VETcpd - Surgery Peer Reviewed Surgical management of thyroid and parathyroid tumours When approaching cases of thyroid or parathyroid tumours in dogs and cats in practice, an understanding of the potential endocrinological effects of such tumours, as well as the likelihood of benign versus malignant transformation, is important. Following diagnostic investigations to determine whether the tumour is functional, to anatomically localise the tumour and, in some cases, to screen for metastatic spread, surgical excision is frequently indicated in the management of thyroid and parathyroid tumours. A thorough knowledge of the anatomy of the neck and Julia Riggs delicate surgical approach are essential. Prognosis is ultimately dependant on the MA VetMB AFHEA DipECVS MRCVS tumour type and stage, as well as the ability to restore homeostatic mechanisms. EBVS European Specialist in Key words: thyroid gland, thyroid adenoma, thyroid carcinoma, parathyroid Small Animal Surgery gland, parathyroid adenoma, hypercalcaemia, thyroidectomy, parathyroidectomy RCVS Specialist in Small Animal Surgery Introduction Having completed a surgical residency at Dogs and cats have paired thyroid glands, thyroid hormone tightly regulate thyroid the University of Cambridge and passed normally located in the mid cervical hormone production and secretion in a the certifying examinations, Julia became region (Hullinger 2013, Sissener 2014), healthy patient (Scott-Moncrieff 2015), a European Specialist in Small Animal Figure 1. Each thyroid gland is closely Figure 2. If thyroid tumours disrupt Surgery in 2018. She subsequently associated with two parathyroid glands: these feedback loops through increased, spent two years working as a Soft Tissue one parathyroid, often termed the or less commonly decreased, production Surgeon in a private referral practice external parathyroid gland, tends to be of thyroid hormones, they are classed as before returning to work in the Soft located towards the cranial pole of the functional. Tissue Surgery team at the University of thyroid gland and sits on its surface, The parathyroid glands are responsible Cambridge in 2019. whilst the other, often termed the for calcium and phosphorus homeostasis internal parathyroid gland, tends to be E: [email protected] through secretion of parathyroid located towards the caudal pole of the hormone (PTH) from chief cells (Figure thyroid gland and is usually embedded 3). PTH acts to increase blood calcium within the thyroid tissue (Nicholas levels through multiple mechanisms, and Swingle 1925; Hullinger 2013). including increased absorption in the The exact number and position of the kidneys and gastrointestinal tract (via 1, parathyroid glands relative to the thyroid 25-dihydroxycholecalciferol, otherwise glands can vary between individuals, and known as calcitriol) and increased identification of parathyroid tissue at resorption of bone. Feedback loops surgery can be challenging even when involving PTH, calcitonin (thyroid origin) neoplastic transformation is present and circulating levels of ionized calcium (Nicholas and Swingle 1925; Ham et al. are responsible for maintaining the serum 2009; Hullinger 2013). calcium concentration within a relatively To recognise the possible sequelae of narrow range (Feldman 2015). Functional thyroid and parathyroid neoplasia, the parathyroid tumours result in increased normal physiological functions of these production and secretion of PTH with glands must be understood. The thyroid loss of negative feedback control leading glands produce thyroglobulin which is to hypercalcaemia. This is referred to as hydrolysed into thyroxine (T4) and, to primary hyperparathyroidism. a lesser extent, triiodothyronine (T3) prior to release into the circulation. Surgical approach T3 is the most biologically active form A thorough knowledge of neck anatomy and exerts an effect on all body systems, is required if surgical removal of the SUBSCRIBE TO VETCPD JOURNAL regulating metabolic pathways. Feedback thyroid or parathyroid gland(s) is being loops involving thyrotropin-releasing considered. Patients should be positioned Call us on 01225 445561 hormone (TRH, hypothalamus origin), in dorsal recumbency with the neck elevated over a towel or sandbag; care or visit www.vetcpd.co.uk thyroid-stimulating hormone (TSH, pituitary origin) and free circulating should be taken to minimise the risk of

Page 42 - VETcpd - Vol 8 - Issue 3 VETcpd - Surgery Cricothyroid muscle

Hypothalamus Parathyroid gland Parathyroid TRH gland Thyroid gland

Thyroid gland Anterior Pituitary Inhibitory feedback Inhibitory

TSH

Internal Inhibitory feedback jugular vein Carotid artery

Carotid Recurrent artery laryngeal nerve Thyroid

Recurrent laryngeal Trachea nerve T4 and T3 Sternohyoid muscle

Figure 2: Feedback loops involving thyrotropin-releasing hormone (TRH), Figure 1: Schematic anatomy of the thyroid glands in a cat. Dogs and cats thyroid-stimulating hormone (TSH) and free circulating thyroid hormone tightly have paired thyroid glands, normally located in the mid cervical region regulate thyroid hormone production and secretion in a healthy patient

Low ionized calcium

FOUR PARATHYROID GLANDS

Feedback inhibition of PTH synthesis Feedback inhibition of PTH secretion

KIDNEY BONE PTH released into circulation

• Stimulates reabsorption of calcium • Stimulates osteoclasts • Stimulates production of leading to increased calcium resorption from bone 1, 25-dihydroxycholecalciferol INTESTINE (calcitriol)

• Increases calcium absorption from food

Increases serum calcium

Figure 3: How do parathyroid hormone and calcitonin maintain blood calcium levels? PTH is released from the parathyroid glands and calcitonin is released from cells in the thyroid gland; these hormones act in opposite ways to maintain calcium levels in the blood. When calcium levels decrease, this stimulates the parathyroid gland to release parathyroid hormone. Conversely, when the calcium level is high in the bloodstream, the thyroid gland releases calcitonin. Calcitonin slows down the activity of the osteoclasts found in bone, decreasing blood calcium levels.

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important to inspect all glandular tissue for abnormalities, it is critical that the vascular supply to the unaffected glands is preserved and important local neurovascular structures are not inadvertently damaged. Fine surgical instrumentation (including iris scissors, Debakey forceps, curved haemostats and sterile cotton buds) is essential and bipolar electrosurgery should be applied cautiously throughout the procedure (Ehrhart 2003; Séguin et al. 2012). Bipolar electrosurgery is preferred over monopolar for surgical procedures of the head and neck as electrical current is passed through tissues more selectively, minimising inadvertent thermal injury to adjacent anatomical structures. Tumours of the thyroid glands are removed by complete thyroidectomy whilst tumours of the parathyroid glands are excised via complete parathyroidectomy; the aim is to remove all affected glandular tissue so Figure 4: Positioning of a dog in dorsal recumbency that normal endocrinological function is for neck surgery (thyroid neoplasia). The thoracic restored or maintained. Surgical details Figure 5: Adenomatous hyperplasia causing thyroid limbs should be pulled and tied caudally against specific to the species, affected gland gland enlargement in a cat. the thoracic cavity and a sandbag or towel is placed and tumour type will be outlined later under the neck to optimise visualisation of cervical structures. It is important to ensure the patient stays in the article. Following completion of Pre-operative management straight throughout surgery – taping the maxilla to the thyroidectomy or parathyroidectomy, the Cats with suspected hyperthyroidism table as shown here may help with this. Placement separated sternohyoid muscles are re- should undergo comprehensive of a wide-bore orogastric tube may help delineate the apposed along the midline and the skin is biochemical and haematological analysis oesophagus for left-sided tumour excisions. closed in a routine fashion (Flanders 1999). including serum total T4 analysis – the There is usually no requirement for drain latter is elevated in the vast majority of the patient rolling under anaesthesia to placement, though mild seroma formation affected cats. Repeat testing of T4 levels ensure anatomical orientation is not lost at the surgical site may be encountered a few weeks later is recommended if the (Figure 4). The thyroid and parathyroid in the early post-operative period. Post- T4 value is within reference range when glands are always approached via a midline operative management is specific to the first assessed despite supportive clinical ventral neck incision so that the adjacent species, affected gland and tumour type, signs (Scott-Moncrieff 2015). Urinalysis neurovascular structures (vagosympathetic and will be explored further below. should also be performed to determine trunk, recurrent laryngeal nerve, common renal function. The main aim of diagnostic carotid artery and internal jugular vein) Thyroid tumours in cats imaging in hyperthyroid cats is to localise can be readily identified and preserved. A Pathophysiology the abnormal thyroid tissue to facilitate midline approach also permits exploration The vast majority (75-95%) of thyroid surgical decision-making. Both thyroid of the contralateral thyroid and parathyroid tumours in cats (Figure 5) are functional glands will be affected in the majority glands to ensure accurate localisation of and benign (Gerber et al. 1994; Birchard of cats and the possibility of functional diseased tissue. The incision should be 2006; Naan et al. 2006; Scott-Moncrieff ectopic thyroid tissue, most commonly generous, extending from the larynx to the 2015). Adenomatous hyperplasia of the located in the thoracic cavity, should be manubrium, again to ensure that all of the thyroid gland results in increased secretion considered (Ehrhart 2003; Naan et al. thyroid, parathyroid and local lymphoid of thyroxine which in turn has an effect 2006; Harvey et al. 2009). Scintigraphy tissue can be effectively visualised (Séguin on multiple organ systems causing is the most sensitive imaging modality et al. 2012). Following the skin incision, characteristic clinical signs in affected for this purpose; however, this is not the sternohyoid and sternothyroid cats. These most commonly include widely available (Ehrhart 2003; Naan muscles are separated via their median weight loss despite polyphagia, polydipsia et al. 2006; Harvey et al. 2009). Instead, raphe to expose the trachea, adjacent to and polyuria, gastrointestinal signs and ultrasonography and CT are more which the thyroid and parathyroid glands restlessness, though hyporexia, lethargy, commonly used to determine thyroid should be located (Flanders 1999). Self- hair coat changes and neurological signs gland size and investigate whether there retaining, blunt-tipped Gelpi retractors can also be encountered (Thoday and is any ectopic thyroid tissue present are inserted to maintain exposure of the Mooney 1992). Clinical examination (Wisner et al. 1994; Barberet et al. 2010; deeper neck structures at this stage. The may reveal a palpable mass in the ventral Bush et al. 2017). This is particularly right thyroid-parathyroid gland complex cervical region (reported in over 90% of important in cases of persistent or is usually located more cranially than the cats), cardiac abnormalities and poor body recurrent hyperthyroidism following left, within the paratracheal fascia (Séguin condition (Thoday and Mooney 1992; bilateral thyroidectomy (Naan et al. 2006). et al. 2012; Hullinger 2013); whilst it is Scott-Moncrieff 2015). In theory, unilateral functional thyroid

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