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An Urban Experience
Prognosis: Guarded to poor depending on the size of
the tumor at the time of diagnosis, extent of disease,
and possibly histologic type, as dogs with medullary tumors may have a better prognosis. Long-term
survival is possible as dogs with freely movable tumors without evidence of metastasis have a median survival time of 20.5 months with surgical excision alone. At diagnosis, tumour diameter, tumour volume, tumour fixation, ectopic location, follicular cell origin and Ki67 were positively associated with local invasiveness.
Tumor diameter, tumour volume and bilateral location were positively associated with the presence of distant metastasis. Macroscopic and histologic vascular invasion were independent negative predictors for disease free survival. Sublingual ectopic thyroid tumours appear to have a less aggressive biological behavior compared to ectopic sublingual tumours. Many of these dogs have long survival even without treatment.
Background: Thyroid neoplasia in cats is a much different disease than in dogs. These tumors are almost always benign adenomas/adenomatous hyperplasia and are functional. They produce excessive amounts of thyroxine, and affected cats show clinical signs of hyperthyroidism (tachycardia, PU/PD, weight loss, polyphagia). There
is bilateral involvement approximately 80% of the time and 5% are found to be ectopic (thoracic inlet or cranial mediastinum). Malignant thyroid tumors are rare.
Diagnosis: Based on history, clinical signs, elevated serum thyroxine levels and palpation of a thyroid mass. In normal cats you cannot feel the thyroid glands. 85-90% of affected cats have palpable tumors. Many cats also have hypertrophic cardiomyopathy and will have a gallop rhythm and sometimes a heart murmur. This can be confirmed by ultrasound of the heart. Diagnosis can be aided by radionuclide scanning of the thyroid glands, and is confirmed by increased uptake and size of the affected lobes. Because the disease is usually seen in older cats and older cats have a higher incidence of chronic renal failure, renal function (BUN and creatinine levels and urine specific gravity) should be evaluated pre-operatively. Following therapy and the return to a euthyroid state, patients with renal failure or insufficiency should be monitored for worsening of renal disease that may result from a lower cardiac output (and therefore glomerular filtration rate).
Surgery: Thyroidectomy is a practical and usually curative procedure for hyperthyroidism. Preoperatively each cat should be treated medically (Methimazole/Tapazole) to make it euthyroid, improving its candidacy for anesthesia and surgery. Patients with hypertrophic cardiomyopathy and tachycardia should be started on propranolol preoperatively to lower the heart rate and lessen the chance of cardiac arrhythmias during surgery. The EGG is monitored closely because premature ventricular
contractions are common.
The thyroid tumor should be removed by a modified extracapsular technique. A nick incision is made with
a #15 blade, or bipolar cautery in an avascular area of the capsule adjacent to the parathyroid gland, and iris scissors used to extend the cut in the capsule around the parathyroid gland, removing tyroid tissue and capsule as one. If both thyroid glands are involved they are each removed, however, removal of both glands during the same procedure increases the likelihood
of post-operative hypocalcemia from damaging
the blood supply to the parathyroid glands. For this reason, some surgeons elect to stage removal of the second gland. The resected tissue is submitted for histopathology.
Postoperative care and Complications: With bilateral thyroidectomy postoperative monitoring for hypocalcemia (panting, nervousness, facial rubbing, muscle spasms, anorexia, depression) is extremely important. Signs of hypocalcemia indicate that the calcium level is probably less than 7.5 mg/dl and should be initially treated
with intravenous 10% calcium gluconate (0.5-1.5 ml/ kg slowly over 10-20 minutes). During the treatment heart rate should be monitored and the infusion stopped if bradycardia is induced. When post-operative hypocalcemia occurs, long-term (usually a few months) treatment is sometimes necessary until the remaining parathyroid tissue revascularizes. Oral calcium (Tums) is often used along with vitamin D (dihydrotachysterol) for as long as clinically warranted based on serum calcium levels.
Even after bilateral thyroidectomy many cats do not require long-term exogenous thyroid supplementation; although, thyroid levels should be monitored periodically and thyroid supplementation instituted if levels
remain low. The cardiac changes associated with the cardiomyopathy usually resolve following thyroidectomy. The prognosis for these cats is generally good, though relapse may occur 1-2 years later due to regrowth of the adenoma due to hypertrophy of tissue not removed during thyroidectomy.
Radioactive iodine treatment (131I) is a viable (and often preferred over surgery) option for treating hyperthyroidism in cats. Especially cats that are a poor anesthetic risk or who have hyperfunctional ectopic thyroid tissue. This is successful in approximately 85% of cats.

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