Page 135 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery

mobile on palpation may be amenable to surgical resection, Diagnosis
palpation alone may not be a good indicator of the like i- Physical examination is often rewarding, as these cats
l
VetBooks.ir palpation and also appear well encapsulated on advanced may be thin and the enlarged thyroid can be easy to pal-
hood of vascular invasion. Tumours that are freely mobile on
pate in the jugular groove. Although up to 70% of cats
imaging would be considered good candidates for surgical
are affected bilaterally, this may not be detected during
excision, and median survival times of 3 years have been
reported. However, most thyroid tumours are highly vas- physical examination because one thyroid gland often
cular and fixed to underlying structures, making surgical slips caudally into the thoracic inlet. Confirmation of diag-
removal extremely difficult, and considerable haemorrhage nosis usually relies on the identification of a raised serum
is common. Thyroidectomy of invasive tumours is asso- total T4 level (see above). If total T4 is high-normal, testing
ciated with a shorter remission time of 6–12 months (Carver should be repeated in 1–2 weeks. Free T4 levels should not
et al., 1995; Klein et al., 1995). Obtaining effective surgical be used as a screening tool for hyperthyroidism as they
margins may be very challenging and in some cases referral can be elevated due to non-thyroidal illness. Pretreatment
may be appropriate. investigations should include screening for occult renal
Debulking or marginal surgery to remove most of the and cardiac disease; a minimum database should include
tumour may be possible by an experienced surgeon and haematology, biochemistry, urinalysis and blood pressure
may have some palliative effect, or it can be combined measurements. Thoracic radiography, renal ultrasono-
with radiotherapy to improve remission times. Some graphy and echocardiography should be recommended.
tumours may be controlled by radiotherapy alone, with Thyroid scintigraphy can be useful to identify ectopic
palliative radiotherapy achieving a median survival time of thyroid tissue or metastatic disease, although this is not
22 months and definitive radiotherapy protocols achieving specific to malignant thyroid disease.
72% progression-free survival at 3 years (Brearley et al.,
1999; Theon et al., 2000). Treatment
Postoperative complications include: hypocalcaemia
due to hypoparathyroidism; laryngeal paralysis; Horner’s There are four therapeutic options in the cat (Figure 10.4).
syndrome; and hypothyroidism.
Histopathological features and Ki67 immunohisto- Medical treatment: This is used as a simple long-term
chemistry of resected tissue may help in determining the or temporary preoperative treatment. It is not curative;
prognosis for individual dogs (Campos et al., 2014). lifelong therapy or subsequent treatment by other means
is required. Regular biochemical monitoring is necessary
to assess total T4 and it is essential that the owner is
Feline thyroid tumours able to administer the tablets consistently two or three
Hyperthyroidism is the most common endocrine disease in times daily.
the cat, affecting both sexes and all breeds, with a mean There are two available drugs: methimazole and carbi-
age at diagnosis of 13 years. Although feline thyroid carci- mazole. These drugs are not cytotoxic and have no anti-
nomas are rare, they are more common in males. They tumour activity. They work by resolving the clinical signs
rarely metastasize and the cat nearly always exhibits of hyperthyroidism by lowering serum thyroid hormone
clinical signs of hyperthyroidism (see above). levels. Side effects are reported for both drugs, although
carbimazole (which is converted after administration to
methimazole) is better tolerated. Reported side effects
Clinical signs include: vomiting; anorexia; depression; eosinophilia;
Although there are a wide range of potential clinical signs, leucopenia; lymphocytosis (5–15%); self-induced excoria-
the typical picture is of a polyphagic, restless cat over tion (2%); agranulocytosis; thrombocytopenia (<5%);
the age of 10 years, with tachycardia and weight loss. hepato pathy (<2%); and positive testing for antinuclear

Treatment Advantages Disadvantages Availability
Oral medication • imple ine pensive lo risk • aily effective medication is essential Prescription-only medication
• Can be used until other therapy available • Not curative dispensed by veterinary
• eversible if renal insu ciency becomes • ome cats become increasingly di cult to surgeons
evident once thyrotoxicosis resolved medicate
Thyroidectomy • No special equipment required • Irreversible Some experience and surgical
• Curative • Postoperative complications (hypocalcaemia) skill re uired
may require intensive management and can be
life-threatening
Radioiodide • Curative • Irreversible Specialist centres only
therapy • Fe complications after treatment • Expensive
• Treats bilateral or malignant disease • e uires hospitali ation for eeks
• o risk to parathyroid or normal thyroid depending on the dose and ionizing radiation
tissue protection rules of the centre
• Treats unidentified ectopic thyroid • Development of any medical disease during
adenomas simultaneously isolation cannot be treated
Prescription • Reversible • ners must be completely compliant and Widely available
lo -iodine diet • Hypothyroidism does not occur feed the diet exclusively
• ot all cats respond at eeks 3 of cats
had normalized T4. No further improvement
after eeks

10.4 Treatments available for hyperthyroidism in the cat.

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