A novel thyroid ultrasound phantom with tissue-equivalent characteristics was designed consisting of two lobes with three lesions each. One set of lesions is manufactured with a -5 dB echo difference to the surrounding tissue, the other with -10 dB. This phantom was used as a standardized measuring object for reproducibility of two-dimensional and three-dimensional ultrasound volumetry and for an interobserver and intraobserver variability study. For the variability study, nine experienced physicians scanned all specimen three times. Each time the volumes were calculated using the ellipsoid method. A three-dimensional ultrasound scan of each specimen was performed to evaluate all volumes by multiplanar volume approximation. The results of these volume data were compared to the known true volumes. The interobserver variability ranged from -13.4% to 11.9% (median, 0.7%); the intraobserver variability from -9.1% to 16.4% (median, 3.6%). The systematic error as calculated from the total mean of all specimens is 0.5% for the interobserver variability and 4.1% for the intraobserver variability. The phantom can be used for training purposes, to improve the skills of the examining physicians by simulating real thyroid morphology, to provide a standardized reference object for long-term quality control of conventional ultrasound scanners, and the determination of the accuracy and reproducibility of volumetry using three-dimensional ultrasound systems.