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% Cl, 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.