A thyroxine dosage of 8 μg/kg per day is appropriate for the initial treatment of the majority of infants with congenital hypothyroidism

Abstract

 The adequate l-thyroxine dosage for the initial treatment of infants with congenital hypothyroidism is a subject of controversy. Some recommend higher dosages (> 10 μg/kg/day) to ensure adequate levels, while others advocate lower dosages to permit normalisation of thyroid status. The aim of this study was to evaluate the␣results of a treatment strategy using an initial dosage of 7.5–8.0 μg/kg per day, TSH measurements being␣taken at 15 and 30 days of treatment. Fifty one newborns infants with primary congenital hypothyroidism␣detected by neonatal screening were treated with the same therapeutic strategy. A mean l-thyroxine dosage of 7.9 μg/kg per day at the onset of treatment and 6.6 μg/kg/d at 2 months, normalised the FT₄ and FT₃ levels at 15 days in 100% and TSH levels at 2 months in 90% of cases. Many patients showed elevated levels of FT₄ and a systematic higher initial dosage could expose many infants to a dangerous hyperthyroidism. Patients with abnormal TSH levels at 2 months already had higher TSH levels in the first 8 weeks of life and, despite higher l-thyroxine dosage, also exhibited lower FT₄ and FT₃ levels. These patients who needed an early increase in dosage had already shown a more profound ante and neonatal hypothyroidism. This subgroup of patients require a higher dosage of thyroxine and early assessment of FT₄, FT₃ and TSH levels are required for optimum dosage choice.

Conclusion Even though a subgroup of patients may require a higher dosage of l-thyroxine, an initial␣dosage of 7.5–8.0 μg/kg per day, with an early assessment of␣FT₄, FT₃, and TSH levels, is adequate for the treatment of the majority of infants with congenital hypothyroidism.