Is Hyperthyroidism Common in Dogs?
- Dr Andrew Matole, BVetMed, MSc
- Mar 31
- 4 min read
Introduction
Hyperthyroidism is an uncommon endocrine disorder in dogs, often secondary to functional thyroid tumors rather than spontaneous gland hyperplasia, as seen in cats (Feldman & Nelson, 2020).
The condition produces excessive thyroid hormone production, leading to multisystemic effects, including metabolic, cardiovascular, and neuromuscular disturbances. Unlike feline hyperthyroidism, which is commonly associated with benign adenomas, canine hyperthyroidism is frequently linked to malignant neoplasia, requiring a different diagnostic and therapeutic approach (Hess et al., 2016).
Causes and Risk Factors
The pathogenesis of hyperthyroidism in dogs is primarily linked to thyroid neoplasia and iatrogenic factors, though environmental and dietary influences may also play a role.
A. Thyroid Carcinoma
Thyroid carcinoma is the most common underlying cause of hyperthyroidism in dogs, accounting for over 90% of cases (Hess et al., 2016). These tumors are often locally invasive, affecting surrounding tissues such as the trachea, esophagus, and blood vessels, and may metastasize to regional lymph nodes or lungs. Unlike their feline counterparts, thyroid tumors in dogs are typically non-functional, with only 10–20% producing excessive thyroid hormones (Higgs et al., 2012). Large-breed dogs, including Golden Retrievers, Beagles, and Boxers, appear to have a higher risk of developing thyroid neoplasms, suggesting a possible genetic predisposition (Reusch, 2015).
B. Iatrogenic Hyperthyroidism
Iatrogenic hyperthyroidism occurs when dogs receive excessive levothyroxine supplementation during the management of hypothyroidism (Nelson & Couto, 2019). Overdosing results in clinical signs resembling naturally occurring hyperthyroidism, which can be reversed by adjusting the dosage.
C. Dietary and Environmental Factors
Though less studied in dogs than cats, dietary iodine intake and environmental pollutants, such as endocrine-disrupting chemicals (EDCs), have been implicated in thyroid dysfunction (Reusch, 2015). Chronic exposure to these factors may contribute to thyroid gland hyperplasia, increased T4 secretion, and a predisposition to neoplastic changes.
Clinical Signs
The clinical manifestations of hyperthyroidism in dogs are variable and depend on the underlying cause and the severity of excess thyroid hormone production.
A. Metabolic and General Signs
Unexplained weight loss despite a normal or increased appetite (polyphagia)
Hyperactivity or restlessness
Muscle wasting and generalized weakness
Heat intolerance
B. Cardiovascular Signs
Excess thyroid hormones increase metabolic rate and sympathetic nervous system activity, leading to cardiovascular changes (Hess et al., 2016):
Tachycardia (increased heart rate)
Hypertension (high blood pressure), predisposing dogs to retinal damage and kidney dysfunction
Cardiac arrhythmias, including atrial fibrillation
Heart murmurs and cardiomegaly due to increased cardiac output
C. Gastrointestinal and Urinary Signs
Polyuria and polydipsia (PU/PD) resulting from increased renal blood flow
Diarrhea and vomiting due to gastrointestinal hypermobility
D. Neuromuscular Signs
Tremors and muscle weakness
Behavioral changes such as aggression, anxiety, or hyperexcitability
Given that many of these signs overlap with other metabolic diseases, including diabetes mellitus and hyperadrenocorticism, thorough diagnostic work is crucial for accurate diagnosis.
Diagnosis
A combination of biochemical testing, imaging, and histopathology is required to confirm hyperthyroidism in dogs.
A. Biochemical Testing
Total T4 (TT4) and Free T4 (fT4): Elevated serum T4 and fT4 concentrations are indicative of hyperthyroidism, though mild increases can be seen in non-thyroidal illness (Feldman & Nelson, 2020).
Thyroid-Stimulating Hormone (TSH) Suppression: Unlike hypothyroidism, TSH is often suppressed in hyperthyroid dogs due to negative feedback from excess thyroid hormones.
T3 Suppression Test: Less commonly used, but may aid in borderline cases.
B. Imaging Studies
Ultrasound: Useful for detecting thyroid enlargement, nodules, and tumor characteristics.
Scintigraphy (Nuclear Imaging): Helps differentiate functional from non-functional thyroid tumors and detect metastatic spread (Peterson et al., 2014).
CT/MRI: Recommended in cases of suspected invasive thyroid carcinoma to assess tumor extent.
C. Histopathology and Cytology
Fine-needle aspiration (FNA) or biopsy may be performed to confirm thyroid carcinoma, though these techniques have variable sensitivity due to tumor invasiveness.
Treatment Options
Treatment choice depends on the underlying cause, disease severity, and presence of metastases.
A. Surgery (Thyroidectomy)
Surgical removal of the affected thyroid gland(s) is the preferred treatment for localized thyroid carcinomas. However, risks include:
Damage to the recurrent laryngeal nerve, leading to laryngeal paralysis
Hypoparathyroidism if the parathyroid glands are inadvertently removed
Hypothyroidism, necessitating lifelong levothyroxine supplementation
B. Radioactive Iodine Therapy (I-131)
Radioactive iodine effectively destroys hyperfunctional thyroid tissue while sparing normal thyroid cells (Higgs et al., 2012). This treatment is advantageous in cases where surgery is not viable, particularly for dogs with bilateral thyroid involvement or metastases.
C. Chemotherapy and Radiation Therapy
For unresectable or metastatic thyroid carcinomas, systemic chemotherapy (e.g., doxorubicin, carboplatin) or external beam radiation therapy may be used. These approaches improve survival times but do not cure the disease (Nelson & Couto, 2019).
D. Medical Management
Unlike in cats, medical management with antithyroid drugs (e.g., methimazole) is rarely used in dogs, as hyperthyroidism is often due to carcinoma rather than benign adenomatous changes (Feldman & Nelson, 2020). Beta-blockers (e.g., atenolol) may be prescribed to control tachycardia and hypertension.
Prognosis
The prognosis depends on the underlying cause and treatment response. Dogs with benign hyperthyroidism or surgically resectable tumors have a good prognosis. In contrast, those with invasive thyroid carcinoma or metastasis have a poorer outcome, with survival times ranging from months to years depending on treatment efficacy (Hess et al., 2016).
Conclusion
Hyperthyroidism in dogs is an uncommon but clinically significant endocrine disorder primarily associated with thyroid carcinoma. While iatrogenic and environmental factors may contribute, most cases require aggressive treatment, including surgery, radioactive iodine therapy, or chemotherapy. Early diagnosis and appropriate management are essential for improving clinical outcomes in affected dogs.
References
Feldman, E. C., & Nelson, R. W. (2020). Canine and Feline Endocrinology (5th ed.). Elsevier.
Hess, R. S., Kass, P. H., Shofer, F. S., et al. (2016). "Evaluation of serum thyroid hormone concentrations in dogs with thyroid carcinoma." Journal of Veterinary Internal Medicine, 30(5), 1450–1456.
Higgs, P., Evering, W., & Mahony, O. M. (2012). "Canine hyperthyroidism: A review of thyroid neoplasia and its management." Veterinary Clinics of North America: Small Animal Practice, 42(2), 327–339.
Nelson, R. W., & Couto, C. G. (2019). Small Animal Internal Medicine (6th ed.). Mosby.
Peterson, M. E., Ward, C. R., & Randolph, J. F. (2014). "Hyperthyroidism in dogs: A retrospective study of 15 cases." Journal of Veterinary Endocrinology, 25(3), 209–218.
Reusch, C. E. (2015). "Thyroid function in dogs: Current perspectives." Veterinary Journal, 203(1), 15–22.
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