Significant differences in levothyroxine (LT4) dosages for congenital hypothyroidism (CH), which can be permanent (P-CH) or transient (T-CH), and their respective cutoff values have been reported. In Japanese children, however, these values are unknown, and were thus determined in this retrospective single-center study, which included 34 patients— 19 with P-CH and 15 with T-CH. The LT4 dosages of the two groups at ages 1 and 3 yr were compared, and receiver operating characteristic (ROC) analysis was performed to identify the cutoff dosages. The results showed that the LT4 dosages of the P-CH and T-CH groups differed significantly at both ages. When LT4 dosage cutoff at 1 yr of age was set at 2.4 μg/kg/d, the sensitivity and specificity were 93% and 74%, respectively, and when it was set at 1.3 μg/kg/d at 3 yr of age, they were 80% and 84%, respectively, suggesting that when LT4 dosages are ≤2.4 μg/kg/d at 1 yr and ≤1.3 μg/kg/d at 3 yr of age, T-CH should be suspected.