Effect of temperature changes on the occurrence of congenital hypothyroidism

To investigate an association between seasonal changes in temperature and the prevalence of congenital hypothyroidism (CH) in the Southwest of Iran.

A prospective two-year study conducted from March 2007 to March 2009 during the nationwide screening programme for CH in Ahvaz, the biggest city in southwest Iran. Blood samples were taken from almost all newborns by heel-stick and used in a thyroid-stimulating hormone (TSH)-enzyme-linked immunosorbent assay (ELISA). Serum thyroxine (T4) and TSH were measured if the ELISA-TSH was >5 mIU/L (suspicious cases). Infants were considered to have CH with T4 <6.0 µg/dL and TSH > 10 mIU/L, or with normal T4 values and persistent high TSH values for >2 months (subclinical hypothyroidism). Date of birth and sex were recorded. Detailed temperature data were obtained from the meteorological organization. The relationship between the monthly incidence of CH and the average monthly temperature was investigated.

From 47,075 (50.92% male) newborns, 1131 were referred (recall rate = 2.4%) and 142 infants (51.4% male) were confirmed to have CH. The seasonal distribution of CH cases was 32.4% in the warm period and 67.6% in the cold period of the year (19%, 13.4%, 32.4% and 35.2% in spring, summer, fall and winter, respectively; P = 0.001). A statistical difference was seen between mid-summer (2.8%, warmest time) and early winter (15.5%, coldest time). The above-mentioned difference was the same for suspicious individuals (P < 0.001). The odds of being affected were increased by 4% for each fall of 1°C.

The prevalence of CH has a significant negative correlation with the temperature in the tropical area of Iran.