Thyroid tumors are relatively uncommon in dogs, accounting for only 1 percent to 4 percent of all tumors. The majority of
diagnosed thyroid tumors in dogs are malignant, because adenomas are clinically silent and found incidentally on necropsy.
There is no documented cause of thyroid tumors in dogs, although certain breeds, such as Beagles, Boxers and Golden Retrievers,
have a higher incidence of them. It has been suggested that hypothyroidism in Beagles, and possibly in other breeds, may contribute
to the development of these tumors because of the chronic stimulation of the thyroid gland from excessive TSH production,
although this is unproven.
The most common presenting complaint is a mass in the neck, although clinical signs such as a change in voice, dyspnea and
facial edema are common, too. Only 10 percent of thyroid tumors are functional, and these dogs typically present with weight
loss, polyphagia, polyuria and polydipsia.
The differential diagnosis for a neck mass should include carotid body tumors, granuloma or abscess, metastasis from other
neoplasia such as tonsillar squamous-cell carcinoma or salivary-gland diseases. Thyroid tumors can be located anywhere in
the neck, although adjacent to the larynx is most common. Ectopic tumors have been reported in sites such as the mediastinum
and base of the tongue.
Obtaining a definitive diagnosis of a thyroid malignancy may be challenging. Malignant thyroid tumors have a well-developed
vascular supply so that blood contamination limits the diagnostic accuracy of cytology to 50 percent. On cytology, the majority
of thyroid tumors appear relatively well differentiated, so that often it is not possible to make a diagnosis of malignancy.
Cytology can be helpful in establishing an alternative diagnosis, such as metastatic squamous-cell carcinoma, allowing a thyroid
tumor to be ruled out.
An incisional biopsy of a thyroid tumor needs to be approached with caution, given the vascularity of these tumors. Tru-cut
biopsies, although less invasive, are not well suited for tumors with a high risk of post-biopsy hemorrhage. There is still
the risk of excessive hemorrhage with an open biopsy, although it may be easier to control than a Tru-cut biopsy.
In addition to being locally invasive, thyroid tumors have a relatively high rate of metastasis, mainly to the lungs and regional
lymph nodes. At the time of diagnosis, as many as 40 percent of the dogs have detectable metastasis. Ultimately, up to 80
percent of thyroid tumors metastasize.
Reported prognostic factors include size, movability and grade.
Multiple studies have shown that smaller tumors (<3 cm) are associated with a better prognosis because they have a lower metastatic
rate and are more amenable to surgical excision. Tumors that are freely movable have a better prognosis, presumably because
they can be removed successfully with surgery.
The histologic subtypes are medul-lary, solid/compact, follicular, papillary and anaplastic.
As would be expected, high-grade tumors are associated with a poor prognosis; however, there is no accepted grading scheme
for thyroid tumors. Histologic features, such as vascular or capsular invasion, a high mitotic index or a pleomorphic population
of cells, are consistent with a high-grade tumor.
Imaging of thyroid tumors
Prior to surgical excision, it may be appropriate to consider more advanced imaging, including a CT scan, cervical ultrasound
and/or nuclear scintigraphy. A CT scan or MRI can identify the extent of the tumor as well as the degree of invasiveness into
surrounding structures such as the larynx and other cervical structures.
This information is critical for surgical planning. The most common uses of nuclear scintigraphy are to identify ectopic thyroid
tumors, residual disease following surgery and occult metastasis. Both I-131 and 99mTc-pertechnetate can be used to image
both functional and non-functional tumors. The only requirement is that the tumor be capable of trapping pertechnetate or
trapping and organifying I-131, but it does not need to produce a functional thyroid hormone.
On pertechnetate scans, it is possible to identify capsular invasion, which appears as a heterogenous and poorly circumscribed
uptake of the pertechnetate. There is no advantage to using a pertechnetate scan for the identification of pulmonary metastasis
compared to thoracic radiographs.
I-131 scans appear to be more sensitive than pertechnetate scans in identifying the primary tumor and metastatic disease;
however, pertechnetate scans are more readily available, less expensive and require shorter holding times.