Levothyroxine Requirement in Congenital Hypothyroidism: 12-year Longitudinal Study

Background: The replacement therapy with levo-thyroxine (LT4) in congenital hypothyroidism (CH) aims to ensure normal growth and neuropsychological development. Few data are available about the appropriate dose during childhood and early adolescence. Objective and hypotheses: i) To evaluate LT4/kg per day requirement from diagnosis until 12 years of age; ii) to assess any differences in relation to the different etiology of CH as concerns the LT4/kg per day requirement. Method: Multicentric observational study; 216 patients (142 females) with permanent CH classified as athyreosis, ectopia, in situ gland, on the basis of the thyroid imaging; LT4 dose was recorded from 6 months to 12 years of age and the LT4/kg per day calculated. Results: The LT4/kg per day requirement statistically decreased year by year, irrespective of etiology. It was about 3–4 mg/kg per day from 1 to 5 and about 2–3 mg/kg per day from 6 to 12 years of age. It was significantly lower in patients with in situ gland than with athyreosis and with ectopic gland from the age of 1 year. Only at 1, 2, and 10 years the LT4/kg per day requirement was higher in athyreotic than in ectopic patients. The LT4/kg per day requirement at 6 months of age was correlated with the requirement at each later time point. The LT4/kg per day dose was modified less frequently in patients with in situ thyroid (40.5%) than in patients with ectopic gland (47.4%) or with athyreosis (48.9%). Conclusion: Euthyroidism may be achieved by 3–4 and 2–3 mg/kg per day of LT4 in preschool and in school CH patients. The LT4/kg per day dose is affected by the etiology: patients with in situ gland require a lower dose than the other ones. The patients with ectopia or athyreosis require more frequently a change in the daily dose, and thus such patients have to be followed up more frequently. Since the age of 6 months, some patients require higher doses than other ones, irrespective of etiology.