In congenital hypothyroidism, an initial L-thyroxine dose of 10-12 μg/kg/day is sufficient and sometimes excessive based on thyroid tests 1 month later.

Background: Current guidelines recommend an initial L-thyroxine (L-T4) dose of 10-15 μg/kg/day for the treatment of congenital hypothyroidism (CH). We analyzed our data for the treatment outcome at 1 month after we noted a frequent overtreatment even at the lower end of this dose range.

Methods: A 3-year chart review of 55 patients with confirmed CH was performed. The patients were divided to three groups based on L-T4 dose: Group 1 (6-9.9 μg/kg), Group 2 (10-11.9 μg/kg), and Group 3 (12-15 μg/kg). Overtreatment was defined as T4>16 μg/dL/free T4>2.3 ng/dL±thyroid-stimulating hormone (TSH) <0.5 μIU/L and undertreatment was defined as TSH>6 μIU/L at 1 month.

Results: At 1 month, 45.8%, 37.5%, and 16.6% in Group 1, 30%, 55%, and 15% in Group 2, and 0%, 75%, and 25% in Group 3 had target labs, overtreatment, and undertreatment, respectively.

Conclusions: An initial L-T4 dose of 10-11.9 μg/kg for TSH>100 μIU/L and 8-10 μg/kg for TSH<100 μIU/L at diagnosis met and often exceeded the target thyroid levels at 1 month. More frequent overtreatment was seen when >12 μg/kg was given.