CONTEXT: The increase in the incidence of congenital hypothyroidism (CH) reported worldwide may in part be explained by an increase in the transient form of CH. OBJECTIVES: We aimed to estimate the proportion of transient CH (TCH) in France, and to identify associated neonatal and young child characteristics. PATIENTS AND METHODS: We used probabilistic record linkage to link children with eutopic gland born between 2006 and 2012 recorded in the national French CH registry and the French national health data system (SNDS). Of the 703 children recorded, 484 (68.8%) were linked. We retrospectively examined reimbursement for oral levothyroxine (LT4) between 01/01/2006 and 31/12/2017. Children who had discontinued treatment for six months or more before 31/12/2017 were classified as having TCH. We used a Cox model to examine the factors associated with TCH. RESULTS: Among the main study sample (n=471), 53.5% were female, 14.2% were preterm, and 13.8% had low birth weight. One-quarter (n=111, 24.3%) had mild CH (thyroid-stimulating hormone (TSH)\textless50mU/L (serum) at diagnosis and a median LT4 dose at treatment initiation of 30 μg/day. One third (n=155, 32.9%) had TCH. Premature birth (adjusted Hazard Ratio=2.1 [1.0-4.2]), a TSH\textless50 mU/L at CH screening (7.4 [3.2-17.1]), LT4 dose received at 12 months of age (0.98 [0.97-0.99, p=0.003]), congenital cardiac malformations (6.6 [1.5-29.0]), and year of birth (1.2 [1.1-1.4]) were all associated with TCH. CONCLUSIONS: One third of the children had TCH and it was associated with several characteristics at birth and post-partum. These data are useful for CH medical management and epidemiological surveillance.