Neonatal TSH levels as an index of iodine sufficiency: differences related to time of screening sampling and methodology.

INTRODUCTION: Current WHO guidelines consider that under adequate iodine intake <3% of newborns should have neonatal TSH levels of >5 mU/l blood when screening is performed in cord blood or at 3 days to 3 weeks of age.
OBJECTIVE: To estimate whether this absolute criterion when applied to newborns older than 48 h of age and native to Buenos Aires coincides with the traditional ones (goiter and urinary iodine in school-age children (SAC)), and if the evaluation varies with either the methodology used for TSH measurements and/or the time of specimen sampling. POPULATION AND METHODS: TSH was measured by an immunofluorometric assay (IFMA) on filter paper blood spots of 186 cord blood samples, 112 babies <48 h of age and 1,500 newborns >48 h of age, and by immunoradiometric assay (IRMA) in 238 newborns. The WHO ICCIDD absolute criteria were applied to each population. Thyroid volume was assessed by direct palpation in 500 SAC, and in 100 of them urinary iodine levels were measured.
RESULTS: TSH levels were >5 mU/l blood in 11.3% of the cord blood samples and in 3.6% of the samples from babies <48 h of age, suggesting mild iodine deficiency. TSH was >5 mU/l in 2.7% of newborns >48 h of age tested by IFMA (iodine sufficient) and in 30% measured by IRMA (moderate iodine insufficiency). Median urinary iodine and goiter prevalence in SAC were 143 mug/l and 4.5%, respectively, as expected in an iodine-sufficient area. conclusion: The TSH levels in Buenos Aires conform with the WHO criterion that defines iodine sufficiency. Application of this criterion, however, to cord blood samples or samples from babies <48 h old and the use of different methodologies may lead to erroneous conclusions.