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The aim of the replacement therapy with levothyroxine in congenital hypothyroidism (CH) is to correct hypothyroidism and ensure normal growth and neuropsychological development. Few data are available about the appropriate dose during childhood and early adolescence; therefore, we performed a multicenter observational study in a large population of patients with CH to assess the required levothyroxine dose to obtain euthyroidism. We recruited 216 patients with permanent CH classified into three groups (agenesia, ectopia, and in situ gland) on the basis of the thyroid imaging. The levothyroxine dose was recorded at 6 and 12 months and then yearly until 12 years of age. The daily levothyroxine requirement progressively decreased during the follow-up, irrespective of etiology. It was significantly lower in patients with in situ gland than in patients with athyreosis during the entire study period and with ectopic gland from the age of 1 year. The levothyroxine requirement at 6 months of age was correlated with the requirement at each later time-point. The daily dose was modified less frequently in patients with in situ thyroid (36 %) than in patients with ectopic gland (41.4 %) or with athyreosis (43.6 %). Patients with in situ gland required a lower dose than the other two subgroups. The dose at 6 months seems predictive of the requirement until 12 years of age. Euthyroidism may be achieved in pre-school and in-school patients by 3-4 and 2-3 µg/kg/day (70-90 and 60-80 µg/m(2)/day) of levothyroxine, respectively.

Background: Linear growth and final height are reported as normal in congenital hypothyroid patients in the neonatal screening era., Methods: We evaluated the final height in 215 patients with congenital hypothyroidism to assess if it improved over the last 2 decades., Results: Final height (-0.1 ± 1.0 SDS) was higher than target height (-0.8 ± 1.0 SDS, p < 0.001) and not different among the 4 quartiles for birthdate. It was correlated with target height (r(2) = 0.564, p < 0.001) and height at puberty onset (r(2) = 0.685, p < 0.001), but not with age at diagnosis or the starting LT4/kg/day dose. The curve fitting analysis showed that the age at diagnosis progressively decreased during the 20-year study period, while the target height and the starting LT4/kg/day increased. Final height was not affected by the birthdate, the age at diagnosis, the starting LT4 dose., Conclusions: The final height is higher than the target height, but despite the improvement in the screening and the treatment, it did not improve over the last 20 years. These findings are in keeping with the described secular trend and suggest that earlier diagnosis and replacement therapy do not significantly modify final height in these patients.