Giovanni Tremolada Department of Clinical Sciences, Colorado State University, Fort Collins, CO, USA In dogs and cats, the thyroid gland is composed of two separate structures, often referred to as lobes. The lobes are rarely connected by a bridge of glandular tissue or isthmus, located ventral to the trachea, called isthmus glandularis.1 The normal thyroid gland lobe appears as an elongated, dark‐red structure located laterally and slightly dorsal to the cranial trachea, usually with the cranial aspect near the first tracheal ring or cricoid cartilage and caudal aspect extending to the fifth to eighth tracheal rings. In dogs, the two thyroid gland lobes are not the only source of thyroid tissue, as ectopic tissue can be found in 23–80% of dogs. In cats, the reported percentage of ectopic thyroid tissue is around 4%. This ectopic tissue can be found anywhere from the base of the tongue to the base of the heart.1 The main arterial supply to the thyroid gland is provided by the cranial thyroid artery, a branch of the common carotid artery, and the caudal thyroid artery, most commonly arising from the brachiocephalic trunk. The main venous drainage of the gland is supplied by the cranial and caudal thyroid veins, which both empty into the internal jugular vein at different levels. Most cats are missing the caudal thyroid vein.2 As suggested by their names, these vessels are located at the cranial and caudal poles of each thyroid lobe (Figure 14.1). The lymphatic drainage of the thyroid gland is represented by the cranial deep cervical lymph nodes (if present) or the medial retropharyngeal lymph nodes for the cranial half of the gland and the caudal deep cervical lymph node for the caudal half of the gland.3 Both cranial and caudal deep cervical lymph nodes, if normal, are usually small and may not be identified on imaging or in surgery. Figure 14.1 Vascular anatomy of the thyroid and parathyroid glands and important structures in the visceral space of the neck. Source: Reproduced with permission from Hullinger1, Elsevier. The thyroid lobes are covered by the sternocephalicus and sternohyoideus muscles ventrally and the sternothyroideus muscle laterally. Anatomically, the right thyroid lobe is often located cranial to the left thyroid lobe and is in closer contact with the recurrent laryngeal nerve, which is dorsal and usually medial to the thyroid glands. The distance between the left recurrent laryngeal nerve and the thyroid gland on the left side is greater because of the presence of the esophagus on its dorsolateral border (Figure 14.1). Other important structures in the visceral space of the neck that a surgeon should be familiar with are the vagosympathetic trunk, the internal jugular vein, and the common carotid artery. The thyroid gland is responsible for the production of thyroid hormones under the regulation of the hypothalamus–hypophysis axis. The thyroid‐releasing hormone (THR) produced by the hypothalamus will activate the hypophysis of the pituitary gland to release the thyroid‐stimulating hormone (TSH or thyrotropin). TSH will trigger the thyroid gland to produce and secrete thyroxine (T4) and triiodothyronine (T3). In the bloodstream, the majority of T4 and T3 are bound to plasma protein and are, therefore, inactive. Only a small percentage of the free T4 and T3 exert a function on the body, with T3 being the most biologically active compound. The amount of T4 and T3 in the bloodstream acts as negative feedback on the hypophysis and inhibits the secretion of TSH. The mechanism of production and release of TRH by the hypothalamus is not well understood. The thyroid gland is responsible not only for production of T3 and T4 but also for secretion of calcitonin through the C‐cells to help regulating the body’s calcium homeostasis. Other texts covering the pathophysiology of the thyroid gland will have details about this topic. The parathyroid glands in dogs and cats are two paired glands with a total of four glands (two per thyroid gland lobe), located at or near the cranial and caudal poles of the thyroid. The normal parathyroid gland appears as a small (~2–3 mm) tan, flat structure. Parathyroid glands are commonly referred to as external and internal based on their location relative to the thyroid parenchyma. External parathyroid glands are located at the cranial poles of the thyroid gland lobes (one on the right, one on the left) and usually are separated from the thyroid parenchyma, while the internal parathyroid glands are in the caudal poles of the thyroid gland lobes and are commonly embedded in the gland parenchyma. Variation in location of both internal and external parathyroid glands is common; therefore, a thorough exploration of the area surrounding the thyroid gland should be performed to identify them. Ectopic parathyroid tissue is uncommon in dogs, but may be more frequent than previously reported.4 On the contrary, in cats, the presence of ectopic parathyroid tissue has been reported in 35–50% of cases.5,6 The function of parathyroid glands is to secrete parathyroid hormone from their chief cells. Because of its direct effect on bones and indirect effect on GI tract and kidneys, this hormone regulates the body’s calcium and phosphorus homeostasis. Increased ionized calcium concentration in the blood acts as negative feedback on the parathyroid’s chief cells, decreasing its secretion. The most common reason for unilateral or bilateral thyroidectomy in dogs is neoplasia, with carcinoma being the most common tumor type. Commonly, dogs present to their veterinarian for the presence of a ventral cervical mass without other clinical signs.7 If the dog is affected by hyperthyroidism, clinical signs, such as tachycardia, restlessness, weight loss, polyphagia, polyuria, and polydipsia, can be present;7 however, the majority of thyroid carcinomas in dogs are non‐functional, meaning excessive thyroid hormone is not being secreted by the neoplastic gland. Thyroid tumors usually affect old, mid‐ to large breed dogs with Golden retrievers and Beagles being overrepresented.8 In the cat, thyroidectomy (unilateral or bilateral) is more commonly performed for benign functional lesions causing hyperthyroidism.9 Bilateral thyroidectomy in cats should only be performed when clinical signs cannot be controlled by medical management or treatment with I131 is not available. Fine needle aspirate, either “blind” or ultrasound‐guided, is usually sufficient to obtain a diagnosis of a neuroendocrine neoplasia of a thyroid gland mass. Blood contamination can preclude obtaining a definitive diagnosis. Biopsy of the mass with a Tru‐Cut needle is usually discouraged because of the risk of profuse and potentially fatal hemorrhage,10,11 but a more recent study did not show evidence of a high complication rate when this was performed.12 A minimum database including complete blood count, blood type, biochemical profile, and urine analysis should be performed before surgery. A total T4 measurement is also routinely evaluated at the author’s practice. While it is important to treat cats for hyperthyroidism before surgery,13 this does not seem to be required in dogs.7,14 Hypercalcemia of malignancy secondary to thyroid carcinoma has been described in a case report and was resolved after surgery.15 Before surgery, patients are staged with cervical imaging (ultrasound or CT), thoracic imaging (radiographs or CT), and abdominal imaging (ultrasound or CT). For small masses that are freely movable,4 cervical ultrasound is considered by the author an acceptable preoperative test. An expert ultrasonographer or radiologist can identify the presence of vascular invasion, the status of the thyroid glands, and the locoregional lymph nodes. In cases of larger or less movable/more “fixed” masses, the author prefers to obtain a CT angiogram of the cervical region to help planning for surgery or radiation therapy. If possible, any abnormal structure identified on imaging should be sampled. Thoracic radiographs or thoracic CT scans can be used to identify the presence of metastatic disease, with CT being a more sensitive test.16 Abdominal ultrasound or CT scan is usually recommended since thyroid neoplasia affects older animals, and the presence of a concomitant abdominal neoplasia was identified in 33% of dogs in one study.4 It is the author’s opinion that surgery in a general practice setting should be considered only for small, freely movable thyroid tumors. Additionally, access to blood products should be available if needed due to the risk of hemorrhage with thyroidectomy.
14
Thyroidectomy
Introduction
Basic Thyroid Gland Anatomy and Physiology
Basic Parathyroid Anatomy and Physiology
Indications/Pre‐op Considerations
Surgical Procedure
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
