What is already known on this topic? Setting a lower thyroid stimulating hormone (TSH) referral cut-off value in neonatal screening programs is increasingly common. However, this had resulted in referral of neonates with lower TSH concentrations and an increase in transient mild TSH elevation. Identifying infants who are likely to have transient TSH elevation and therefore withdrawing replacement therapy earlier would be of clinical benefit. What this study adds? L-thyroxine replacement dose requirement at the sixth month of therapy may be a good marker for predicting those with transient elevated TSH in patients with an eutopic thyroid gland. Objective: The tendency to reduce thyroid stimulating hormone (TSH) referral cut-off values in congenital hypothyroidism (CH) neonatal screening programs has resulted in an increase in the incidence of CH, but also the referral of infants with mild transient elevation of TSH. Therefore, there is a need to develop markers for differentiation of transient elevated TSH and permanent CH as early as safely possible to avoid unnecessary treatment. The aim was to evaluate sixth-month L-thyroxine (LT4) dose as a predictive marker for differentiation of transient elevated TSH and permanent CH. Methods: Data of patients who had been followed after referral from the neonatal screening programme between the year 2010 and 2019 in a tertiary pediatric endocrine centre were examined retrospectively. Results: There were 226 cases referred, of whom 186 (82.3%) had eutopic thyroid gland, and 40 (17.7%) had dysgenetic gland. In patients with a dysgentic gland there was a non-significant tendency to have lower diagnostic free thyroxine concentration but significantly higher TSH compared with those with eutopic gland (p=0.44 and p=0.023, respectively). Patients with thyroid dysgenesis required higher initial and six month LT4 doses compared with those with eutopic glands (p=0.001). Receiver operator curve analysis showed the optimum cut-off value for LT4 at six months for transient vs. permanent CH was 2 microg/kg/day (sensitivity 77% and specificity 55%), regardless of etiology. Similarly, in patients with eutopic glands the optimum cut-off value for LT4 dose at six months for permanent vs. transient patients was 2 microg/kg/day (sensitivity 72% and specificity 54%). Conclusion: Results suggest that LT4 requirement at six months of therapy may be a good marker for predicting transient TSH elevation in patients with eutopic thyroid gland, thus facilitating the decision to halt LT4 therapy. Keywords: Congenital hypothyroidism, transient, permanent, six month L-thyroxine dose