Objective To assess the potential association between congenital hypothyroidism (CH) and third-trimester maternal exposure to nitrate and perchlorate in tap water and to particulate matter (PM) in outdoor air. Methods Using the French National Health Data System (SNDS), a retrospective ecological cohort was created, including all children born in France between 2014 and 2019. Ecological data for each child's municipality were used to examine associations, measured as odds ratios (OR), between mean exposure levels and CH in 1,417,402 newborns . Both single and multipollutant analyses were undertaken. To limit residual bias from the administrative nature of SNDS, analyses were conducted at both national and regional levels. The Benjamini-Hochberg approach was used to account for multiple testing. Results Higher exposure to nitrates was associated with a greater risk of permanent CH (OR [95%CI] = 1.017 [1.001;1.035] in the Nord-Pas-de-Calais region and 1.024 [1.001;1.048] for the Pays-de-la-Loire region). Higher exposure to PM2.5 or PM10 was associated with a lower risk of permanent CH (OR [95%CI] = 0.928 [0.873;0.982] for PM10 in the Nord-Pas-de-Calais). These findings were confirmed in multi-pollutant analyses. Conclusions In some French regions, prenatal exposure to nitrate ions in tap water was significantly associated with an elevated risk of permanent HC, while exposure to PM was significantly associated with a decreased risk. However, using municipal-level exposure proxies may introduce ecological bias, and results were not robust enough to draw firm conclusions about pollutant influence on CH risk.