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.