Difference in the incidence of congenital hypothyroidism among world countries.

To the Editor: The incidence of congenital hypothyroidism (CH), which is one of the most common preventable causes of mental retardation, has been shown to vary among different parts of the world, and the global average of CH is estimated 1 per 3000 to 4000 live births.1,2

Abdelmoktader3 recently conducted a population-based case– control study to investigate the risk factors of CH in Egypt and indicated that birth defects, female gender, gestational age >40 weeks, twins, and gestational diabetes were significantly associated with congenital hypothyroidism. In another population-based case–control study in Italy,4 factors related to CH including birth defects, female gender, maternal diabetes, twins, preterm delivery, and gestational age >40 weeks were also confirmed. Similarly, in a matched case–control study in Iran, we also revealed that factors such as twin, birth season, maturity, jaundice at birth, birth weight, age at pregnancy, maternal anemia and goiter, gestational age, delivery type, father’s education and smoking status, and consanguinity have an effect on the incidence rate of CH.5

All of the mentioned studies have concluded that genetic and environmental factors contribute to the etiology of CH in the region of study that may also be applicable to other developed or developing countries. Questions remain as to why some countries have a higher incidence rate of CH than others while there is a similarity in risk factors of CH among different countries. We do not know the underlying causes of this difference. These hypotheses could be tested in a global case–control study considering all the potential confounding factors. However, the results from the comparison between studies in the different parts of world led us to formulate a new hypothesis: perhaps the interaction of environmental factors and other factors on CH exists higher in some regions than in other. However, geographical differences and climate may play an important role in the occurrence of CH compare to genetic factors. A comparative evaluation in the form of a global case–control study among different countries is recommended to test the abovementioned hypotheses.