Surgical Thyroidectomy in Cats: Thyroid and Parathyroid Anatomy

The thyroid gland, or simply the thyroid, in vertebrate anatomy, is one the largest endocrine glands. The thyroid is a very vascular organ located in the neck, just below the larynx or voice box. The thyroid gland secretes T4 and T3, the two main thyroid hormones.

The small parathyroid glands are located adjacent or embedded within the thyroid gland. These parathyroid glands secrete a hormone (parathyroid hormone or PTH) that regulates calcium levels in the body. If damaged or removed, a dangerous low serum calcium concentration can develop, leading to cramping and twitching of muscles or tetany (involuntary muscle contraction) and sometimes seizures.

Surgical removal of the thyroid glands is relatively straightforward. Successful, uncomplicated excision of unilateral or bilateral thyroid tumors, however, requires knowledge and preservation of the nervous, vascular, and other glandular structures surrounding the thyroid glands for a successful operation.

Normal Anatomy of the Feline Thyroid Gland

Location, size, color

Figure 1: Thyroid and parathyroid glands
of cat

The thyroid gland in the cat is divided into 2 lobes, which are located adjacent to the trachea (windpipe) and just distal to the larynx (voicebox). Each thyroid lobe lies on the lateral aspects of the trachea (windpipe) at first few tracheal rings. Each lobe is approximately 1-cm long, 3- to 5-mm wide, and 1-2 mm thick in the cat. The right thyroid may be located higher than the left, at the caudal aspect of the larynx (1-6).

The glands are closely adhered to the trachea and deep to the neck muscles — the sternohyoideus and sternothyroideus muscles (1-6). The normal thyroid gland is cream to tan in color. Each thyroid lobe is contained in a distinct capsule that can be bluntly separated from the gland. Small blood vessels are found on both the capsule surface and between the thyroid capsule and the gland itself.

In contrast to the human thyroid, where the thyroid lobes are connected by an isthmus, the feline thyroid gland does not have this connecting “bridge” between the two lobes of the thyroid (Figure 2).

Figure 2. Comparative anatomy of the human and feline thyroid gland. Note that cats lack the connecting bridge between the 2 thyroid lobes.

Blood supply
The principle blood supply to each thyroid lobe is the cranial thyroid artery, a branch of the common carotid artery (Figure 1). In contrast to dogs and humans, the caudal thyroid artery is absent in the cat (7). Venous drainage of the thyroid gland occurs through the cranial and caudal thyroid veins.

Surrounding structures
The right gland is closely associated with the structures of the carotid sheath (which contains the carotid artery, internal jugular vein, and vagosympathetic trunk). The recurrent laryngeal nerves pass dorsal to the thyroid glands. The left thyroid gland is closely associated with the esophagus, which lies dorsolateral to the gland and separates it from the carotid sheath. As discussed below, the parathyroid glands are located adjacent to or are embedded within the thyroid gland.

Ectopic thyroid tissue
As described above, the thyroid gland is normally located near the base of the larynx, lateral to the first few tracheal rings. During the few weeks of development of the fetus, the thyroid starts to form higher up, in the region of the cat’s tongue. As the fetus continues its development, thyroid tissue descends to its normal final resting place in the body, lower in the neck.

Sometimes descent of the thyroid gland is not complete and some (or all) thyroid tissue ends up remaining in the tongue. In other cases, the thyroid continues its descent too far and ectopic thyroid tissue ends up in the chest cavity.

In normal cats, the incidence of ectopic thyroid tissue is not known, but approximately 5% of hyperthyroid cats will have ectopic thyroid tissue found on thyroid scintigraphy. Therefore, this condition does not appear to be uncommon in cats.

Normal Anatomy of the Feline Parathyroid Glands

Location, size, color
The 4 parathyroid glands are intimately associated with the thyroid tissue. Two parathyroid glands are usually associated with each thyroid lobe (7).

Figure 3: Anatomy of the parathyroid glands of the cat

The external parathyroid gland usually lies in the loose fascia at the cranial (top) pole of each thyroid lobe (Figure 3). The external parathyroid glands are round and flat and are much smaller than the thyroid lobe (approximately 4 mm in diameter). They can be distinguished from the thyroid tissue by their lighter (white to tan) color and spherical shape. The internal parathyroid gland is usually embedded in the thyroid tissue itself and cannot be seen without dissection of the thyroid gland.

Blood supply
Like the thyroid gland, the blood supply to the parathyroid glands arises from the cranial thyroid artery (5,6,7).

Ectopic parathyroid tissue
A small percentage of cats have ectopic parathyroid tissue in the chest, similar to ectopic thyroid issue. It is not know if this tissue secretes parathyroid hormone, however, since one study found that this ectopic tissue was not capable of maintaining normal calcium levels immediately after removal of both the thyroid and parathyroid glands (8).

Bottom Line

The anatomy of the feline thyroid and parathyroid glands is extremely important when considering surgery to remove thyroid adenomas. Adjacent arteries, veins, and nerves (eg, carotid artery, internal jugular vein, vagosympathetic trunk, recurrent laryngeal nerve, esophagus) make the procedure complicated if the thyroid tumor is large, highly vascular, invasive, or malignant. 

Because of the highly vascular nature of the thyroid, operation carries the potential risk of hemorrhage, especially with larger thyroid tumors. Lastly, the intimate association of the parathyroid glands and the shared blood supply explains the risk of hypoparathyroidism leading to low calcium levels in hyperthyroid cats requiring bilateral thyroid gland surgery.

References:

1. Crile R. The comparative anatomy of the thyroid and adrenal glands in wild animals. Ohio Journal of Science 1937;37:42–61. 
2. Dyce KM, Sack WO, Wensing CJG. The endocrine glands. In: Dyce KM, Sack WO, Wensing CJG (eds): Textbook of Veterinary Anatomy. Philadelphia: WB Saunders, 1987;205–211. 
3. Waters DJ. Endocrine system. In: Hudson LC, Hamilton WP (eds): Atlas of Feline Anatomy for Veterinarians. Philadelphia: WB Saunders, 1993;127–134. 
4. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
5. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
6. Drost WT, Mattoon JS, Samii VF, et al. Computed tomographic densitometry of normal feline thyroid glands. Veterinary Radiology and Ultrasound 2004;45:112-116. 
7. Nicholas JS, Swingle WW. An experimental and morphological study of the parathyroid 
glands of the cat. American Journal of Anatomy 1925;34:469-508. 
8. Flanders JA, Neth S, Erb HN, et al. Functional analysis of ectopic parathyroid activity in cats. American Journal of Veterinary Research 1991;52:1336–1340.