The utility of FT4 serum in newborns at risk for congenital hypothyroidism (CH).

Thyrotropin (TSH) stimulates hormonogenesis using the iodate substrate and tyrosine amino acid. After various enzymatic reactions, thyroxine (T4) and triiodothyronine (T3) are released. Part of the hormone freely circulates in serum as free T3 (FT3) and free T4 (FT4). TSH is released after feedback. A study was undertaken to determine cut-off levels of FT4 to verify hypothalamic and pitutitary hypothyroidism. Both TSH and T4 were determined on blood spots by chemoluminescence and radioimmunology. TSH, T4 and FT4 were determined in control cases using an immuno-luminometric method. All newborns with TSH>3 mIU/l and FT4<0.6 ng/l were followed. Some of the positive cases may have resulted from iodine deficiency as a result of geography and environment. New risk values are being evaluated based on spot testing on the third day of blood sample collection. The ongoing study of new cut-off values in relation to birth day and the introduction of FT4 control serum level seems to prevent repeated measurements and promote quicker intervention by physicians thus preventing difficult genetic investigations.