Our previous results show that high fetal free thyroxine (fT4) levels measured by cordocentesis are unexpectedly frequent in women with autoimmune thyroid disease, including maternal autoimmune hypo- and hyperthyroidism. Increasing awareness that even some mild fetal disorder can have an impact on later neurophysiologic development and the health of an individual makes the recognition and therapy of fetal hypo- or hyperthyroidism an increasingly significant domain of interest. According to our results, fetal fT4 concentrations did not correlate neither with dose of medication nor with ultrasound biometric parameters; the range for maternal thyroid-stimulating hormone (TSH) correlated predominantly with normal fT4 can not be marked off. The type and concentration of antithyroid antibodies might have some prognostic value.