Increased iodine deficiency in Victoria, Australia: analysis of neonatal thyroid-stimulating hormone data, 2001 to 2006

To the Editor: Rahman and colleagues suggest that iodine deficiency in Victoria increased between 2001 and 2006, based on the findings of thyroid-stimulating hormone (TSH) levels in neonates at routine newborn screening.1 Indeed, their data as presented suggest a doubling of the percentage of mothers with iodine deficiency to over 9% during that period. This could be correct. Certainly, as they state, there is much evidence to suggest that there is mild iodine deficiency in Australia. However, there are caveats about the data they report which are not mentioned.

Data from New South Wales do not show this trend. While they do suggest a degree of mild iodine deficiency, there was no increase in the percentage of neonates with TSH levels > 5 mIU/L of whole blood from 2002 to 2009 (Box), although the average age at sampling falls slightly (from 2.96 to 2.32 days) over this period.

The World Health Organization has defined iodine sufficiency as being indicated, inter alia, when more than 3% of newborns aged 3–4 days have a TSH level > 5 mIU/L of whole blood.2 Factors that affect the TSH level in a newborn screening program include the precise age at sampling, and any changes to the method of TSH analysis used. In a Swiss study assessing the efficacy of iodine supplementation, there was a small but significant decrease in the TSH level from Day 3 to Day 4 of age.3 This is unsurprising: following the TSH surge in the first hours after birth, TSH levels decline gradually to a steady level at about Day 7.4

There could well have been a trend to earlier sampling in Victoria, within the bounds of the 2–4 days of age assay that Rahman and colleagues mention, over the period studied, but these crucial data are not given. The dried blood spot TSH assay method is not described either. A change in any aspect of the methodology; for example, if the manufacturer modified the antibody used, may produce a small, clinically insignificant but numerically significant, change in results.

If the data presented by Rahman and colleagues for Victoria do not have these biases, then the situation warrants further investigation, but whatever is happening in Victoria seems not to be replicated over the border.