['Untreated hypothyroidism is known to impair fecundity. Patients treated early for congenital hypothyroidism (CH) have yet to be evaluated in adulthood, because screening programs have been running for only the last 30 years in most industrialized countries.', 'Our objective was to assess the fecundity of young adults treated early for CH and its determinants.', 'Of the 1748 subjects diagnosed with CH in the first 10 yr after the introduction of neonatal screening in France, 1158 completed a questionnaire on fecundity at a mean age of 25.3 yr. This self-administered questionnaire focused on first attempts to have a child and time to pregnancy. The control group was that used in an analogous study on subjects born between 1971 and 1985.', 'Fecundability hazard ratios (HR) were estimated with Cox regression models and adjusted for known fecundity confounders (age, smoking, and reproductive history).', 'Fecundability was similar for the CH and control groups: HR = 1.14 (0.89-1.47) for women, and HR = 0.98 (0.58-1.66) for men. In women, the most severe initial forms of the disease, athyreosis, absence of bone maturation at the knee epiphyseal ossification centers, and a low serum free T(4) concentration at diagnosis (<5 pmol/liter), were associated with lower fecundity: HR = 0.68 (0.50-0.98) (P = 0.02); HR = 0.65 (0.45-0.94) (p = 0.02) and HR = 0.70 (0.50-0.97) (P = 0.03), respectively. However, fecundability was not associated with age at the start of treatment, initial levothyroxine dose, or the adequacy of hypothyroidism control.'] Is fecundity in young adults treated early for congenital hypothyroidism related to the initial severity of the disease : a longitudinal population-based cohort study?