A comprehensive assessment of urinary iodine concentration and thyroid hormones in New Zealand schoolchildren: a cross-sectional study

Background: Insufficient iodine in children's diets is of concern because thyroid hormones are needed for normal growth and development, particularly of the brain. This study aimed to carry out a comprehensive assessment of the iodine status of New Zealand schoolchildren using a range of biochemical indices suitable for populations (i.e. urinary iodine concentration) and individuals (i.e. thyroid hormones). Methods: The New Zealand National Children's Nutrition Survey was a cross-sectional survey of a representative sample of schoolchildren aged 5-14 years. Children were asked to provide a casual urine sample for the determination of urinary iodine concentration (UIC) and a blood sample for the determination of thyroglobulin (Tg), Thyroid Stimulating Hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3). Results: The median UIC was 68 mu g/L (n = 1153), which falls between 50-99 mu g/L indicative of mild iodine deficiency. Furthermore, 29% of children had an UIC < 50 mu g/L and 82% had an UIC < 100 mu g/L. The median Tg concentration was 12.9 mu g/L, which also falls between 10.0-19.9 mu g/L indicative of mild iodine deficiency. The Tg concentration of children with an UIC < 100 mu g/L was 13.9 mu g/L, higher than the 10.3 mu g/L in children with an UIC > 100 mu g/L (P = 0.001). The mean TSH (1.7 mU/L), fT4 (14.9 pmol/L), and fT3 (6.0 pmol/L) concentrations for these mildly iodine deficient New Zealand children fell within normal reference ranges. Conclusions: The UIC and Tg concentration indicate that New Zealand schoolchildren were mildly iodine deficient according to WHO/UNICEF/ICCIDD, and both are suitable indices to assess iodine status in populations or groups. The normal concentrations of TSH, fT4 and fT3 of these children suggest that these thyroid hormones are not useful indices of mild iodine deficiency.