had consumed 3 to 4 bowls of brown seaweed (Undaria pinnatifida) soup daily from the time of birth. The infant's hypothyroidism was probably caused by the high iodine content of the seaweed soup.[10]

Effects on Lactation and Breastmilk

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


Brodribb W. ABM Clinical Protocol #9. Use of galactogogues in initiating or augmenting maternal milk production, second revision 2018. Breastfeed Med. 2018;13:307–14. [PubMed: 29902083]
Breastfeeding challenges: ACOG Committee Opinion, Number 820. Obstet Gynecol. 2021;137:e42–e53. [PubMed: 33481531]
Kopec K. Herbal medications and breastfeeding. J Hum Lact. 1999;15:157–61. [PubMed: 10578793]
Consolini AE, Ragone MI. Patterns of self-medication with medicinal plants and related adverse events - a South American survey. Curr Drug Saf. 2010;5:333–41. [PubMed: 20615178]
Rhee SS, Braverman LE, Pino S, et al. High iodine content of Korean seaweed soup: a health risk for lactating women and their infants? Thyroid. 2011;21:927–8. [PubMed: 21745110]
Dasgupta PK, Kirk AB, Dyke JV, et al. Intake of iodine and perchlorate and excretion in human milk. Environ Sci Technol. 2008;42:8115–21. [PubMed: 19031911]
Moon S, Kim J. Iodine content of human milk and dietary iodine intake of Korean lactating mothers. Int J Food Sci Nutr. 1999;50:165–71. [PubMed: 10627832]
Chung HR, Shin CH, Yang SW, et al. Subclinical hypothyroidism in Korean preterm infants associated with high levels of iodine in breast milk. J Clin Endocrinol Metab. 2009;94:4444–7. [PubMed: 19808851]
Emder PJ, Jack MM. Iodine-induced neonatal hypothyroidism secondary to maternal seaweed consumption: A common practice in some Asian cultures to promote breast milk supply. J Paediatr Child Health. 2011;47:750–2. [PubMed: 21276114]
Hulse T. Transient neonatal hypothyroidism resulting from maternal ingestion of a traditional Korean seaweed soup. Horm Res Paediatr 2012;78 (Suppl 1):127-8. Abstract. doi: 10.1159/000343182. [CrossRef]

Substance Identification

Substance Name


Scientific Name

Fucus vesiculosus Undaria pinnatifida

CAS Registry Number


Drug Class

Breast Feeding


Complementary Therapies



Plants, Medicinal

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