[Utility of congenital hypothyroidism screening in neonates for monitoring iodine deficit disorders in the Canary Islands (Spain)]

Abstract

Background: Newborns in European cities where iodine intake is low have been demonstrated to present high frequencies of transitory hypothyroidism. Because the neonatal period is critical for cerebral development, this is a cause for concern. Published studies (WHO/UNICEF/ICCIDD) indicate that neonates with a thyroid-stimulating hormone (TSH) concentration of more than 5 mU/ml revealed by screening for congenital hypothyroidism present mild iodine deficiency.

Objective: To analyze the utility of TSH values as an indicator of the prevalence of iodine deficiency in the general population.

Material and methods: We prospectively evaluated 19 809 neonates, corresponding to all the neonates screened from May 2001 to April 2002 in the Canary Islands.TSH determination in whole blood dried on filter paper was performed using immunofluorescence (Delphia) in the Center for the Detection of Metabolic Disorders in the Canary Islands. The percentage of neonates in each island with TSH values of > 5 mU/l was calculated. Samples of cord blood were not used.

Results: A total of 19 809 infants were analyzed. Of these 1800 had values of TSH > 5 mU/L, representing 9.08 % of neonates. The mean age at blood extraction was 4.31 6 3.78 days (range: 0.5-40). The percentage of neonates with values of THS > 5 mU/L in each island was 13.1 % in Gran Canaria, 5.1 % in Lanzarote, 7.3 % in Fuerteventura, 6.0 % in Tenerife, 6.2 % in La Palma, 6.6 % in Gomera and 10.1 % in Hierro. In 77.5 % of neonates in Gran Canaria blood was extracted for screening within the first 72 hours of life and 15.2 % of these neonates had TSH concentrations of > 5 mU/L. In 22.5 % of neonates blood was extracted on the third day of life or later and 7.9 % of these neonates had TSH values of > 5 mU/L.

Conclusions: In the Canary Islands, the percentage of neonates with iodine deficiency, according to elevated TSH levels detected screening for congenital hyperthyroidism, was small. The validity of TSH level as an indicator of the prevalence of iodine deficiency in the general population is influenced by the days of life at which the blood sample is taken.