Page 134 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

Chapter 10 · Surgery of the thyroid and parathyroid glands

the diagnosis. However, in some cases, total T4 is persis- tachycardia); hypertension; hyperactivity or lethargy;
tently within the normal range, despite a high index of weak ness; and panting. Clinical signs associated with
VetBooks.ir measuring the free T4 level may be more sensitive, as this myopathy may be seen in cats, and these animals may
congestive heart failure secondary to hypertrophic cardio-
clinical suspicion for hyperthyroidism. In these cases,
decompensate if stressed during examination or blood
is less affected by non-thyroidal factors. In combination
with a total T4 level in the upper half of the normal range,
glomerular filtration rate (GFR), which may compensate for
an elevated serum level of free T4 may be considered diag- sampling. Hyperthyroidism also results in an increased
nostic of hyperthyroidism. However, measurements of chronic renal failure in older cats. When these patients
free T4 alone can still be misleading, and some cats with are treated for thyrotoxicosis, the GFR drops and renal
non-thyroidal disease may have a high free T4 level compromise is unmasked. On initiation of treatment, serial
with normal levels of total T4 and T3. Finally, a T3 suppres- measurements of serum urea and creatinine combined
sion test or a thyroid-releasing hormone (TRH) stimulation with urine specific gravity and protein:creatinine ratios
test may be used to confirm the diagnosis. are therefore recommended. Definitive treatment such as
thyroidectomy or radioactive iodine treatment may be
Hypothyroidism contraindicated in patients with concurrent renal disease.
Large masses or retropharyngeal metastases may cause:
Hypothyroidism is the most common thyroid disorder coughing; dysphagia; facial oedema due to lymphatic
affecting dogs. Most cases are due to primary destruction obstruction; or a neuropathy such as Horner’s syndrome
of thyroid gland tissue due to lymphocytic thyroiditis or or laryngeal paralysis.
primary atrophy, although some animals may present sub-
sequent to thyroidectomy or radiation therapy to the neck. Diagnosis
Hypothyroidism is important to surgeons as a cause of
delayed wound healing, poor postoperative recovery and The enlarged thyroid gland is often palpated in the neck.
wound dehiscence. In the cat, it can be felt as a smooth nodule that ‘pops’
Clinical hypothyroidism is rare in cats, even when bilat- between the fingers as they are slid down the tracheal
eral thyroidectomy has been performed. It is occasionally groove.
seen after radioiodine treatment.
Diagnosis is through confirmation of persistently low Canine thyroid tumours
total T4 accompanied by high TSH. Dogs with non-thyroid
disease or receiving phenobarbital may show decreased Thyroid tumours account for up to 3.8% of all tumours in
dogs, and whilst up to 50% may be benign adenomas
T4 that is not associated with clinical hypothyroidism; if
there is any suspicion of this the diagnosis should be more than 90% of those that present clinically are malig-
nant. Thyroid tumours usually present as a firm painless
confirmed with a TRH or TSH stimulation test (Figure 10.3).
Clinical signs of hypothyroidism include: skin and hair mass in the neck, often associated with the larynx, but
they can be found at any site along the ventral cervical
coat abnormalities; peripheral neuropathy; bradycardia;
obesity; lethargy; dullness; heat-seeking behaviour; infer- and mandibular region and up to 60% may have bilateral
involvement. A small number of thyroid tumours may
tility; exercise and cold intolerance; normochromic
normocytic non-regenerative anaemia; hypercholesterol - present as heart-base masses, arising from ectopic
thyroid tissue. Only about 6–10% of dogs with thyroid
aemia; delayed recovery from anaesthesia; and delayed
wound healing. masses show clinical signs of hyperthyroidism. Pre-
operative staging and screening for other disease is
Drug therapy important and up to 40% will have detectable metastases
at the time of presentation.
• Phenobarbital
• Corticosteroids
• Propranolol Diagnosis
• Furosemide
• ome non-steroidal anti-inflammatory drugs A minimum database should include: three-view thoracic
• Sulphonamides radiographs (or CT scan); a full haematology and biochem-
Diseases istry blood screen; and ultrasonographic or CT/magnetic
resonance imaging examination of the neck mass and
• Hyperadrenocorticism regional lymph nodes. Advanced imaging is strongly
• Diabetes mellitus
• Renal disease recommended for preoperative planning in the dog as
• Hypoalbuminaemia palpation has been shown to be a poor indicator of invasive
Circumstances that may give a false result of a lo total behaviour; it also improves the sensitivity for detection of
10.3 thyroxine (T4). metastatic disease (Taeymans et al., 2013). Ultrasono-
graphic investigation of the liver, spleen and mediastinal
lymph nodes may also assist in identifying metastatic
Hyperthyroidism disease. The mass should be aspirated, under ultrasound
Hyperthyroidism is most commonly associated with guidance, to confirm the diagnosis, although haemodilution
thyroid adenoma in the cat; dogs occasionally present may make aspirates difficult to interpret. Trucut biopsy
with a thyroid carcinoma that is producing active hormone. carries a significant risk due to local coagulopathy and the
Clinical signs are secondary to excessive production of high vascular component of thyroid tumours. Fine-needle
T4 and T3, with a slow onset. aspirates of regional lymph nodes may be helpful in making
a diagnosis and staging.
Clinical signs
Treatment
These include: weight loss; polyphagia or anorexia; poly-
dipsia; diarrhoea; vomiting; skin and hair coat abnormal- Surgical resection is the treatment of choice for thyroid
ities; cardiac abnormalities (heart murmur, gallop rhythm, tumours. However, whilst thyroid tumours that are freely

125

Ch10 HNT.indd 125 31/08/2018 11:41