PurposeThe incidence of congenital hypothyroidism (CH) has increased in several countries. Lower cut-off in screening programs have led to an increase in the proportion of transient hypothyroidism (TH) cases diagnosed, leading to debate on the associated clinical and economic impact. This study aimed to identify factors that would allow discrimination between TH and permanent CH (PH) in patients with a eutopic thyroid gland.MethodsSixty-six patients with CH from 3 different hospitals were studied: 26 cases of TH, and 40 cases of PH. Laboratory findings and clinical parameters were analysed in 56 patients with eutopic thyroid gland.ResultsInitial serum thyroid stimulating hormone levels and L-thyroxine dose at 12 and 24 months of age were significantly higher in PH than TH patients with a eutopic thyroid gland. The area under the curve for the 12-month and 24-month dose for the prediction of TH in eutopic CH was 0.799 (95% confidence interval [CI], 0.678-0.919; P<0.001) and 0.925 (95% CI, 0.837-1.000; P<0.001), respectively. The optimum 12-month and 24-month dose in predicting TH is 3.25 µg/kg (12-month: sensitivity, 87.1%; specificity, 68.0%; 24-month: sensitivity 93.5%, specificity 88%).ConclusionInfants with CH requiring lower L-thyroxine doses (<3.25 µg/kg) are likely to have TH, and thus might be re-evaluated at 12 months or 24 months rather than 3 years of age.