One of the strengths of ultrasound imaging is its ability to resolve very small soft-tissue structures. The thyroid and parathyroid
glands are prone to clinical disease such as hyperthyroidism and hyperparathyroidism that ultrasound can help to diagnose.
The neck may be unfamiliar territory to many practitioners who focus their skills on abdominal ultrasound, so landmarks become
very important for locating the thyroid glands during the scan.
Location of the thyroid glands
There are two thyroid glands in the dog and the cat. The lobes are located on either side of the trachea, on the ventral aspect
of the neck. The thyroid is fusiform in shape in the sagittal plane, and oval or triangular in the transverse plane (Image
1). The long axis is oriented parallel to the long axis of the neck about 5-10 mm deep to the skin surface.
Image 1a: The thyroid gland in a Beagle appears fusiform or oval in the sagittal plane. In transverse
To locate the right thyroid gland, place your high-frequency transducer in the transverse plane on the larynx. The dog should
be in dorsal recumbency with the nose extended parallel to the table and the neck straight. From this position, slide the
transducer caudally, keeping the trachea in the center of the screen. The jugular vein and carotid arteries should be visible
lateral to the trachea. The thyroid will appear in cross-section lateral to the trachea, and medial to the carotid artery.
It has a finely granular echotexture that is isoechoic to muscle. To view it in the sagittal plane, keep the thyroid in your
image as you rotate very slowly 90 degrees. This takes some practice so you do not lose view of the thyroid during rotation.
The left thyroid has similar landmarks, but is located slightly more caudally, and you can usually see the esophagus dorsal
or lateral to it.
Image 1b: it is triangular in shape and sits between the trachea and common carotid artery (CCA).
There are four parathyroid glands in each thyroid. They all may be visible, or only one or none. This may depend on the frequency
of your transducer, as they are usually 2-3 mm in diameter and can be too small for some transducers to resolve. The parathyroids
appear as a hypoechoic, round structure on the craniodorsal edge of the thyroid (cranial parathyroid), or the caudal aspect
of the thyroid (caudal parathyroid).
Thyroid and parathyroid conditions
The most common endocrine conditions involving the thyroid and parathyroid glands are hyperplasia and neoplasia. Thyroid carcinoma
in dogs often presents as a large neck mass. On ultrasound these are extremely vascular, and may be located more caudally
than a normal thyroid gland (Image 2). The key to examining a thyroid mass is to determine that there is no normal thyroid
gland on that side by using landmarks to examine the region carefully. The thyroid on the opposite side also should be evaluated.
Because of the vascularity, tissue sampling should be performed with caution.
Image 2: Thyroid carcinomas are large, irregular, vascular masses.
Parathyroid hyperplasia, adenoma or adenocarcinoma is suspected in cases of primary hyperparathyroidism. These are single
nodules hypoechoic to the thyroid tissue, and are larger than the normal 2-3.3 mm in diameter (Image 3). Nodules greater than
4 mm in diameter are more likely to be adenomas or carcinomas. These can be surgically removed or treated using heat ablation.
Image 3: This parathyroid adenoma is hypoechoic to the thyroid and surrounding tissue.
Cats with hyperthyroidism may have hyperplasia or adenomas. Seventy percent of cats are affected bilaterally. Hyperplastic
thyroid lobes are well marginated and slightly hypoechoic to normal thyroids. Thyroid adenomas may be focal or involve the
entire lobe, with a uniform or mottled echotexture (Image 4). If only one gland is affected, the normal gland may be atrophied
and not visible.
Image 4: In this sagittal image of a feline thyroid adenoma, the nodule occupies the central portion of the lobe.
Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.