An area ratio of thyroid gland to common carotid artery for evaluating the thyroid gland size.

The feasibility of ultrasonographic measurement of thyroid gland area to common carotid artery (TG:CCA) was investigated. Twenty-one healthy, 12 hypothyroid and 18 non-thyroid illness (NTI) dogs were evaluated. The area of thyroid lobe and common carotid artery in right and left sides were measured using the same ultrasonographic images in transverse plane. The average of the right and left ratio was calculated as TG:CCA. The median TG:CCA of 21 healthy dogs was 1.53, and it did not correlate either body weight or age. The median TG:CCA of 12 hypothyroid dogs was 0.81, which was significantly lower than that of 18 NTI dogs (1.81, P<0.001). If the cut off value <1.12 was used, TG:CCA indicated hypothyroidism with a sensitivity of 100%, specificity of 83%, and accuracy of 90%. Our data indicated that TG:CCA was independent of both body weight, which may contribute to consistent measurement of thyroid size. The results of this study suggest that TG:CCA is a promising tool for diagnosing canine hypothyroidism.