Longitudinal study of serum free thyroxine and thyrotropin levels by chemiluminescent immunoassay during infancy.

Abstract

From October 1996 to December 1997, ninety normal infants born at full-term were enrolled in this study. There were 59 female infants and 31 male infants. Both serum free thyroxine and thyrotropin levels were measured by chemiluminescent immunoassay in the first week (n = 75), 3 months (n = 66), 6 months (n = 79), and 12 months (n = 77) of age. Serum free thyroxine levels (ng/dL) (mean +/- standard deviation) were 2.2 +/- 0.5, 1.3 +/- 0.2, 1.2 +/- 0.2, 1.0 +/- 0.2 for each age group, respectively. Serum thyrotropin levels (mIU/L) (mean +/- standard deviation) were 9.7 +/- 6.7, 3.0 +/- 1.6, 2.7 +/- 1.3, 2.5 +/- 1.2 for each age group, respectively. There was no significant difference between the levels of these two parameters between male and female. The present study provides the reference data for the normal range of thyroid function during infancy. Our results also show that the serum free T4 and TSH levels tend to decline during the first year of life.