Fetal and neonatal thyroid function: review and summary of significant new findings.

PURPOSE OF REVIEW: The purpose of this review is to briefly summarize current knowledge of fetal and neonatal thyroid function, and then to summarize the most significant new findings over the last year that add to our knowledge of the cause, diagnosis, and management of fetal and neonatal thyroid disorders.

RECENT FINDINGS: Significant findings from publications in the last year include a report that inadequate iodine intake during pregnancy exists in many parts of the world. Conversely, maternal exposure to iodinated contrast agents did not affect neonatal thyroid function. A small lowering in the screening of thyroid-stimulating hormone cutoff resulted in nearly a doubling of the birth prevalence of congenital hypothyroidism, but more cases had a thyroid gland 'in situ'. Partial iodination defects are relatively common causes of dyshormonogenesis. Tailoring the initial starting levothyroxine dose to severity of hypothyroidism resulted in rapid normalization of thyroid function. Although consensus guidelines recommend an initial starting dose in the 10-15-mug/kg/day range, the Cochrane collaborative did not find sufficient evidence from randomized controlled trials to confirm the high-dose recommendation. Under or overtreatment of childhood hypothyroidism appears to adversely impact adult cardiovascular function. Adults with congenital hypothyroidism are more likely to have quality of life issues.

SUMMARY: Investigations of the impact of iodine and thyroid hormone transfer continue to improve our knowledge of maternal-fetal thyroid relationships. Screening programs to detect and treat newborns with congenital hypothyroidism have resulted in a dramatic improvement in neurocognitive outcome. Nevertheless, debate continues on the optimal screening test approach and thyroid hormone treatment.