OBJECTIVE: To investigate the intake of iodine in mother-infant pairs. DESIGN: An exploratory, cross-sectional study. Iodine intake was estimated using Nutritics nutritional analysis software, following 24-h dietary recall. Iodine-rich foods were grouped and compared between those women who met the UK reference nutrient intake (RNI) for iodine (140 µg/d) and those who did not. SETTING: Online and telephone questionnaires. PARTICIPANTS: Self-selecting caregivers of infants aged 6-12 months. RESULTS: Ninety-one mother-infant pairs with a mean (sd) age of 33·2 (4·1) years and 8·4 (1·3) months, respectively, were included. Most mothers were exclusively breast-feeding (54·9 %). The estimated maternal median iodine intake from food and supplements (median 140·3 µg/d, just meeting the UK RNI for women of reproductive age, but not the World Health Organisation (WHO) or British Dietetic Association (BDA) recommendations for lactating women (250 µg/d and 200 µg/d, respectively). Forty-six (50·5 %) of mothers met the UK RNI. Estimated intakes of fish, eggs, cow's milk and yoghurt/cream/dairy desserts were significantly greater, whilst intakes of plant-based milk alternative drinks were significantly less in mothers who met the RNI for iodine ( CONCLUSIONS: Women in the UK may not consume enough iodine to meet the demands of lactation. Guidance on iodine-containing foods, focussed on intake before and during pregnancy and lactation and mandatory fortification of plant-based milk-alternatives could all serve to avoid deficiency.