Neonatal thyroid-stimulating hormone concentrations in northern Sydney: Further indications of mild iodine deficiency?

Objective: To determine whether thyroid-stimulating hormone (TSH) concentrations in a large sample of neonates meet World Health Organization criteria for an iodine-replete population (<3% of neonates with whole-blood TSH concentrations > 5 mlU/L), and, in a small subset of neonates, to examine the correlation between maternal urinary iodine and neonatal TSH concentrations. Design: Cross-sectional study of neonatal whole-blood TSH values obtained as part of a routine newborn screening program. Setting: Royal North Shore Hospital (RNSH) in northern Sydney. Participants: Two anonymous samples of neonates born at RNSH (1316 infants born between August 1998 and April 1999 and 1457 infants born between 1 March and 31 December 2000); and 84 infants whose mothers had attended RNSH between September 1998 and August 1999 and supplied a urine sample for iodine measurement. Main outcome measures: Iodine status of neonates (proportion with whole-blood TSH values > 5 mlU/L), and urine iodine concentrations of pregnant women. Results: In the two large population samples of neonates, 8.1% (95% CI, 6.6%-9.5%) and 5.4% (95% CI, 4.3%-6.6%), respectively, had whole-blood TSH values > 5 mlU/L (prevalence range for mild thyroid deficiency, 3%-19%). Comparing the TSH values of the 1316 anonymous infants and the 84 identified infants showed no difference between the proportions with TSH values > 5 mlU/L (8.1% v 10.7%, respectively; P = 0.39). Urine iodine concentrations in the 84 pregnant women indicated borderline mild iodine deficiency. TSH values in their 84 infants were positively correlated with maternal urine iodine concentrations. Conclusions: Our results suggest that the population of northern Sydney may have mild iodine deficiency. However, the expected relationship between maternal urine iodine levels and neonatal TSH concentrations was not found.