Developing an accurate method for estimating thyroid volume in calves using ultrasonography.

ABSTRACT

Diseases of the thyroid gland with struma formation can occur in calves suffering from iodine deficiency. The aim of the study was to develop a guideline for standardized ultrasonographic examination of the thyroid gland in order to determine its volume. Sonographic measurements of the thyroid lobes were carried out on three standardized axes in calves up to 3 months of age (n = 83). Total volume (V SON ) was calculated using the formula for an ellipsoid body. Forty three of the enrolled animals were euthanized. The lobes of their thyroid glands were dissected and measured with a slide gauge, and V SLG was calculated accordingly. In addition, thyroid volume was measured using the method of water displacement (V AQU ) and this was used as a gold standard in correlation and regression analyses with sonographic and slide gauge values. Intraobserver variability of sonographic measurements was dependent on side and axis, and ranged between 3.04% and 7.35%. In the euthanized calves with a body mass of 52.4 ± 17.1 kg, V SLG was 4.5 ± 1.4 ml, V SON was 6.5 ± 1.7 ml, and V AQU was 7.9 ± 2.4 ml (mean ± standard deviation). Instead of the formula for ellipsoid bodies which underestimates thyroid volume, we developed a new formula derived from regression analysis. Body mass and thyroid volume correlated significantly. With the formula presented, thyroid volume of calves can be estimated by ultrasonographic measurement of three axes and compared to predicted values based on body mass.