NORMAL FETAL THYROID VOLUME

A prospective study of 289 normal fetal thyroids was undertaken betwee n 20 and 36 weeks of gestation in 289 mothers without any history of t hyroid disease or maternal disease that could cause intrauterine growt h restriction. The fetal thyroid volume was calculated by the ellipsoi d equation, with the assumption that each lobe was an ellipse. A nomog ram relating fetal thyroid volume to gestational age was established. The mean fetal thyroid volume was found to be 0.23 +/- 0.18 cm(3) (med ian 0.17 cm(3); range 0.022-0.85 cm(3)). The ratio of fetal thyroid vo lume to estimated fetal weight (V/W ratio) (at 0.163 +/- 0.079 cm(3)/k g) was constant throughout gestation. The value was smaller than those of the child and the adult reported in other studies, indicating that fetal thyroid enlargement is likely to be underestimated if based on postnatal criteria. It was found that the mean fetal thyroid volume wa s significantly correlated with estimated fetal weight and gestational age, in which the former dominated the effect. The fetal thyroid incr eased in size much faster after 32 weeks, suggesting that evaluation o f possible fetal goiter should take into account gestational age and t he nomogram developed in this study.