High and low dose initial thyroxine therapy for congenital hypothyroidism

Objective: To assess factors influencing thyroxine (T-4) levels 1 month after initiating replacement therapy for congenital primary hypothyroidism. Methodology: A retrospective review of 41 children with congenital hypothyroidism who received either high or low dose initial T-4 therapy. Thyroid scintiscan was performed, and T-4 levels determined before starting treatment and after 1 month. Results: T-4 levels at 1 month were correlated (r(2) = 0.38, P<0.001) with the pretreatment T-4 level (r = 0.48), as well as with the T-4 dose (r = 0.46). Suboptimal treated T-4 levels (<130 nmol/L) were seen with greater frequency in infants with thyroid agenesis (7/11) rather than ectopia (7/28, P<0.03), despite receiving similar doses of thyroxine. Infants with suboptimal treated Ttl levels had lower pretreatment T-4 levels than those with optimal levels (21+/-7 vs 48+/-34 nmoI/L, P<0.02). Biochemical hyperthyroidism (T-4>216 nmol/L) occurred in six patients; four of six had ectopia. Conclusions: These data suggest that infants with little residual thyroid function should receive higher initial T-4 doses than those with significant ectopia.