Measuring levels of neonatal thyroid-stimulating hormone (TSH) may not be sufficiently sensitive for inferring the extent of iodine deficiency in a given population, study results suggest.
Stefanie Vandevijvere (Scientific Institute of Public Health, Brussels, Belgium) and co-workers found that, despite pregnant women in Belgium being mildly iodine deficient, neonatal TSH values were unexpectedly low.
"Although neonatal TSH is useful to detect severe iodine deficiency it should not be recommended presently for the evaluation of iodine status in mildly iodine deficient regions," the team comment in PLoS One.
The utilization of neonatal TSH is an attractive method because it is assumed that the thyroid of the newborn is very sensitive to iodine status and even mild iodine deficiency during pregnancy will cause an increase in neonatal TSH secretion. In addition, in countries where such a screening program exists, the utilization of neonatal TSH concentrations does not incur an extra cost.
Indeed, the World Health Organization proposes that neonatal TSH concentrations above 5 mU/L in less than 3% of the population indicate iodine sufficiency.
Several studies have attempted to apply neonatal TSH values in determining population iodine status, with some successful and others providing conflicting or uncertain data.
Vandevijvere and colleagues assessed the matter in Belgium, where a recent study has shown that pregnant women are mildly iodine deficient with a median urinary iodine concentration of 124.1 mg/L.
They gathered data on all newborns born in Belgium during the period 2009 to 2011 (n=377,713), except those suffering from congenital hypothyroidism and premature neonates.
Analysis revealed that the frequency of neonatal TSH concentrations above 5 mU/L from 2009 to 2011 in Belgium fluctuated between 2.6% and 3.3% in the centers that used the same TSH assay.
There was a significant inverse association between neonatal TSH level and birthweight, while there was a significant positive association between neonatal TSH level and pregnancy duration. The longer the duration between birth and screening or birth and analysis, the lower the TSH level.
Neonatal TSH levels were significantly lower in winter than in spring or autumn, and lower in spring and summer than in autumn, while they were significantly higher in spring compared with summer.
"Several factors, other than iodine intake, influence neonatal TSH concentration and may explain to some extent the conflicting results when applied for the monitoring of iodine status," Vandevijvere et al comment, adding that mode of delivery, maternal smoking status, use of iodine-containing antiseptics during delivery, and multiple pregnancies could all play a role.
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