Abstract: FRI 047

Does Initial Dosing of Levothyroxine in Infants with Congenital Hypothyroidism Lead to Frequent Dose Adjustments Secondary to Iatrogenic Hyperthyroidism on Follow-up Laboratory Evaluation?

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Abstract


BACKGROUND: Congenital hypothyroidism is the most common preventable cause of intellectual disability.  The current guidelines recommend starting levothyroxine at an initial dose between 10-15 mcg/kg, an extremely wide range. 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. view more

BACKGROUND: Congenital hypothyroidism is the most common preventable cause of intellectual disability.  The current guidelines recommend starting levothyroxine at an initial dose between 10-15 mcg/kg, an extremely wide range. 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.
HYPOTHESIS: We hypothesized that a sizable proportion of the newborns treated for congenital hypothyroidism at the higher end of the dosage range become biochemically hyperthyroid at follow-up visit, requiring a reduction of the dose. 
DESIGN/METHODS: Single-center retrospective chart review of infants newly diagnosed with congenital hypothyroidism at Cohen Children’s Medical Center from June 1st, 2004 through December 31st, 2012. Inclusion criteria were infants presenting to the outpatient endocrinology office within the first month of life with primary congenital hypothyroidism identified by New York State newborn screen with a confirmatory elevated TSH. Infants with Down Syndrome and transient hypothyroidism (defined as children who were able to be taken off levothyroxine by 3 years of age) were excluded from the study, as well as patients lost to follow up. Dose of levothyroxine expressed in mcg/kg based on weight at intial visit.
RESULTS: 117 patients were identified, of which 17 were excluded based on above criteria. Of the 100 patients included in this analysis, the average age at diagnosis was 11 days (SD ± 6 days) and the average starting dose of levothyroxine was 12 ± 2.6 mcg/kg. At 4-6 week follow-up, 40% required a dose reduction because of iatrogenic hyperthyroxinemia, 46 % required no dose adjustment, and 15% required an increase in the dose for an elevated TSH. The starting dose of levothyroxine for those requiring a dose reduction, those not requiring an adjustment, and those requiring an increase in the dose were 13.2 ± 2.3, 11.4 ± 2.2 and 10.6 ± 3.0 mcg/kg/day, respectively (p = 0.00039). Of the 64% of infants treated with an initial dose of > 12.5 mcg/day, 58% required a dose reduction at follow-up, compared to 28% of those whose initial starting dose was ≤ 12.5 mcg/kg/day (p = 0.0058).
CONCLUSIONS: When following the guidelines for initiating therapy for congenital hypothyroidism, 40% of the infants required a dose reduction for iatrogenic hyperthyroxinemia. These infants received a higher dose of levothyroxine than the infants who either required no adjustment or required an increase in the dose. A narrower range for initial dosing in congenital hypothyroidism may be appropriate. Further prospective studies will be needed to determine this range.

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