Neonatal thyroid screening using serum thyrotropin (TSH) as the primary screening test detects not only permanent sporadic congenital hypothyroidism, whose incidence is about 1 per 4000 births, but also compensated or transient primary hypothyroidism, whose incidence can be as high as 1 in 10 neonates and whose main cause is iodine deficiency. Elevated serum TSH in the neonate indicates insufficient supply of thyroid hormones to the developing brain, and therefore, constitutes the only indicator that allows prediction of possible impairment of mental development at a population level, which is the main consequence of iodine deficiency. Therefore, the World Health Organization (WHO), United Nations International Children's Emergency Fund (UNICEF), and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) included neonatal TSH as one of the indicators for assessing iodine deficiency disorders (IDD) and their control. In the absence of iodine deficiency, the frequency of neonatal TSH above 5 mU/L whole blood (or 10 mU/L serum) is less than 3%. A frequency of 3%-19.9% indicates mild IDD. Frequencies of 20%-39.9% and above 40% indicate moderate and severe IDD, respectively. Neonates exhibit elevated serum TSH more frequently than adults for a similar degree of iodine deficiency. Consequently, they appear hypersensitive to the effects of iodine deficiency. This characteristic is explained by a particularly low iodine content of the thyroid of neonates and, consequently, by an accelerated turnover rate of their intrathyroidal iodine reserves. This turnover rate is 1% in adults. It is 17% in the neonate in conditions of iodine repletion, but is as high as 62% and 125% in conditions of moderate and severe iodine deficiency, respectively. Such an accelerated turnover rate requires thyroid hyperstimulation by TSH that is morphologically evidenced even in moderately iodine deficient neonates. Neonatal screening using primary TSH is implemented in most countries with mild IDD where it detects the cases of sporadic, permanent congenital hypothyroidism and where it is also used as a monitoring tool for IDD evaluation and control. However, the implementation of such programs in countries affected by moderate or severe IDD is still insufficient because of lack of resources of the countries. This should be considered in the framework of the external support often provided to these countries for the implementation of programs of universal salt iodization. Monitoring of these programs in order to achieve the goal of sustainable elimination of IDD now constitutes an absolute priority.
