CONTEXT:

Thyroid hormone is essential for normal brain development. Limited data are available regarding whether thyroid function in neonates influences later cognitive development.

OBJECTIVE:

Our objective was to study associations of newborn T4 levels with maternal thyroid function and childhood cognition.

DESIGN AND SETTING:

We studied participants in Project Viva, a cohort study in Massachusetts.

PARTICIPANTS:

We studied a total of 500 children born 1999--2003 at 34 wk or more.

MAIN OUTCOME MEASURES:

We determined cognitive test scores at ages 6 months and 3 yr.

RESULTS:

Mean newborn T4 at a mean age of 1.94 d was 17.6 (sd 4.0) microg/dl, and levels were higher in girls [1.07 microg/dl; 95% confidence interval (CI) 0.38, 1.76] and infants born after longer gestation (0.42 microg/dl; 95% CI 0.17, 0.67 per wk). Newborn T4 levels were not associated with maternal T4, TSH, or thyroid peroxidase antibody levels. On multivariable linear regression analysis, adjusting for maternal and child characteristics, higher newborn T4 was unexpectedly associated with poorer scores on the visual recognition memory test among infants at age 6 months (-0.5; 95% CI -0.9, -0.2), but not with scores at age 3 yr on either the Peabody Picture Vocabulary Test (0.2; 95% CI -0.1, 0.5) or the Wide Range Assessment of Visual Motor Abilities (0.1; 95% CI -0.2, 0.3). Maternal thyroid function test results were not associated with child cognitive test scores.

CONCLUSIONS:

Newborn T4 concentrations within a normal physiological reference range are not associated with maternal thyroid function and do not predict cognitive outcome in a population living in an iodine-sufficient area.