Objective: to compare the estimated thyroid volume, as determined by two-dimensional (2D) and three-dimensional (3D) ultrasonography, with real volume. Design: Descriptive Study Animals: A total of 7 mixed breed dogs Procedures: On 2D ultrasound, the thyroid volume is estimated by the ellipsoid equation and on 3D ultrasound; it is estimated by outlining the boundaries of the thyroid lobe in image planes and then calculated with the inbuilt software. The real volume was measured after Euthanasia. Correlation between actual and estimated size were computed by using statistical software. All of the dogs must be euthanized because of clinically end-stage condition. Results: The study demonstrates that there is a high agreement between 3D ultrasound and real volume measured by graduated cylinder method (r=0.97, P<0.000). Conclusion and Clinical Relevance: Evaluation of the size of thyroid gland is useful in diagnosis of several situations and in follow-ups monitoring. Ultrasonography is supposed to provide a reliable estimate of thyroid volume. In this study, 3D ultrasound was a useful image method in the estimating of canine thyroid volume. Keywords: Thyroid lobe, Canine, 3D Ultrasonography, volumetry.

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