Recommended levothyroxine dosing may be too high in congenital hypothyroidism

BOSTON — Some infants require a levothyroxine dose reduction for iatrogenic hyperthyroxinemia when following guidelines for initiating therapy for congenital hypothyroidism.

“The current guidelines recommend starting levothyroxine at an initial dose between 10 to 15 mcg/kg, an extremely wide range,” the researchers wrote. “The dosage is influenced by the available tablet strengths [25 mcg, 37.5 mcg (1/2 75-mcg scored tablet), 44 mcg (1/2 88-mcg scored tablet), and 50 mcg]. In general, a 2.4-kg infant will receive 25 mcg daily (10.4 mcg/kg), while a 2.6-kg infant will receive 37.5 mcg daily (14.4 mcg/kg), a dose that is 42% larger than the 2.4-kg newborn’s dose.”

Meghan Elizabeth Craven

Meghan Elizabeth Craven, MD, and Graeme R. Frank, MD, of Cohen Children’s Medical Center in New Hyde Park, New York, evaluated 104 infants newly diagnosed with congenital hypothyroidism (average age at diagnosis, 11 days; average starting dose of levothyroxine, 12 mcg/kg) at Cohen Children’s Medical Center from 2002 to 2012. Researchers sought to evaluate whether newborns treated for congenital hypothyroidism at the higher end of the dosage range become biochemically hyperthyroid and might require a lower dose of levothyroxine.

Fifty-one percent of newborns did not require dose adjustment, 36.5% required dose reduction because of iatrogenic hyperthyroxinemia, and 12.5% required a dose increase because of elevated thyroid-stimulating hormone. Starting doses among the newborns were as follows: 13.2 mcg/kg for those who required a reduction, 11.5 mcg/kg for those who did not need dose adjustment and 10.3 mcg/kg for those who required an increase. Fifty-seven percent of newborns with an initial dose of more than 12.5 mcg per day required a dose reduction at follow-up compared with 26.1% of those with a starting dose of 12.5 mcg per day or less (P = .006573).

“A significantly large portion of patients being treated for congenital hypothyroidism are requiring dose reduction on follow-up due to becoming hyperthyroid on laboratory evaluation, especially when dosed at the higher end of the dosage range,” Craven told Endocrine Today. “Further studies are needed, but we do advise practitioners to think carefully when dosing in the higher end of the dosage range and to make sure they are following laboratory values closely at the recommended 2-week mark.” – by Amber Cox

Reference:

Craven ME, Frank GR. Poster Board FRI 047. Presented at: The Endocrine Society Annual Meeting; April 1-4, 2016, Boston.

Disclosure: The researchers report no relevant financial disclosures.