Determination of thyroxine in the hair of newborns by radioimmunoassay with high-performance liquid chromatographic confirmation

Hair analysis is often used in forensic toxicology to study, retrospectively, chronic exposure of individuals to drugs, and consequently newborn hair may become an ideal sample to study intrauterine exposure to xenobiotics as well as to endogenous compounds. As a tool to investigate a supposed maternal thyroxine (T 4 ) supply to the congenital hypothyroid fetus, we devised to use the analysis of T 4 extracted from newborn hair. In the present paper, the analytical method based on T 4 extraction from hair followed by a radioimmunoassay is described. To verify the nature of the T 4 -like immunoreactive material present in newborn hair, it was further studied by HPLC fractionation with radioimmunoassay of the eluted fractions. On the basis of a clear correspondence between retention times of T 4 standard and T 4 -immunoreactive compound extracted from hair, we assigned this immunoreactive material to T 4. Then, we determined T 4 hair concentrations in 19 control newborns at birth and 12 congenital hypothyroid infants at 22 days of life. Values obtained from hypothyroid infants (31.47±8.8 pg/mg hair, mean±S.D.) were not significantly lower than those obtained from healthy newborns at birth (36.10±13.2 pg/mg hair ). Such results are in agreement with the hypothesis of a maternal supply of thyroxine to the fetus through placental crossing.