Thyroid carcinomas are relatively common dogs, particularly in breeds such as Beagles, Boxers, Golden Retrievers, and Siberian Huskies. The majority of thyroid carcinomas in dogs are non-functional, with < 25% of dogs presenting with either hypothyroidism or hyperthryoidism. Thyroid carcinomas can either be unilateral (67%-75%) or bilateral (25%-33%), and they can either be non-invasive or invasive. Thyroid carcinomas can also reported in ectopic locations in 7.5% of dogs with thyroid carcinoma. The hyoid apparatus and cranial mediastinum are the two most common locations for ectopic thyroid carcinoma, but they can occur anywhere from the tongue to the heart.
Thyroid carcinomas are rare in cats (2% of all thyroid diseases), whereas functional benign thyroid adenomas and hyperplasia are common and cause hyperthyroidism.
Thyroid carcinomas are generally diagnosed by palpation of a mass in the ventral or ventrolateral cervical region. An important step in determining treatment options is to evaluate whether the tumor is mobile or fixed. Fine-needle aspirates may be diagnostic, but blood contamination is common and this may result in a misdiagnosis or unrepresentative sample. Biopsy is usually not required because the biopsy procedure is just as invasive as surgical excision of the thyroid gland.
Thoracic radiographs or CT scans are recommended to assess the lungs for metastasis. Palpation and possibly aspiration of the regional lymph nodes should be performed to check for nodal metastasis. In one study, 33% of dogs with thyroid carcinoma had multiple unrelated tumors and hence preoperative abdominal and thoracic imaging should be considered for the identification of second tumors.
Thyroidectomy is recommended surgical treatment for dogs with mobile thyroid tumors. Radiation therapy is recommended for dogs with fixed thyroid tumors. Chemotherapy should be considered in dogs in which there is either macroscopic or microscopic evidence of vascular invasion. However, an effective chemotherapy protocol has not been identified. In one study, postoperative treatment with isotretinoin 9-cis significantly improved survival times in dogs with thyroid carcinomas.
The prognosis is very good for dogs with treated thyroid carcinomas. The median survival time for dogs with untreated thyroid carcinoma is 6-12 weeks. The majority of dogs with mobile thyroid tumors are cured with thyroidectomy with a median survival time > 3 years, and 1- and 2-year survival rates of 75% and 70%, respectively.
In contrast, the prognosis for dogs with fixed thyroid carcinomas treated surgically is guarded with a median survival time of 10 months and 1- and 2-year survival rates of 25% and 10%, respectively. As a result, radiation therapy or radioactive iodine are recommended for dogs with fixed thryoid carcinomas.
The prognosis following radiation therapy for dogs with fixed and invasive thyroid carcinomas is also very good. Complete responses (i.e., resolution of the palpable tumor) are reported in 8% of dogs, and partial responses (ie, reduction in tumor size by > 50%) in 69% of dogs; but 6-22 months are required to achieve maximal response. The 1- and 3-year progression-free survival rates following radiation therapy are 80% and 72%, respectively. Hypothyroidism is a common complication following radiation therapy and has been reported in 48% of dogs.
High-dose radioactive iodine has also been investigated in the treatment of dogs with fixed and invasive thyroid carcinomas. This treatment modality is preferred for dogs with functional hyperthyroidism, but dogs with non-functional thyroid carcinomas may also respond to radioactive iodine treatment. In one clinical study (2005) of 43 dogs treated with radioactive iodine therapy either alone or in combination with surgery, the median survival times were 30 and 34 months, respectively. In another clinical study (2006) of 39 dogs treated with radioactive iodine therapy, the median survival time was significantly longer in dogs with either non-metastatic thyroid carcinomas or thyroid carcinomas with lymph node metastasis (839 days) was significantly longer than dogs with distant metastasis (366 days). Of these 39 dogs, three dogs died of radio iodine-associated myelosuppression within 3 months of treatment.
Local tumor recurrence is reported in up to 16%-21% of dogs following thyroidectomy and 24% of dogs following radiation therapy. The metastatic rate for dogs treated with either thyroidectomy or radiation therapy is < 50%. Prognostic factors for the development of metastasis following treatment include tumor diameter > 5cm, bilateral thyroid carcinomas (controversial), follicular cell thyroid carcinomas (controversial), and macroscopic or microscopic evidence of vascular invasion. Dogs with thyroid carcinomas < 5cm have a 20% risk of developing metastasis, whereas larger tumors have a significantly increased risk of distant metastasis. In one study, dogs with bilateral thyroid carcinomas had a 16-times greater risk of developing metastatic disease compared to dogs with unilateral thyroid carcinoma; however, other studies have not identified bilateral thyroid carcinoma as a poor prognostic factor. In one clinical study of 15 dogs with bilateral thyroid carcinomas treated surgically, the median survival time was 38.3 months with no dog developing either local tumor recurrence or distant metastasis. In one clinical study (2014) of 44 dogs treated with thyroidectomy, macroscopic or histologic evidence of vascular invasion had a significant impact on disease-free survival times: disease-related death was 48-times greater in dogs with macroscopic vascular invasion and 3-times greater in dogs with microscopic vascular invasion.
Thyroid carcinoma is rare in cats and hence there is a paucity of information on treatment options and success rates. In one study of thyroidectomy in 101 cats (2006), only three cats had thyroid carcinoma with one cat dying of unknown causes 12 days postoperatively and the other two cats being alive and disease-free at 6 and 22 months postoperatively. In an earlier study of seven cats with thyroid carcinoma treated with thyroidectomy and postoperative radioactive iodine, hyperthyroidism resolved in all cats with one cat alive and disease-free at 33 months post-treatment and the remaining six cats dying of unrelated reasons 10-41 months post-treatment. In a 2009 clinical study of eight cats with thyroid carcinoma treated with radioactive iodine alone, hyperthyroidism resolved in six cats and these cats had survival times ranging from 181-2381 days.
Last updated on 6th March 2017