High and low dose initial thyroxine therapy for congenital hypothyroidism

OBJECTIVE: To assess factors influencing thyroxine (T4) 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 T4 therapy. Thyroid scintiscan was performed, and T4 levels determined before starting treatment and after 1 month. RESULTS: T4 levels at 1 month were correlated (r2 = 0.38, P < 0.001) with the pretreatment T4 level (r = 0.48), as well as with the T4 dose (r = 0.46). Suboptimal treated T4 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 T4 levels had lower pretreatment T4 levels than those with optimal levels (21 +/- 7 vs 48 +/- 34 nmol/L, P < 0.02). Biochemical hyperthyroidism (T4 > 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 T4 doses than those with significant ectopia.