Ultrasonographic Examination of Endocrine Glands: The Endocrinologists View

Claudia E. Reusch, DECVIM-CA
Clinic for Small Animal Internal Medicine, University of Zuerich, Zuerich

During the last 10 to 15 years enormous progress has been made with regard to quality of ultrasound equipment. The improved resolution has enabled consistent visualisation of endocrine glands in dogs and cats and today ultrasonography is routinely used in the work-up of endocrine diseases. It is important to remember, that ultrasonographic imaging gives information on size, shape and parenchyma of the endocrine organs, but not their endocrine function. Therefore, ultrasonography should be regarded as a test that is useful in addition (not as replacement) to hormone testing. The presentation will focus on the ultrasonographic examination of adrenal glands, thyroid and parathyroid glands.

Adrenal Glands

In almost all dogs and cats it is possible to visualize the adrenal glands. Usually, a 7.5 MHz transducer is adequate, in general the highest frequency that will penetrate to the adrenal region should be used. A systematic approach is critical for a high detection rate. In our clinic the left adrenal gland is evaluated in dorsal recumbency, the right adrenal in left lateral recumbency. The left adrenal gland is located ventrolateral to the aorta between the origin of the left renal artery and the cranial mesenteric artery; therefore it is advisable to use those 3 blood vessels as anatomical landmarks. In some animals the right adrenal gland is more difficult to find, since it is located deeper and more cranial than the left adrenal and therefore ribs and bowl gas can compromise visualisation. The caudal vena cava serves as landmark, since the right adrenal gland is firmly attached to its lateral side. In healthy dogs the left adrenal gland appears as a peanut-shaped hypoechoic structure (in the longitudinal plane). The right adrenal is hypoechoic and comma-shaped and it may be difficult to visualize the entire length in a single-long axis view. Due to its special shape the cranial pole of the right adrenal gland may appear thickened and resemble an adrenal tumour. Therefore, careful imaging of the right adrenal and conservative interpretation of a thickened cranial pole is of particular importance, especially for beginners.

In cats adrenal glands are also hypoechoic compared to surrounding tissue, and they are oval to bean-shaped. A distinct waist between the cranial and caudal pole is usually not identified. In dogs and cats the parenchyma may appear uniform or two layers may be seen.

Measurement of the dorsoventral dimension (thickness) appears to be the most accurate means of assessing adrenal size. Normal thickness in dogs is between 3 and 6 (-7) mm, in cats 2-5 mm. It should be noticed, however, that size measurements depend on the imaging position, therefore own reference ranges should be established. Adrenal ultrasound is most often used in dogs and cats with hyperadrenocorticism (HAC) to differentiate between pituitary-dependent disease (PDH) and HAC due to an adrenocortical tumour (AT). In the typical case of a dog with PDH the adrenal glands have a bilateral symmetrical appearance. They may be enlarged (increase in thickness), or normal-sized. Normal-sized adrenal glands are relatively frequent (about 30% cases with PDH) and we assume that they are mostly seen in dogs with short term or mild disease. In dogs with PDH the shape of the adrenals is usually preserved, a plump appearance is sometimes seen. With regard to echogenicity adrenal glands in dogs with PDH are most often hypoechoic and homogenous. However, the parenchyma may also appear irregular, show focal areas of increased echogenicity or a nodular appearance. These findings may indicate nodular hyperplasia, however they are indistinguishable from AT or other adrenal lesions. In some dogs with PDH adrenal glands are not symmetrical (one gland is thicker than the other) a situation which resembles AT. In those latter cases further endocrine tests (such as cACTH) are needed to specify if the HAC is due to PDH or AT.

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