Despite the relative absence of iodised salt (only ∼4% of table salt is iodised), Ireland has not been regarded as a region with a high prevalence of iodine deficiency disorders. However a recent and worrying decline in urinary iodine (UI) excretion, most marked in the summer months, was observed with median UI of 6183 ug/L between 1988 and 1999; falling to 45.0 ug/L in 2004 and to 42.5 ug/L in 2005 with UI values <50 ug/L (64.5% and 73.9% respectively in summer; winter equivalent 21.423.8%). A slight improvement in iodine status was observed in Summer 2006 (Median 59.5 ug/L). A similar trend towards declining maternal urinary iodine (UI) excretion reported from the US and Australia has reawakened concerns about its consequences for the developing fetus. The objective of this study was to investigate if readily available blood neonatal TSH could be used to provide a bioassay for dietary iodine intake. Although the primary purpose of neonatal TSH screening is the detection of congenital hypothyroidism, the finding of an increased proportion (>3%) of blood TSH values >5.0 mU/L has been recommended by the WHO as an index of iodine deficiency. Analysis of 72,400 neonates screened in Ireland between 1995 and 2006 showed a consistent and highly significant shift towards higher TSH recorded over this period (P<0.001), although the proportion of TSH >5.0 mU/L remained relatively constant (2.352.83%), thus excluding severe iodine deficiency. In view of the observed decline in maternal UI it is surprising that neonatal TSH levels were not even higher. The findings demonstrate that blood neonatal TSH data can be utilised to detect the effects of altered trends in maternal iodine nutrition, even in areas considered iodine replete, without resorting to contentious pregnancy thyroid screening programmes.