Nomograms of fetal thyroid measurements estimated by 2-dimensional sonography

Purpose. To build nomograms of fetal thyroid circumference (FTC), fetal thyroid area (FTA), and fetal thyroid transverse diameter (FTTD) throughout gestational age (GA). Method. Between January 2006 and July 2006, FTC, FTA, and FTTD were measured once in 196 normal fetuses examined at a GA of 22-35 weeks. Inclusion criteria were a healthy mother with normal maternal thyrotropin level during pregnancy, a singleton pregnancy with normal fetal morphology on sonography, and GA confirmed via first-trimester sonographic examination. Results. Mean FTC, FTA, and FTTD ranged from 3.21 cm, 0.58 cm², and 1.19 cm at 22 weeks to 5.11 cm, 1.69 cm², and 1.89 cm at 35 weeks, respectively. Linear regression analysis yielded the following formulas for FTC, FTA, and FTTD according to GA: FTC (cm) = 0.146 x GA (weeks); FTA (cm ²) = -1.289 + 0.085 x GA (weeks); FTTD (cm) = 0.054 x GA (weeks). The following logarithmic formulas were obtained for the expected fetal thyroid measurements according to estimated fetal weight (FW): FTC (cm) = -4.791 + 1.265 x logN FW; FTA (cm²) = -1.676 + 0.455 x logN FW; and FTTD (cm) = 0.399 + 0.001 x logN FW. Conclusion. We describe new nomograms of fetal thyroid measurements throughout gestation that may be useful in case of thyroid dysfunction.