Objective: To present data on the relationship between the concentration of thyroid-stimulating hormone (TSH) in whole blood or serum from neonates and the concentration of iodine in their mother's urine collected at birth to contribute to the contention that the recommended iodine intake during pregnancy should be increased.
Design and setting: Data were provided by current programmes of neonatal screening of congenital hypothyroidism in Bangkok and rural areas of Thailand.
Subjects: A total of 5144 cord serum samples were collected in 2003 and measured for TSH concentrations. Paired samples of blood and urine were collected in 2000 from 203 infants and their mothers and from 1182 infant-mother pairs in 2002-03 in six rural provinces. Iodine was measured in the urine and TSH was measured in cord serum.
Results: The urinary iodine concentration of mothers in rural Thailand is adequate, with a median of 103 microg l-1. However, in 2000, the median urinary iodine concentration of mothers in Bangkok was only 85 microg l-1. The concentration of TSH in whole blood collected on filter paper from neonates was not sensitive enough to be used as a monitoring tool for iodine nutrition in the neonates, as there was no relationship with the concentration of iodine in the urine of the children's mothers. This was in contrast to the concentration of TSH in serum collected from cord blood.
Conclusions: Several conclusions were drawn from this data: 1) Neonatal TSH screening using whole blood collected from a heel prick at 3 days of age is not sensitive enough to assess the iodine nutrition of neonates; 2) Neonatal TSH screening using cord sera can be used to assess iodine nutrition in neonates; 3) The optimum median maternal urinary iodine concentration in Thailand appears to be 103 microg l-1; 4) The criteria proposed by WHO, UNICEF, and ICCIDD to assess iodine nutrition using data on neonatal TSH concentrations should be reassessed; and 5) Neonatal TSH screening can be effectively performed by collecting cord serum in district hospitals in Thailand.