Can neonatal TSH screening reflect trends in population iodine intake?

Background: The distribution of neonatal blood thyroid-stimulating hormone (TSH) concentrations has been used as an index reflecting population dietary iodine intake, with higher concentrations being indicative of lower iodine intake. We examined this distribution in neonates born in Ireland, where the pregnant population has shown a recent decline in urinary iodine (UI) excretion. Our objectives were to determine if any alteration was observed in the percentage of values > 5.0 mIU/L and whether a trend in neonatal blood TSH was apparent. Methods: Samples drawn from the National Neonatal Screening Programme were assessed during the years 1995 - 2006 from winter (January n = 35,079) and summer (August n 37,940) months, respectively, in view of the known seasonal variation in Irish dietary iodine intake. Results: Apart from the first years studied (1995 - 1996), the proportion of individual blood TSH values > 5.0 mIU/L did not exceed 3%, a value believed to be indicative of iodine deficiency. A significant declining trend in the proportion of blood TSH > 5.0 mIU/L was observed in subsequent years (p < 0.01). While excluding severe iodine deficiency, these analyses failed to detect the slight but highly significant (p < 0.001) tendency toward increasing blood TSH within the 0 - 5.0mIU/L interval in the study population between 1999 and 2006, which was greater in summer than in winter months (p 0.001). Conclusions: These data support a link between fetal thyroid function and a fall in maternal iodine intake. While the findings of the proportion of blood TSH values > 5.0 mIU/L exclude severe maternal or fetal iodine deficiency, a trend toward increasing TSH may provide an early indication of impending iodine deficiency. The findings assume greater importance in the context of declining UI reported from many developed countries even where the proportion of blood TSH values > 5.0 mIU/L is < 3%, thus excluding severe maternal and fetal iodine deficiency.