Effect of temperature changes on the occurrence of congenital hypothyroidism

Objectives To investigate an association between seasonal changes in temperature and theprevalence of congenital hypothyroidism (CH) in the Southwest of Iran.Methods A prospective two-year study conducted from March 2007 to March 2009 during thenationwide screening programme for CH in Ahvaz, the biggest city in southwest Iran. Bloodsamples were taken from almost all newborns by heel-stick and used in a thyroid-stimulatinghormone (TSH)-enzyme-linked immunosorbent assay (ELISA). Serum thyroxine (T4) and TSH weremeasured if the ELISA-TSH was .5 mIU/L (suspicious cases). Infants were considered to have CHwith T4 ,6.0 mg/dL and TSH . 10 mIU/L, or with normal T4 values and persistent high TSHvalues for .2 months (subclinical hypothyroidism). Date of birth and sex were recorded. Detailedtemperature data were obtained from the meteorological organization. The relationship betweenthe monthly incidence of CH and the average monthly temperature was investigated.Results From 47,075 (50.92% male) newborns, 1131 were referred (recall rate ¼ 2.4%) and 142infants (51.4% male) were confirmed to have CH. The seasonal distribution of CH cases was 32.4% inthe warm period and 67.6% in the cold period of the year (19%, 13.4%, 32.4% and 35.2% inspring, summer, fall and winter, respectively; P ¼ 0.001). A statistical difference was seen betweenmid-summer (2.8%, warmest time) and early winter (15.5%, coldest time). The above-mentioneddifference was the same for suspicious individuals (P , 0.001). The odds of being affected wereincreased by 4% for each fall of 18C.Conclusion The prevalence of CH has a significant negative correlation with the temperature in thetropical area of Iran.