Immunocytochemistry of canine thyroid tumors

Follicular thyroid carcinoma in an inbred family of mongrel dogs in Trinidad & Tobago

Thyroid tumors occur in many domestic species, but are most common in the dog, in which they are classified as follicular or medullary. During 2012-2016, we received tissue specimens or whole carcasses of 4 dogs with variable enlargement of the thyroid glands. The 2 males and 2 females were of mixed (mongrel) inbreeding, 3-4.5-y-old. All tumors had lobulated architecture forming follicular structures variably containing colloid. On immunohistochemistry of the tumors from 3 of the dogs, 2 were thyroglobulin positive, and all 3 were negative for calcitonin, confirming follicular thyroid carcinoma in 2 of the dogs. Thyroid carcinomas have not been reported previously in related mongrel dogs, to our knowledge.

Canine follicular cell and medullary thyroid carcinomas: Immunohistochemical characterization

Research on modulation of iodine uptake by thyroid cells could help improve radioiodine treatment of dogs with thyroid tumors. The aim of this study was to characterize the immunohistochemical expression of thyroid transcription factor-1 (TTF-1), thyroglobulin, thyrotropin receptor (TSHR), sodium iodide symporter (NIS), pendrin, thyroid peroxidase (TPO), vimentin, and Ki-67 in follicular cell thyroid carcinomas (FTCs) and medullary thyroid carcinomas (MTCs), and to compare protein expression between FTC causing hyperthyroidism and FTC of euthyroid dogs. Immunohistochemistry was performed in 25 FTCs (9 follicular, 8 follicular-compact, and 8 compact) and 8 MTCs. FTCs and MTCs were positive for TTF-1, and expression was higher in FTCs of euthyroid dogs compared with FTCs of hyperthyroid dogs (P= .041). Immunolabeling for thyroglobulin was higher in follicular and follicular-compact FTCs compared with compact FTCs (P = .001), while vimentin expression was higher in follicular-compact FTCs compared with follicular FTCs (P = .011). The expression of TSHR, NIS, pendrin, and TPO was not significantly different among the different subtypes of FTCs or between FTCs causing hyperthyroidism and FTCs in euthyroid dogs. TSHR, NIS, pendrin, and TPO were also expressed in MTCs. Ki-67 labeling index was comparable between FTCs and MTCs, and between FTCs causing hyperthyroidism and FTCs in euthyroid dogs. Proteins of iodine transport were also expressed in canine MTCs, which could have implications for diagnosis and treatment. The different expression of thyroglobulin and vimentin between FTC histological subtypes could reflect variations in tumor differentiation.

Mixed medullary and follicular cell thyroid carcinoma in a dog

A 12-y-old, castrated male Weimaraner dog was presented for a wellness examination. A 7-cm, firm mass was palpated on the left, ventral, mid-lateral neck. The neck mass was removed surgically and submitted for histopathology. A thyroid carcinoma was diagnosed based on microscopic examination. Immunohistochemistry for chromogranin-A, calcitonin, and thyroglobulin identified dual immunoreactivity of the latter two, and a final diagnosis was of a well-differentiated, compact, mixed medullary and follicular cell thyroid carcinoma. These neoplasms are rare in humans and have not been reported in dogs, to our knowledge.

Comparative Histology of C Thyrocytes in Four Domestic Animal Species: Dog, Pig, Horse, and Cattle

Simple Summary

In this study we have proved that dogs, pigs, cattle, and horses, species belonging to four distinct families, differ in regard to microscopical characteristics of their C thyrocytes. Although the total number of C thyrocyte profiles and their localization within thyroid lobes were comparable, each of the examined species displayed their unique morphological characteristics and distribution pattern. The differences described in our study, easily recognizable microscopically, can be used as a reference material for further studies focused on C thyrocytes biology in physiological and pathological conditions.

Abstract

The number, morphology, and distribution of C thyrocytes within the thyroid gland vary among species; however, studies in domestic animals are limited. In this study we compared the morphology, distribution pattern, and percentage of C thyrocytes in four domestic species: dogs, pigs, horses, and cattle. Eighty thyroid glands, 20 per species, were examined. C thyrocytes were visualized immunohistochemically with anti-calcitonin rabbit polyclonal antibody alone and combined with the periodic acid Schiff method to simultaneously visualize C thyrocytes with the basement membranes of thyroid follicles. C thyrocyte morphology varied considerably between species, from oval- (dogs) and spindle-shaped (pigs) to polymorphic (cattle and horses). Bovine C thyrocytes demonstrated cytoplasmic protrusion. C thyrocytes were located intrafolliculary (all species), epifollicularly (dogs, horses, cattle), or interfolicularly (cattle). Most porcine and bovine C thyrocytes existed individually whereas canine C thyrocytes usually formed clusters. In horses, they tended to form groups of various shapes and sizes or even rims encompassing whole follicles. In all species, the number of C thyrocyte profiles increased from the periphery to the central area of the thyroid lobe. The mean total fraction of C thyrocytes in the superficial, intermediate, and central areas were as follows: 2.55%, 8.43%, and 12.48% in dogs; 3.81%, 7.66%, and 10.79% in pigs; 1.55%, 7.44%, and 8.87% in horses; and 2.62%, 10.75%, and 12.96% in cattle. No statistical differences in the total number of C thyrocyte profiles were observed among species (8.87% in dogs, 8.58% in cattle, 7.98% in pigs, and 5.83% in horses). Our results indicated that the studied species displayed their own morphological characteristics and distribution pattern of C thyrocytes; however, total numbers of C thyrocyte profiles and their localization within the thyroid lobe are comparable.

Pathological and Immunohistochemical Characterization of Thyroid Neoplasms in Cats

There are few pathological and immunohistochemical descriptions of thyroid neoplasms in cats. We investigated neoplasms of the thyroid in 31 cats (average age 15.6 years) in Southern Brazil. Most (96.8%) of these cases were classified as follicular adenomas and were predominantly unilateral and multinodular. Histologically, macrofollicular adenomas were the most common type observed. Carcinomas represented 3.2% of the investigated tumours. By immunohistochemical labelling, thyroglobulin was expressed more commonly than paired box gene 8 or thyroid transcription factor 1 in the follicular adenomas. One carcinoma was immunopositive for thyroglobulin, pancytokeratin, chromogranin A and synaptophysin, suggestive of a calcitonin-negative neuroendocrine carcinoma.

Canine Thyroid Cancer: Molecular Characterization and Cell Line Growth in Nude Mice

Thyroid cancer is the most common endocrine malignancy in dogs. Dogs and humans are similar in the spontaneous development of thyroid cancer and metastasis to lungs; however, thyroid cancer has a higher incidence of metastasis in dogs. This study developed a preclinical nude mouse model of canine thyroid cancer using a canine thyroid adenocarcinoma cell line (CTAC) and measured the expression of important invasion and metastasis genes in spontaneous canine thyroid carcinomas and CTAC cells. CTAC cells were examined by electron microscopy. Short tandem repeat analysis was performed for both the original neoplasm and CTAC cells. CTAC cells were transduced with luciferase and injected subcutaneously and into the tail vein. Tumors and metastases were monitored using bioluminescent imaging and confirmed with gross necropsy and histopathology. Invasion and metastasis genes were characterized in 8 follicular thyroid carcinomas (FTCs), 4 C-cell thyroid carcinomas, 3 normal thyroids, and CTAC cells. CTAC cells grew well as xenografts in the subcutis, and they resembled the primary neoplasm. Metastasis to the kidney and lung occurred infrequently following subcutaneous and tail vein injection of CTAC cells. STR analysis confirmed that CTAC cells were derived from the original neoplasm and were of canine origin. Finally, 24 genes were differentially expressed in spontaneous canine thyroid carcinomas, CTAC, and normal thyroids. This study demonstrated the usefulness of a nude mouse model of experimental canine thyroid carcinoma and identified potential molecular targets of canine follicular and C-cell thyroid carcinoma.

Bilateral thyroid follicular compact-cellular carcinoma in a llama

An 18-y-old female llama (Lama glama) in Saskatoon, Saskatchewan was examined during a routine herd check, and a mass was detected in the ventral cervical area just below the angle of the jaw. No clinical signs were evident except for the mass and chronic loss of body condition. Postmortem examination revealed bilateral enlargement of the thyroid gland with multiple cysts. Histopathology of the thyroid gland revealed follicular compact-cellular carcinoma lesions, with infiltration of neoplastic thyroid follicular cells in regional lymph nodes.

Establishment and characterization of a novel cell line of medullary thyroid carcinoma from a dog

A novel cell line of canine medullary thyroid carcinoma (MTC) was established from the neck mass, diagnosed histopathologically and immunohistochemically as ectopic MTC. The neoplastic cells arranging trabecular structures were characterized as pleomorphic cells with eosinophilic cytoplasm and nucleus, containing often clear nucleolus. These tumor cells were immuno-positive for calcitonin gene-related protein (CGRP), somatostatin, and chromogranin A. In addition, 8th passaged cultured cells were also immuno-positive for CGRP, somatostatin, and chromogranin A. The cloned tumor cells showed logarithmic cell growth with a doubling time of 33.3 h. From the results of DNA sequencing of rearranged during transfection (RET) proto-oncogene, the cloned tumor cells had four single base substitution, including exon 5 codon 82, exon 16 codon 750, exon 17 codon 777, and exon 24 codon 1085, all of which were single nucleotide polymorphism reported in RET gene of dogs. After the xenotransplantation into severe combined immunodeficiency (SCID) mice, the cloned cells showed tumorigenicity potentials. The morphological and immunohistochemical features of the xenotransplanted tumor were almost in conformity with those of the original tumor, including positive immunoreactivity for calcitonin, CGRP, and chromogranin A. To our knowledge, this is the first report of canine MTC cell line, which provides useful in vitro tool for understanding oncogenic mechanism and pathophysiological state of MTC in dogs.

CEREBELLAR AND MESENCEPHALON NEOPLASIA IN A NILE HIPOPPOTAMUS (HIPPOPOTAMUS AMPHIBIOUS)

A 52-yr-old female Nile hippopotamus ( Hippopotamus amphibious ) was presented for acute onset anorexia, depression, lethargy, instability, and weakness in the pelvic limbs. Clinical signs were rapidly progressive, despite empiric therapy with anti-inflammatory medications, resulting in the death of the animal. Gross necropsy evaluation revealed two tan, firm masses in the cerebellum and mesencephalon and a single mass in the right cranial adrenal gland. All three masses had a similar histologic morphology, and immunohistochemical investigation confirmed the general diagnosis of an adenocarcinoma, but the exact cell of origin remains unclear. In addition, there was evidence of neuroendocrine differentiation in the adrenal gland and not in the brain. These findings suggest either two distinct neoplastic populations or a metastasizing adenocarcinoma with focal endocrine differentiation. In dogs, anal sac and clitoral adenocarcinomas have been reported to undergo focal endocrine differentiation, and both can cause widespread metastasis while the primary lesion can be small. A small neoplasm of these glands may have been missed on gross examination.

Calcitonin-negative primary neuroendocrine tumor of the thyroid (nonmedullary) in a dog

The Calcitonin-negative neuroendocrine tumor of the thyroid (CNNET) or “nonmedullary” in humans is a rare tumor that arises primarily in the thyroid gland and may be mistaken for medullary thyroid carcinoma; it is characterized by the immunohistochemical (IHC) expression of neuroendocrine markers and the absence of expression for calcitonin. An Argentine dogo bitch showed a solid, compact thyroid tumor, which was IHC negative for the expression of calcitonin, carcinoembryonic antigen, thyroglobulin and S100 protein, and positive for synaptophysin and cytokeratin AE1-AE3. The Ki-67 proliferation index was low. We cite this case not only because it is the first case report of calcitonin-negative primary neuroendocrine tumor of the thyroid in dogs but also because we want to highlight the diagnostic importance of IHC in this regard.

Clinical, pathologic, and immunohistochemical prognostic factors in dogs with thyroid carcinoma

Background

Prognostic markers for dogs with thyroid tumors are limited.

Hypothesis/Objectives

To identify clinical, pathologic, and immunohistochemical prognostic factors for dogs with thyroid tumors.

Animals

Seventy dogs with thyroid neoplasia.

Methods

Retrospective study. Dogs with thyroid neoplasia were included when follow‐up information and formalin‐fixed paraffin‐embedded tumor samples were available. Immunohistochemistry (IHC) was performed for thyroglobulin, calcitonin, Ki‐67, and E‐cadherin. Correlation of tumor variables (diameter, volume, localization, scintigraphic uptake, thyroid function, IHC) with local invasiveness and metastatic disease was performed on all tumor samples. Forty‐four dogs treated by thyroidectomy were included in a survival analysis.

Results

Fifty dogs (71%) had differentiated follicular cell thyroid carcinoma (dFTC) and 20 (29%) had medullary thyroid carcinoma (MTC). At diagnosis, tumor diameter (P = .007; P = .038), tumor volume (P = .020), tumor fixation (P = .002), ectopic location (P = .002), follicular cell origin (P = .044), and Ki‐67 (P = .038) were positively associated with local invasiveness; tumor diameter (P = .002), tumor volume (P = .023), and bilateral location (P = .012) were positively associated with presence of distant metastases. Forty‐four dogs (28 dFTC, 16 MTC; stage I–III) underwent thyroidectomy. Outcome was comparable between dogs with dFTC and MTC. Macroscopic (P = .007) and histologic (P = .046) vascular invasion were independent negative predictors for disease‐free survival. Although time to presentation, histologic vascular invasion and Ki‐67 were negatively associated with time to metastases, and time to presentation was negatively associated with time to recurrence, no independent predictors were found. E‐cadherin expression was not associated with outcome.

Conclusions and Clinical Importance

Prognostic factors have been identified that provide relevant information for owners and clinicians.

Clinical features and treatment outcomes of 41 dogs with sublingual ectopic thyroid neoplasia

Background

Thyroid neoplasia is common in dogs, but there are few reports of dogs with ectopic, sublingual thyroid tumors.

Objectives

To describe clinical features and outcomes of dogs with ectopic, sublingual thyroid neoplasia.

Animals

Five hundred and forty‐four dogs with thyroid neoplasia.

Methods

This retrospective study reviewed the medical records of dogs referred for thyroid neoplasia between 1995 and 2013. Data extracted included signalment, extent of thyroid disease (eutopic or ectopic; metastasis), serum thyroxine (T₄) concentration, treatment, and survival.

Results

Of 544 dogs with thyroid neoplasia, 41 (7.5%) dogs had ectopic sublingual thyroid tumors. The clinical features of these 41 dogs were similar to the cohort group of 503 dogs with eutopic or ectopic mediastinal thyroid tumors, but dogs with sublingual tumors were younger and less likely to have metastatic disease (15% versus 30%, P < .05). Of the 41 dogs, 28 received treatment: 21 with surgery (which included partial hyoidectomy in 13), 7 with radioiodine alone, and 13 with surgery followed by administration of radioiodine. Overall median survival was 562 days (range, 1‐1,850 days).

Conclusions and Clinical Importance

When compared with eutopic thyroid carcinomas, ectopic sublingual thyroid tumors generally have a less aggressive biologic behavior. Many dogs have prolonged survival, even without treatment, although death because of local tumor invasiveness or metastasis can develop in some dogs. Surgical thyroidectomy, including partial hyoidectomy, is generally effective for control of local disease. Administration of radioiodine, alone or in combination with surgical treatment, is recommended for multifocal disease or metastasis.

Histopathological and immunohistochemical findings of primary and metastatic medullary thyroid carcinoma in a young dog

This report describes the gross, histological, and immunohistochemical features of medullary thyroid carcinoma (MTC) with pulmonary metastases in a young dog. Sheets of pleomorphic cells supported by fibrous stroma characterized the primary mass, while metastatic nodules had a neuroendocrine pattern. Despite differing histologic features, all masses showed marked immunoreactivity against calcitonin and multiple neuroendocrine markers consistent with MTC. Although MTC is a well-recognized entity, it may be difficult to distinguish this mass from other thyroid neoplasms, necessitating immunohistochemical characterization.

Thyroid Tumors in Dogs and Cats

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.

Non-functional C-cell Adenoma in Aged Horses

Thyroid tumours occur in older horses, and most such tumours have been considered to be of follicular epithelial origin. However, their immunohistochemical characterization has not been investigated. The purpose of this study was to confirm a suspicion that most of these tumours are in fact parafollicular cell (C cell)-derived adenomas, and to evaluate their pathogenesis and functional state. Thyroid glands from 38 horses aged 10-29 years were evaluated, all tissue samples being examined histologically, immunohistochemically and ultrastructurally. Nodular tumour masses were found in the thyroids of 12 of 38 horses older than 10 years (31.6%), and in nine of 12 horses older than 20 years (75.0%), regardless of sex or breed. Nodular lesions were composed of solid proliferations of polygonal cells with eosinophilic granular cytoplasm. Immunohistochemically, tumour cells were positive for calcitonin and neuron specific enolase, but negative for thyroglobulin. Ultrastructurally, few if any secretion granules were found in tumour cells. On the basis of these results it was concluded that the nodular lesions were C-cell adenomas, not follicular adenomas. It was suspected that the C-cell adenomas were non-functional and unlikely to lead to calcitonin hypersecretion-related diseases.

Paratracheal cervical mass in a dog

A 14-year-old female Welsh Terrier was presented for evaluation of multiple skin masses. Two of them were diagnosed as a follicular (epidermal) cyst and a sebaceous epithelioma by cytology and histopathology. The third mass was located in the subcutis adjacent to the cervical trachea. Clinical findings, thoracic radiography, and laboratory results were otherwise unremarkable. Cytologically, the cervical mass was characterized by the presence of round to polygonal cells with distinct cell borders, mild to moderate anisocytosis, round to oval eccentric nuclei with prominent nucleoli, and a variable amount of finely granular, eosinophilic cytoplasm. Differential diagnoses included endocrine/neuroendocrine tumors such as thyroid adenocarcinoma, medullary thyroid carcinoma, carotid body chemodectoma, and parathyroid carcinoma. The mass was removed surgically and submitted for histopathologic and immunohistochemical examination. Histologically, the mass was composed of rows and nests of densely packed cells separated by thin fibrovascular stroma. Neoplastic cells had moderate to large amounts of pale eosinophilic dusty or finely vacuolated cytoplasm and round to oval nuclei with finely dispersed chromatin and 1 or 2 small nucleoli. On immunohistochemistry, neoplastic cells were positive for chromogranin A, calcitonin, and neuronspecific enolase, and were negative for cytokeratin and thyroglobulin. The final diagnosis was medullary thyroid carcinoma, an uncommon endocrine tumor of dogs that has cytologic and histopathologic features similar to other endocrine and neuroendocrine tumors. Immunohistochemistry or immunocytochemistry should be performed to differentiate medullary thyroid carcinoma from other endocrine/neuroendocrine tumors of dogs.

Immunohistochemical detection of thyroid transcription factor-1, thyroglobulin, and calcitonin in canine normal, hyperplastic, and neoplastic thyroid gland

Immunohistochemistry for thyroid transcription factor-1 (TTF-1), thyroglobulin, and calcitonin was done in normal, hyperplastic, and neoplastic canine thyroid glands that had been fixed in formalin and embedded in paraffin. Prolonged fixation did not significantly alter the immunostaining for TTF-1. Staining for TTF-1 was always nuclear and usually strong. One of two C-cell adenomas, five of five follicular cell adenomas, 5 of 11 C-cell carcinomas, 38 of 42 follicular cell carcinomas, two of five cases of C-cell hyperplasia, two of two cases of follicular epithelial hyperplasia, one of two metastatic C-cell carcinomas, and three of four metastatic follicular carcinomas were positive for TTF-1. One follicular carcinoma that was positive for TTF-1 was negative for thyroglobulin. Thirty-nine of 42 follicular cell carcinomas were positive for thyroglobulin, including two cases that were negative for TTF-1. All C-cell lesions were positive for calcitonin. Of a variety of normal canine tissues and 278 nonthyroid tumors, only normal lung (airway and alveolar epithelial cells) and four of five pulmonary carcinomas were positive for TTF-1. TTF-1 is a good marker of thyroid differentiation and can be used in conjunction with thyroglobulin and calcitonin to increase the detection and differentiation of thyroid tumors and their metastases.

Endocrine tumors

Because of the diverse nature of endocrine organs, and their vast range of physiologic functions, endocrine tumors encompass a wide range of origination sites and disease entities. The clinical picture of affected individuals is highly dependent on the tissue of origin, and the presence or absence of functional hormone secretions. Identification, localization, and therapeutic strategies, as well as prognosis can vary greatly. Many endocrine tumors have been described in human as well as veterinary patients. This article focuses on endocrine tumors of dogs and cats. Various tumors affecting the pancreas, thyroid, parathyroid, adrenal and pituitary glands are described, including insulinoma, gastrinoma, glucagonoma, and thyroid carcinoma, as well as parathyroid hormone- and growth hormone-secreting tumors. The syndrome of multiple endocrine neoplasia is also described.

Thyroid C-cell carcinoma with amyloid in a red fox (Vulpes vulpes schrenchki)

An amyloid-producing medullary thyroid carcinoma (MTC) in a red fox (Vulpes vulpes schrenchki) bred in a zoo was examined using histopathologic and immunohistochemical techniques. The neoplastic cells had an ill-defined cytoplasmic membrane and abundant, finely granular eosinophilic cytoplasm, containing numerous argyrophilic granules. The neoplastic tissues were divided into various sizes by a vascular connective stroma, which was partly fibrovascular with broad areas of hyalinization containing varied amounts of amyloid. Immunohistochemically, neoplastic cells showed reactivity to anti-calcitonin, neuron-specific enolase, somatostatin, and keratin antibodies. However, amyloid in the stroma did not show immunoreactivity to the antibodies used. Histologic and immunohistochemical features of MTC in the present animal were analogous to those of the C-cell carcinoma derived from thyroid C cells (parafollicular cells) reported in humans and dogs.

Intrapericardial neoplasia in dogs

A retrospective study was conducted from 1,919 necropsies of dogs, and 52 cases were selected on the basis of primary or metastatic neoplasia in the pericardium, the heart base, or the myocardium. Primary intrapericardial neoplasms were more frequent than intrapericardial metastases. Hemangiosarcomas were the most frequent primary neoplasms, followed by chemodectomas. Lymphomas were the most frequent metastatic neoplasms, followed by fibrosarcomas and rhabdomyosarcomas. Other neoplasms, either primary or metastatic, occurred rarely.

Immunophenotype of the canine transmissible venereal tumour

The canine transmissible venereal tumour is a naturally occurring contagious round-cell neoplasia which is primarily located in the mucous membrane of the external genitalia in dogs of either sex. In order to specify the controversial cytogenetic origin of this round-cell tumour, 14 cases of canine transmissible venereal tumour, formalin- or Bouin-fixed and paraffin-embedded, were subjected to extensive immunophenotypic analysis using reagents specific to a variety of cytoplasmic or surface antigens: lysozyme, ACM1 antigen, vimentin, neuron-specific enolase, glial fibrillary acidic protein, desmin, alpha smooth muscle actin, CD3, IgG, kappa and lambda light chains, and keratin. Lysozyme immunoreactivity was detected in all cases, ACM1 antigen in 11 of 14, neuron-specific enolase in 11 of 14, vimentin in 10 of 14, glial fibrillary acidic protein in 4 of 14 and desmin in 1 of 14. All the sections were negative to keratins, alpha smooth muscle actin and CD3, whereas in five cases, perivascular tumour cells contained Ig G, kappa and lambda light chains. The immunoreactivity to lysozyme and ACM1 antigen supports the hypothesis of a histiocytic immunophenotype for the canine transmissible venereal tumour.

A comparison of medullary thyroid carcinoma and thyroid adenocarcinoma in dogs: a retrospective study of 38 cases

The medical records of 38 dogs with thyroid neoplasia that were treated by surgical excision of the tumor, or had an incisional biopsy performed as a diagnostic procedure, were reviewed. Of the 38 dogs, 21 (55%) had resectable tumors, whereas 17 (45%) had an incisional biopsy as the tumors were nonresectable. All dogs had an initial diagnosis of thyroid carcinoma. The type of carcinoma was confirmed in 33 dogs by histological and immunohistochemical examination. Twelve dogs (36%) had medullary thyroid carcinoma, and 21 dogs (64%) had thyroid adenocarcinoma. Of the 12 dogs with medullary thyroid carcinoma, 10 (83%) had resectable tumors. Of the 10, three (30%) had at least a 1-year survival. None had radiographic evidence of metastasis at the time of surgery. Of the 21 dogs with thyroid adenocarcinoma, 11 (52%) had resectable tumors. Of the 11 dogs, five (45%) had at least a 1-year survival. Three dogs had radiographic evidence of metastasis at the time of surgery. Of 10 dogs with nonresectable thyroid adenocarcinoma, two dogs (20%) had at least a 1-year survival. In the dogs in this study, medullary thyroid carcinoma was more prevalent than previously reported. Most of the medullary thyroid carcinomas were well circumscribed and resectable. Medullary thyroid carcinoma may possess gross and histological characteristics of a less malignant nature when compared with other thyroid carcinomas.