Chapter 6 – Disorders of Calcium: Hypercalcemia and Hypocalcemia

The clinical presentation and biologic behavior of thyroid tumors vary widely among dogs, cats, and human beings. Although thyroid tumors in dogs are rare, they are most likely to be malignant. Clinical signs are usually the result of impingement on surrounding structures, and clinical hyperthyroidism is rare. In contrast, hyperthyroidism resulting from benign thyroid proliferation is relatively common among older cats. Malignant tumors are extremely uncommon but have high metastatic potential. Irrespective of the tumor's ability to produce functional thyroid hormone, scintigraphy is often helpful in the diagnosis and staging of thyroid tumors in all three species. Treatment with surgery is a reasonable treatment option for noninvasive tumors. Iodine 131 is a well-established treatment for thyroid nodules in cats, but its effectiveness in dogs is controversial. In dogs, external beam radiation therapy has produced more consistent results in affording local tumor control when surgery is not possible.

Thyroid tumors in dogs and cats.

The goal of this study was to update the descriptive statistics of thyroid cancer by using data from multiple institutions collected through the Veterinary Medical Database (VMDB). Information was collected and reported from cases of canine thyroid cancer submitted to the VMDB between January 1, 1995 and December 31, 2005. Odds ratio (OR) analysis was performed on breeds that had > or =3% of the total number of dogs with thyroid cancer; ORs for each age category were also determined. Thyroid cancer represented 1.1% of all neoplasms during the time period of interest. Golden retrievers, beagles, and Siberian huskies all had significantly increased ORs for developing thyroid cancer. No sex predisposition was evident, but dogs between 10 and 15 years of age had a significantly increased chance of developing thyroid disease. Carcinomas and adenocarcinomas represented 90% of thyroid cancers, while adenomas represented 9.3%. Thyroid carcinoma and adenocarcinoma continue to be uncommon in our canine population. Older dogs are still more commonly affected, and this study is in agreement with previous studies that golden retrievers and beagles are overrepresented. A new finding is that Siberian huskies are also overrepresented. Carcinomas represent a much higher proportion of thyroid cancers than previously reported, and adenomas are likely incidental findings on necropsy. Thyroid cancer should be high on the list of differentials for a neck mass in older, large-breed dogs, as they make up 1.1% of the cancer cases reported. The overwhelming majority of thyroid cancers are carcinomas, and they are most common in golden retrievers, beagles, and Siberian huskies.

Thyroid cancer in dogs: an update based on 638 cases (1995-2005).

OBJECTIVE: To determine quality and duration of progression-free survival (PFS) time in dogs with unresectable thyroid carcinomas treated with definitive megavoltage irradiation and analyze prognostic factors of PFS and patterns of failure (local recurrence vs metastasis). DESIGN: Prospective clinical trial. ANIMALS: 25 dogs with locally advanced thyroid carcinomas and no evidence of metastasis. PROCEDURE: Dogs were treated with 48 Gy during 4 weeks on an alternate-day schedule of 4 Gy/fraction. RESULTS: Irradiation was safe and effective for treatment of large unresectable thyroid carcinomas. Progression-free survival rates were 80% at 1 year and 72% at 3 years. Time to maximum tumor size reduction ranged from 8 to 22 months. Factors affecting PFS were not found. Twenty-eight percent (7/25) of dogs developed metastasis. Dogs with bilateral tumors had 16 times the risk of developing metastases, compared with dogs with a single tumor. Dogs with no evidence of tumor progression had 15 times less risk of developing metastases. Radiation-induced hypothyroidism was suspected in 2 dogs 13 and 29 months after irradiation. CONCLUSIONS AND CLINICAL RELEVANCE: Irradiation is effective for local control of thyroid tumors, despite their slow regression rate. Results provided evidence that local tumor control affects metastatic outcome in dogs with thyroid carcinomas and is a strong basis for the development of new approaches that include irradiation in the management of dogs with advanced thyroid carcinomas. Improvements in local tumor control alone may be insufficient to improve survival times because of the high risk of metastatic spread before an initial diagnosis is made, which warrants initiation of early systemic treatment.

Prognostic factors and patterns of treatment failure in dogs with unresectable differentiated thyroid carcinomas treated with megavoltage irradiation.

Canine thyroid carcinoma.

Of 82 dogs with thyroid carcinoma seen between January 1981 and October 1989, 20 had freely movable tumors without evidence of metastasis and were treated with surgical excision alone. Uncensored mean and median survival times for these 20 dogs were both 20.5 months. Kaplan-Meier survival analysis, which censors for nontumor-related deaths and dogs lost to follow-up, indicated that median survival time was greater than 36 months. Seven dogs died of tumor-related causes: 2 died because of metastasis or local recurrence of the tumor, 5 died of treatment-related complications (eg, laryngeal paralysis, hypocalcemia, tracheostomy complications). Eight dogs died of unrelated causes; 1 dog was lost to follow-up at 26 months after surgery; 3 dogs were alive 19, 24, and 26 months after surgery. Cause of death could not be determined in the remaining dog. Long-term survival is possible following surgical removal of mobile thyroid carcinomas in dogs.

Treatment of thyroid carcinoma in dogs by surgical resection alone: 20 cases (1981-1989).

Determination of the effective dosage of tiletamine-zolazepam-ketamine-xylazine, with or without methadone, in dogs.

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Treatment of thyroid carcinoma in dogs by surgical resection alone: 20 cases (1981-1989).

Determination of the effective dosage of tiletamine-zolazepam-ketamine-xylazine, with or without methadone, in dogs.