Iodine concentrations in human milk after maternal high therapeutic potassium iodide dosing during breastfeeding: a contribution from the ConcePTION project.

Iodine is an essential element of human milk. However, high therapeutic maternal doses of potassium iodide (KI) during breastfeeding and its effect on the infant have been insufficiently studied. Therefore, human milk samples were collected within the UmbrelLACT study to determine human milk iodine concentration (HMIC) and estimate infant exposure during and after high maternal KI intake. One mother used KI 3 × 50 mg/day as preparation for thyroidectomy at 5 months postpartum. Human milk samples were collected opportunistically during and within 2 weeks after KI discontinuation. Furthermore, milk samples were collected over 24-h periods at 3 weeks post-discontinuation and monthly from 1 month up to 5 months after discontinuation of KI intake. Breastfeeding was temporarily interrupted during maternal treatment, with partial resumption of breastfeeding 1 week after KI discontinuation, complemented with infant formula upon the participant's decision. HMIC were analysed using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) after basic extraction with tetra-methyl ammonium hydroxide (TMAH) and infant exposure was estimated. Median HMIC were 70.8 (46.6-80.8) µg/mL during and 0.064 (0.016-0.227) µg/mL from day six after maternal treatment stop onwards. After discontinuation of KI intake, HMIC decreased quickly and remained within reference range (0.1-0.2 µg/mL). Using the infant requirement of 15 µg/kg/day, relative infant recommended dose (RIDrecommended) during and after KI intake was 70805% and 98%, respectively. The infant's thyroid values were within normal ranges at 3 months after discontinuation of maternal treatment and no adverse events were reported for the infant. Post-discontinuation HMIC are in line with mean concentrations in Nordic countries (68-90 ng/mL). High therapeutic doses of KI should not be combined with breastfeeding, but based on these results (exclusive) breastfeeding could be resumed from 3 weeks after finalisation of maternal treatment. Further research is warranted to determine exposure and safety for breastfed infants. Registration UmbrelLACT study: NCT06042803 (https://clinicaltrials.gov/study/NCT06042803?cond=umbrellact&rank=1).