Initial catch-up growth in children with hypothyroidism on thyroxine replacement therapy

Background: There is a paucity of observational data from India on initial catch up growth in hypothyroid children started on thyroxine replacement therapy. Methods: We retrospectively studied 44 children and adolescents (32 girls) with a diagnosis of primary hypothyroidism i.e., (thyrotropin > 15 μIU/mL and thyroxine < 55ng/mL). All were started on treatment with oral thyroxine. The dose was adjusted to maintain thyrotropin between 0.5-5 μIU/mL. Height was measured at baseline and at each follow-up visit and height standard deviation scores (HtSDS) were calculated. We studied the therapeutic benefit of thyroxine replacement therapy on growth in the initial couple of years of treatment. Results: Dose of thyroxine required to restore euthyroidism was 4.1±2.5 μg/kg body weight. The dose fell from 9.7±2.4 μg/kg body weight in infants to 3.0±1.5 μg/kg body weight in adolescence (p<0.001). Likewise, the dose per unit body surface area also fell consistently from 207 ± 70.3 μg/m2 in infants to only 89.3±17.9 μg/m2 in adolescents (p<0.001). The initial HtSDS was -2.0±1.5 and this improved by 0.4 to final value of -1.6±1.3 (p < 0.001) after an average follow up of 14.1±2.5 months. The mean catch-up growth velocity, weighted for the duration of follow-up was 7.7 cm/year. Conclusions: The HtSDS deficit because of hypothyroidism is partially regained in the first few years after treatment.