Introduction

Diagnostic imaging is one of the most important tools for the diagnosis of endocrine disorders in dogs and cats. Multiple modality approaches are often required for the diagnosis of various diseases.

Thyroid Diseases

Canine Hypothyroidism

Ultrasonography can be used to assess the size and symmetry of the thyroid glands. In primary and secondary forms of hypothyroidism, the glands are both reduced in size. They can also become inhomogeneous. Scintigraphically, a reduction in tracer uptake is often detected. In dogs with space-occupying lesions due to hormonally inactive tumours, the thyroid tissue may become enlarged and asymmetrical. In the case of a suspected tumour, thoracic radiographs are indicated to screen for metastases.

Dogs with congenital hypothyroidism will show delayed growth plate closure and irregular shape of the epiphyses of the vertebrae and long bones, all of which are readily detectable with conventional radiography. Long bones appear short and wide. The skull will also appear short and broad.

Feline Hyperthyroidism

Thyrotoxicosis in cats often leads to hypertrophic cardiomyopathy, which may or may not be evident radiographically. If suspected, echocardiography is more sensitive for its detection. Radiographically the thorax appears lucent, mainly due to cachexia, and there is a loss of detail in the abdomen. The thyroid glands are often not visualised radiographically unless they become quite large. Sonographically, the affected gland is unilaterally enlarged 10-15% of the time, but they are more often bilaterally enlarged and symmetrical. If unilateral enlargement is present, the contralateral gland may be greatly reduced in size and not visible. The parenchyma of the enlarged gland often appears somewhat hypoechoic to the surrounding tissue. Cysts can be identified in approximately 10% of the cases. Scintigraphy with Tc-99M is the diagnostic imaging method of choice in cats with suspected or confirmed hyperthyroidism. It not only delivers definitive diagnosis as to the function of the gland, but is also valuable in detecting ectopic tissue. Even a gland that is reduced in size may show activity during the scintigraphic examination. Any tissue that shows tracer activity greater than that of the salivary gland uptake is considered active. The presence of unilateral versus bilateral activity directly affects the therapeutic options (surgery versus radioactive iodine treatment).

Recently, a new ultrasound-guided therapy for hyperthyroidism has been described. Ultrasound-guided percutaneous ethanol injection in the abnormal gland has been used successfully as has percutaneous radiofrequency heat ablation.

Tumours of the Thyroid and Hyperthyroidism in Dogs

Hyperthyroidism in dogs is often due to neoplastic infiltration and carcinomas are diagnosed in 90% of the cases. Although usually hormonally inactive, 10% may lead to hyperthyroidism. Ultrasonography is useful for identifying the thyroid gland as the source of any space-occupying lesion of the cervical region. Furthermore, the retropharyngeal lymph nodes are commonly affected in cases of carcinoma and they should also be examined. When extremely large or diffusely invasive masses are present, either computed tomography (CT) or magnetic resonance imaging (MRI) are indicated prior to any surgical intervention. Scintigraphy should ideally be performed in dogs with hyperthyroidism and the thorax should be screened for ectopic tissue. Metastases do not always take up iodine and will therefore also not take up pertechnetate. Therefore, metastases of the thorax may go undetected scintigraphically.

Parathyroid Diseases

Ultrasonography is the method of choice for examining the parathyroid glands. Linear array probes of at least 10 MHz are required for visualisation of the parathyroids. The thyroid glands are identified in the usual location between the trachea and the carotid arteries. Parathyroid tissue appears as 2-5 mm diameter, well marginated, hypoechoic structures at the cranial and caudal poles of the thyroid gland.

Diagnostic imaging can play an important role in patients with hypercalcaemia. Secondary hyperparathyroidism is more common than the primary form. The secondary forms, either nutritional or renal induced, lead to a disturbance in the calcium-phosphorus homeostasis. Other causes of hypercalcaemia are hypervitaminosis D, hypoadrenocorticism, granulomatous diseases and paraneoplastic diseases. Radiography and ultrasonography are the most important imaging modalities for screening patients with hypercalcaemia. Mineralisations, urolithiasis, osteopenia and pathological fractures can be quickly diagnosed radiologically. The parathyroids themselves can be examined with ultrasonography. Both methods can be used to scan the thorax and abdomen for neoplastic processes.

Thoracic Radiographs

The cranial mediastinum, perihilar and sternal lymph nodes can be detected if enlarged in thoracic radiographs. Thoracic and abdominal lymphadenomegaly is common in patients with lymphoma. Lytic skeletal lesions can also be detected in patients with multiple myeloma or primary or secondary bone tumours.

Disease of the Adrenal Glands: Hyperadrenocorticism in Dogs

Radiographs of the canine patient with hyperadrenocorticism often show an enlarged abdomen with good contrast due to an increased amount of intra-abdominal fat. The urinary bladder may be large (polydipsia and polyuria). Calcinosis cutis may be present and pulmonary mineralisation may be recognised by an increased opacity of the lungs. The bronchi may show increased mineralisation as can the kidneys (nephrocalcinosis). Patients may also show generalised osteopenia.

On ultrasound examination, the liver is often enlarged and diffusely hyperechoic. In older animals, hypoechoic nodules may also be present. These are most commonly hyperplastic nodules.

The appearance of the adrenal glands in ultrasonography depends on the form of hyperadrenocorticism. Pituitary-dependent disease often results in bilateral enlargement of the glands which appear rounded and diffusely hypoechoic. Unilateral and asymmetric enlargement can also occur. Dogs with adrenal gland hyperfunction due to adrenal gland tumours (adenomas and adenocarcinomas) usually have unilateral enlargement or nodular changes of one gland. The contralateral gland is not always reduced in size as might be expected due to negative feedback. Furthermore, both adrenal glands may be affected at the same time by two different processes, making interpretation of the ultrasound findings very difficult. Adrenal gland tumours such as phaeochromocytomas can become quite large and even mineralised. They have the tendency to infiltrate the local vasculature and can grow into the vena cava or aorta as well as the kidneys. Colour Doppler examination of the aorta and vena cava is indicated when a large adrenal mass is detected. The method of choice for assessing vascular invasion prior to surgical excision of a tumour is CT or MRI.

Dogs receiving steroid therapy will usually have small adrenal glands in ultrasonography. However, those being treated with trilostane for Cushing's will show enlargement of both glands following therapy.

Pituitary Diseases

Cross-sectional imaging such as either CT or MRI are the methods of choice for detecting pituitary disease.

Computed Tomography

Pituitary macroadenomas may be difficult to recognise in survey views as they are generally isodense to the surrounding nervous tissue. Contrast-enhanced CT is necessary for accurate detection and tumours will show diffuse and strong enhancement following intravenous injection of iodine-containing contrast medium. The tumours have a supracellar location and are generally well marginated. The CT also serves for the planning of radiation therapy. Microadenomas (<10 mm) are more difficult to identify in conventional contrast-enhanced CT studies. Therefore, dynamic studies are generally indicated. These are performed by making slices directly following the contrast injection and quantifying the degree of enhancement over time. Microadenomas do not show the early pituitary flush phase as is normally expected to occur in the first 20 seconds. Often, no contrast uptake can be detected.

Magnetic Resonance Imaging

Macroadenomas can be recognised as supracellar space-occupying lesions of variable size. In T2 sequences, a signal iso- to hyperintense to the thalamus can be seen. In T1 sequences, they are iso-to hypointense. Typically, macroadenomas show a strong and homogeneous contrast enhancement. Detection of microadenomas is similar to that of CT.

Neuroendocrine Tumours of the Pancreas

Insulinomas are the most common neuroendocrine tumours in dogs and are more common in ferrets. Insulinomas metastasise to the liver and regional lymph nodes and a complete examination of the abdomen with ultrasound is indicated in patients with hypoglycaemia and above normal insulin levels. Insulinomas typically appear as round, hypoechoic and well marginated nodules within the pancreatic parenchyma.

Depending on their size, degree of overlying intestinal gas and obesity, thoracic conformation, as well as experience of the sonographer, insulinomas may not be visualised. Endosonographic ultrasound can also be used to scan the pancreas for insulinomas. This method requires general anaesthesia and specialised equipment, however.

References

1.  Bromel C, Pollard RE, et al. Ultrasonographic evaluation of the thyroid gland in healthy, hypothyroid, and euthyroid Golden Retrievers with nonthyroidal illness. Journal of Veterinary Internal Medicine 2005; 19(4): 499-506.

2.  Feldman EC, Nelson RW. Canine and Feline Endocrinology and Reproduction (second edition). Philadelphia: WB Saunders 1996.

3.  Mantis P, Lamb CR, et al. Changes in ultrasonographic appearance of adrenal glands in dogs with pituitary-dependent hyperadrenocorticism treated with trilostane. Veterinary Radiology and Ultrasound 2003; 44(6): 682-685.

4.  Van der Vlugt-Meijer RH, Meij BP, Voorhout G. Dynamic helical computed tomography of the pituitary gland in healthy dogs. Veterinary Radiology and Ultrasound 2007; 48(2): 118-124.

5.  Wisner ER, Nyland TG, et al. Ultrasonographic evaluation of the parathyroid glands in hypercalcemic dogs. Veterinary Radiology and Ultrasound 1993; 34(2): 108-111.