Iodine is an essential trace nutrient for all infants that a normal component of breastmilk. Infant requirements are estimated to be 15 mcg/kg daily in full-term infants and 30 mcg/kg daily in preterm infants.[1] Breastmilk iodine concentration correlates well with maternal urinary iodine concentration and may be a useful index of iodine sufficiency in infants under 2 years of age, but there is no clear agreement on a value that indicates iodine sufficiency, and may not correlate with infant thyroid function tests.[2-4] A milk iodine value of 100 to 200 mcg/L has been suggested to indicate adequate iodine status in lactating women,[5] although a range as wide as 60 to 465 mcg/L has been proposed.[4] Systematic reviews and studies on iodine nutrition found that iodine in breastmilk is adequate in iodine-sufficient countries, but in countries with iodine fortification of foods, many mothers did not obtain adequate iodine and that additional supplementation was desirable.[6-10] In iodine-deficient areas, supplementation of breastfeeding mothers with iodine appears to be more effective than direct supplementation of the infant in reducing infant iodine deficiency.[11] In the United States, a total iodine intake of 290 mcg daily is recommended during lactation; the WHO recommends a total intake of 250 mcg daily.[9] The American Thyroid Association recommends that breastfeeding women should supplement their diet with a daily oral supplement that contains 150 mcg of iodine, but sustained iodine intake while breastfeeding that exceeds 500 to 1100 mcg daily should be avoided.[12] Vegan and vegetarian mothers appear to have reduced iodine concentrations in their milk and should use iodine supplements.[13] A survey in the United States between 2011 and 2014 found that only 19% of lactating women used a dietary supplement that contained iodine.[14] One study found improved neurodevelopment is the infants of mothers who received 150 mcg of iodine daily during breastfeeding, but a dose of 300 mcg daily did not further improve the outcome.[15]
The use of excessive amounts of iodine in the mother near term and during breastfeeding (e.g., seaweed soup) can increase breastmilk iodine levels and cause transient hypothyroidism in breastfed infants.[16-19] Follow-up at 3 years of age found no long-term thyroid dysfunction in infants exposed to high iodine levels in breastmilk.[20] Supplementing mothers with Grave disease in appropriate doses potassium iodide usually does not adversely affect their breastfed infants, although a few may develop mild hypothyroidism.[21,22] The absorption of iodine can be marked after application to open wounds or mucous membranes. Exposure of mothers to unnecessary iodine who are or will be breastfeeding should be avoided or minimized to the extent possible by avoiding its use on maternal mucous membranes (e.g., vaginal use, wound therapy), avoiding prolonged contact time, avoiding repeated applications, and applying it to the smallest possible surface areas of the body. It is possible that maternal exposure to iodine near term could interfere with thyroid studies done as a part of newborn screening tests.
When treating lactating people after radiation exposure, one dose of potassium iodine 130 mg is recommended at the lowest threshold exposure of >5 Gy to minimize the risk of blocking thyroid function in the fetus or nursing infant. In cases of severe contamination in lactating people, repeat dosing may be given and nursing neonates should be monitored for signs of illness.[23]