Thyroid Functions in Healthy Infants during the First Year of Life

Objective To study the pattern of thyroid function testing in healthy newborns during the first year of life. Study design We used the computerized database of a health management organization. Among the 18 507 infants insured by the Clalit Health Services born in the Sheba Medical Center between 2002 and 2007, 652 full-term healthy newborns with birth weight >2 kg and no significant perinatal morbidity underwent thyrotropin (TSH) determination as outpatients in their first year of life. The Clalit Health Services database provided demographic data, laboratory results, and dispensed medications for the newborns and their mothers. Results Initial serum TSH levels were within normal range (0.35-5.5 mIU/L) in 91.1%, elevated (>5.5-≤10 mIU/L) in 8.3%, and highly elevated (>10 mIU/L) in 0.6% of the studied cohort. The 97.5 and 2.5 percentile values were 7.4 and 0.74 mIU/L, respectively. TSH measurements were repeated in 34.2%, 72.2%, and 100% of children with normal, elevated, and highly elevated initial levels, respectively; results were normal in 96%, 74%, and 50% of patients with initial normal, elevated, and highly elevated TSH, respectively; repeated TSH levels were >97.5 percentile in 35% of patients with initial TSH >97.5 percentile compared with 1% with first results <97.5 percentile (P =.005). Only 4 (0.6%) of the 652 newborns included in the study received thyroxin treatment. Conclusion The normal TSH levels found in most healthy infants with normal thyroid screening and the spontaneous normalization of TSH values below 7.4 mIU/liter, substantiate the reliability of the screening, reduce unnecessary work-up and unnecessary thyroxin treatment of neonates meeting these criteria.