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% of 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.

ATs, which are the most common endocrine tumours, may have a large variation with regard to ultrasonographic appearance. As a general rule, ultrasonography does not allow to differentiate between benign and malignant lesions nor can the type of tumour be specified. They may appear as focal increase in adrenal thickness (= nodule) or a diffuse increase in thickness and/or length, resulting in various degrees of distortion of the normal shape (= adrenal mass). The parenchyma may be homogenous or heterogenous. The larger the tumour the more likely it is malignant; however, also small tumours may be malignant. Mineralization may occur in benign and malignant tumours, however, it may also be seen in hyperplastic adrenal glands and is therefore not a marker for AT. Anechoic lesions are preferably seen in large tumours, they are often part of a heterogenous or mixed pattern of echogenicity. They may represent foci of adrenal necrosis or hemorrhage and may be associated with fast tumour growth. Identification of vascular invasion (most often into the vena cava caudalis) is specific for a malignant tumour. Although ultrasonography in general is a fast and easy tool to detect vascular invasion it may not be possible to differentiate vascular involvement from compression or a blot clot in some cases.

A variety of other tumours (pheochromocytoma, aldosteronoma, sex-steroid-producing tumour, metastasis, myelolipoma, lipoma) or other lesions (cyst, hematoma, abscess, granulomatous disease) may occur in the adrenal glands and can not be differentiated from AT by means of ultrasonography. In a dog with an adrenal nodule/mass, a smaller than normal contralateral gland strongly supports the suspicion of an AT. However, in some dogs atrophy of the contralateral gland is not visible on ultrasonography. In this situation differential diagnosis are: unilateral AT without visible atrophy of the contralateral gland, AT and concurrent PDH, PDH and another adrenal tumour/lesion (as mentioned above), PDH with (irregular) nodular hyperplasia. Most AT are unilateral, however, bilateral tumours occur and may be impossible to differentiate from PDH with nodular hyperplasia. Endocrine tests may be helpful to make the exact diagnosis.

The second most common adrenal tumours are pheochromocytomas. Dogs with pheochromocytomas may be presented with various clinical signs; some of them are similar to those of HAC. Pheochromocytomas have a very wide range of size (mm-cm) and may reveal any of the ultrasonographic patterns described above, rendering differentiation from AT impossible. Mineralization within a pheochromocytoma seems to be a very rare finding; therefore, this finding would make pheochromocytoma less likely than AT.

Up till now information is scarce on the ultrasonographic appearance of the adrenal glands in cats with PDH or AT. So far, it appears that all statements made for the dog are also valid for the cat.

Ultrasonography of the adrenal glands may also be helpful in the workup of patients with hypoadrenocorticism. It is currently believed that most cases result from an autoimmune destruction of the adrenal cortices with bilateral atrophy of all three adrenal zones. It has been demonstrated that this atrophy is visible in the form of a bilateral, symmetrical reduction in size. In most dogs with hypoadrenocorticism adrenal gland thickness is less than 3 mm. However, due to their small size those adrenals are difficult to find and a high level of experience is required on the part of the operator. So far, no reports are available on the adrenal gland size in dogs with partial hypoadrenocorticism.

Thyroid and Parathyroid Glands

The superficial location of the thyroid gland allows ultrasonographic examination with high-frequency transducers. Recently, thyroid ultrasonography has been evaluated as a diagnostic tool for hypothyroidism. It can be performed in dorsal recumbency or in a sitting position, usually without sedation using a 10 MHz linear transducer. The head and neck are moderately stretched and the point of the mandible and sternum are kept in a straight line. Larynx (cranial), trachea (media) and common carotid artery (lateral) serve as anatomical landmarks. The transducer is placed directly caudal to the larynx in the jugular groove and the carotid artery is identified in the longitudinal plane. From this position, the transducer is tipped slightly medially in the direction of the trachea until the boat-shaped thyroid gland is apparent. Rotation of the transducer by 90° allows the visualization in transverse plane. In healthy dogs thyroid lobes appear fusiform or elliptical in the longitudinal plane and round to oval in the transverse plane. Echogenicity compared with surrounding musculature is hyper- or isoechoic in most cases. From the few data available it appears that thyroid size depends on body weight, therefore, reference ranges need to be established for different breeds resp. different weight categories. Primary hypothyroidism is thought to be the result of immune-mediated destruction of the thyroid gland in the majority of cases. This correlates well with the finding that thyroid volume is significantly lower in hypothyroid dogs than in healthy or sick-euthyroid dogs. Echogenicity in hypothyroid dogs is either hypoechoic or heterogenous compared to surrounding musculature. So far it is unknown at which stage of the disease differences in size and echogenicity can be diagnosed by ultrasonography. The diagnostic value of thyroid ultrasonography in dogs with subclinical and early hypothyroidism is currently under investigation.

In dogs with suspected thyroid tumour ultrasonography is helpful to evaluate size, uni-or bilateral involvement, possible invasion into surrounding tissue and to guide fine-needle aspiration.

By means of a 10 MHz high resolution linear transducer it is also possible to visualize the parathyroid glands. There is a positive correlation between body weight and size of the parathyroid glands and the likelihood to detect all 4 glands increase with an increase in body weight. The size of the parathyroid gland is determined by measuring the maximal length of the gland when it is imaged in longitudinal plane. In dogs < 10 kg the longest dimension is 3.0 mm, in dogs weighing between 10 and 19 kg 3.5 mm, in dogs between 20 and 29 kg 4.0 mm and dogs > 30 kg 4.6 mm.

Ultrasonographic evaluation is an extremely valuable tool in the workup of patients with hypercalcemia. In dogs with hypercalcemia of malignancy parathyroids are either small or not detectable. In contrast visualisation of parathyroid masses in dogs with primary hyperparathyroidism is usually easy due to their increased size and anechoic appearance. In our latest case series size of parathyroid adenomas in dogs with primary hyperparathyroidism ranged between 6 and 30 mm. Others have described smaller sizes, which would then make differentiation from normal parathyroid glands more difficult. As in the other endocrine disorders discussed above, ultrasonography is not a test of parathyroid function and should only be used additionally to endocrine tests (such as PTH, PTHrP). Parathyroid ultrasonography may also be helpful to differentiate between acute and chronic renal failure. In a recently performed study in dogs with severe azotemia (serum creatinine > 500 mmol/l) we could demonstrate that dogs with chronic renal failure had significantly larger parathyroid glands than dogs with acute renal failure. The number of abnormal parathyroids correlated with the duration of the disease. Furthermore, the parathyroid glands of dogs with chronic renal failure were more prominent than those of healthy dogs and dogs with acute renal failure and they were consistently anechoic.

References

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