Seasonal variation of hypothyroidism and recall rate in neonates

Awareness of the distribution of recall rate and CH incidence in different seasons could provide a better understanding of their etiologies and appropriate allocation of resources in a CH screening program. This report illustrates the distribution of recall rate and CH incidence in different seasons during the program of screening for CH in Tehran and Damavand. From March 1998 to March 2002 (48 months), cord blood spot samples from 8 hospitals and a rural birth center in Tehran and Damavand were collected on filter papers. TSH values were measured using a two-site IRMA method. Cord TSH levels  20 mU/L were considered abnormal and recalled for serum confirmatory tests. CH was diagnosed by abnormal serum TSH and T4 levels using age adjusted reference values for the age. The odds ratios and 95% confidence intervals (CI) for the occurrence of cord TSH levels  20 mU/L and CH in winter and other seasons were assessed. Also, the effect of using iodine-containing antiseptics for the maternal preparation prior to delivery was assessed. Out of 29908 cord blood samples, 314 had TSH levels of  20 mU/L (recall rate: 10.5‰). The recall rate was 13.1‰ and 9.5‰ of live births in winter and other seasons, respectively. The odds ratio of occurrence of cord TSH  20 mU/L in winter to other seasons was 1.39 with a 95% CI of 1.10-1.76 (P<0.01). The effect of iodine-containing antiseptics on the increased recall rate in winter as compared to other seasons was not significant. Sixteen (1: 502 live births) and 15 (1: 1458 live births) CH neonates were born in winter and other seasons, respectively (odds ratio: 2.91 95% CI: 1.44-5.89 [p<0.01]). The recall rate and CH occurrence are higher in winter than in other seasons.