Iodine deficiency in infants leads to delayed growth and development. Some studies have reported iodine deficiency among infants and lactating women. We assessed iodine status in infants and lactating women, as well as the iodine content in breast milk. A cross-sectional study enrolled mother-infant pairs (infants aged 4-6 months), who visited Well Child Clinic at Ramathibodi Hospital, Bangkok, Thailand. Infants were classified by feeding type as breastfed (BF), mixed breastfed and formula-fed (MF), and formula-fed (FF). Demographic and perinatal data were collected. The urinary iodine concentration (UIC) of infants and lactating women, and breast milk iodine concentration (BMIC) were analyzed. Seventy-one infants were enrolled. The median UIC of infants was 282 mcg/L. Breastfed infants had higher median UIC than formula-fed infants (553 vs. 192 mcg/L; p = 0.002). Forty-eight percent of infants had a UIC more than 300 mcg/L. The median UIC and BMIC of lactating women were 149 and 255 mcg/L, respectively. Among the BF group, the infant UIC was correlated with maternal UIC (r = 0.857, p = 0.014). Multiple linear regression showed the BMIC to be associated with maternal UIC (β = 4.03, 95% CI [1.34, 6.71]) and maternal weight (β = 8.26, 95%CI [2.76, 13.77]). Iodine nutrition among our study population was adequate. The median UIC of infants and lactating mothers were 282 and 149 mcg/L, respectively. Breastfed infants had a significantly higher median UIC than formula-fed infants. The BMIC was associated with maternal UIC and maternal weight.