Population-based differences in thyrotropin and thyroxine distributions in healthy newborns revealing results from...

An independent evaluation of reagents for the determination of thyroxine and thyrotropin from dried blood spot samples taken from newborns between the third and fifth day of life revealed striking differences in the thyrotropin distribution among newborns from Byelorussia. An analysis of the thyrotropin distribution from Byelorussian newborns showed that 40% of samples had over 5 mIU/l blood. In other European populations comparable in respect to timing of blood collection, this fraction varied from only 1% (Stockholm, Sweden) to 3.7% (Lille, France). The reason for the "shift right" in Byelorussian newborns remains to be further investigated. This shift can be attributed to the synergetic effects of mild-moderate iodine deficiency and/or as yet unidentified environmental factors. The differences observed in cumulative distribution patterns obtained by two commercial methods question the use of absolute figures (such as the proportion of samples over 5 mIU/l) for the purpose of inter-population comparisons.