PHOENIX — Neonatal thyroid-stimulating hormone data can be used to detect epidemiologic trends in iodine sufficiency in pregnant women, even in countries where iodine intake is usually adequate, based on a study of 54,400 neonates presented at the annual meeting of the American Thyroid Association.
“Readily available neonatal TSH can be used to track the effects of altered trends in maternal iodine nutrition,” said Dr. Peter Smyth of the Conway Institute of Biomolecular and Biomedical Research at the University College Dublin.
Steps can be taken to increase iodine intake in pregnant women (which is important for proper fetal cognitive development) if the neonatal TSH in a population suggests low levels of maternal dietary iodine. The fetus depends on maternal thyroid hormones for normal development during the first 13–15 weeks of pregnancy, Dr. Smyth noted.
To assess the potential role of neonatal TSH as an indicator of a mother's iodine status, researchers screened a birth cohort of babies born in Ireland between 1988 and 2006.
Overall, TSH levels in newborns increased slightly but steadily during the study period, although the proportion of infants with severe iodine deficiency (TSH less than 5 mU/L) remained constant and stayed in a range of 2.35%–2.83%.
Notably, data from routine neonatal TSH screening showed a seasonal variation: Infants born in August had consistently higher TSH levels than infants born in January. Most dietary iodine in Ireland comes from milk and dairy products, and dietary iodine intake is disproportionately lower during the summer because the herd animals are out grazing and not receiving any nutritional supplements, Dr. Smyth explained.
Iodine levels in pregnant women were assessed using urinary iodine (UI) excretion values, and the decline in these values during the study period confirmed that the pregnant female population was borderline iodine deficient but relatively stable, although the UI values reflected the seasonal variation in dietary iodine intake.
From 1988 to 2003, the mean maternal UI values ranged from 70 to 83 mcg/L during the summer months and from 82 to 137 mcg/L during the winter months.
But findings from 2004 and 2005 showed a significant drop in maternal UI levels, which fell to a mean of 45 mcg/L in 2004 and 42.5 mcg/L in 2005. That trend has raised concerns about the need for dietary iodine supplementation in pregnant women in Ireland, said Dr. Smyth.
The study results support the link between declining urinary iodine levels in pregnant women and fetal thyroid function, and the trend data for maternal UI can be used to decide whether to initiate thyroid screening programs during pregnancy.