Hyperthyroidism

From Dog
Alopecia in the ventral region of the neck of a beagle 3 months after completion of radiation therapy for a thyroid carcinoma[1]

Hyperthyroidism (thyrotoxicosis) is a neuroendocrine disease of older dogs characterized by increased thyroid gland activity, polyphagia and weight loss[2].

This is a less frequently observed disease than is seen in cats, and is far less common than hypothyroidism.

Hyperthyroidism accelerates several physiologic processes, a fact which is reflected in the decreased systemic vascular resistance, increased cardiac output, increased renal blood flow, hypertrophic and hyperplastic tubuli, and increased glomerular filtration rate.

Chronic renal disease can occur due to glomerulosclerosis, proteinuria and oxidative stress[3].

Causes of canine hyperthyroidism include:

Approximately 90% of clinically apparent thyroid tumors in the dog are carcinomas[11]. Breeds that have been reported to be at increased risk of thyroid tumors include the Beagle, Golden Retriever and Boxer. benign thyroid tumors in dogs are relatively rare and usually discovered at postmortem consisting of nonfunctional small nodules.

Many hyperthyroid dogs are asymptomatic, but clinical signs can include polyuria (due to loss of medullary hypertonicity), compensatory polydipsia, polyphagia, weight loss, hyperthermia, aggressiveness, tachycardia, panting and restlessness.

Thyroid carcinomas may be detected as a firm mass in the ventral region of the neck; they may be minimally invasive and freely moveable, but more commonly they invade into adjacent structures and are therefore fixed in place[12].

Chest auscultations may reveal thyrotoxic heart changes such as sinus tachycardia and atrial fibrillation. Additionally, hypertension may be evident using doppler oscillometroscopy (often >180 mmHg)[13].

Diagnosis is based on presenting clinical signs, imaging (ultrasonography or scintigraphy)[14] and demonstration of elevated total thyroxine or free thyroxine (normal 19·3 - 51·5 nmol/L) and thyroid-stimulating hormone (TSH) concentrations (normal < 0·30 ng/mL) using fluoroimmunoassays[15]. However, dogs with thyroid carcinomas are usually euthyroid, but may be hypothyroid or hyperthyroid[16].

A definitive diagnosis requires thyroid biopsy.

A differential diagnosis in older dogs would include chronic renal disease, diabetes mellitus and hyperadrenocorticism[17]. A differential diagnosis for thyroid carcinoma would include mast cell tumor, lymphoma or metastasis from a malignant oral tumor such as tonsillar squamous cell carcinoma.

Treatment of benign thyroid adenomas requires thyroidectomy[18] or thyroid ablation with radioiodine[19]. Clinical symptoms usually resolve after correction of metabolic thyrotoxicosis.

Thyroid carcinomas require surgical debulking followed by palliative radiation[20] and/or chemotherapy.

Drugs such as doxorubicin[21], cisplatin[22], carboplatin, mitoxantrone[23] and actinomycin D[24] have been used with a 20% to 50% partial response.

In older canine patients, regularly blood monitoring is recommended to detect early signs of hyperthyroidism.

References

  1. Mayer MN & MacDonald VS (2007) External beam radiation therapy for thyroid cancer in the dog. Can Vet J 48(7):761-763
  2. Scott-Moncrieff JC (2012) Thyroid disorders in the geriatric veterinary patient. Vet Clin North Am Small Anim Pract 42(4):707-725
  3. van Hoek I & Daminet S (2009) Interactions between thyroid and kidney function in pathological conditions of these organ systems: a review. Gen Comp Endocrinol 160(3):205-215
  4. Liptak JM et al (2008) Cranial mediastinal carcinomas in nine dogs. Vet Comp Oncol 6(1):19-30
  5. Stassen QE et al (2007) Hyperthyroidism due to an intrathoracic tumour in a dog with test results suggesting hyperadrenocorticism. J Small Anim Pract 48(5):283-287
  6. Olson EE et al (2007) Hyperthyroidism associated with probable struma cordis in a young dog. J Vet Intern Med 21(2):332-335
  7. Bigazzi PE & Rose NR (1975) Spontaneous autoimmune thyroiditis in animals as a model of human disease. Prog Allergy 19:245–274
  8. Köhler B et al (2012) Dietary hyperthyroidism in dogs. J Small Anim Pract 53(3):182-184
  9. Fine DM et al (2010) Cardiovascular manifestations of iatrogenic hyperthyroidism in two dogs. J Vet Cardiol 12(2):141-146
  10. Heseltine JC et al (2005) Effect of levothyroxine administration on hemostatic analytes in Doberman Pinschers with von Willebrand disease. J Vet Intern Med 19(4):523-527
  11. Capen CC (2002) Tumors of the endocrine glands. In: Meuten DJ, ed. Tumors in Domestic Animals. 4th ed. Ames: Iowa State Press. pp:607–696
  12. Thrall MA (2007) Diagnostic cytology in clinical oncology. In: Withrow SJ, Vail DM, eds. Withrow & MacEwen’s Small Animal Clinical Oncology. 4th ed. St. Louis: Saunders Elsevier. pp:112–133
  13. Simpson AC & McCown JL (2009) Systemic hypertension in a dog with a functional thyroid gland adenocarcinoma. J Am Vet Med Assoc 235(12):1474-1479
  14. Barber LG et al (2007) Thyroid tumors in dogs and cats. Vet Clin North Am Small Anim Pract 37(4):755-773
  15. Parra MD et al (2004) Cortisol and free thyroxine determination by time-resolved fluorometry in canine serum. Can J Vet Res 68(2):98-104
  16. Bailey DB & Page RL (2007) Tumors of the endocrine system. In: Withrow SJ, Vail DM, eds. Withrow & MacEwen’s Small Animal Clinical Oncology. 4th ed. St. Louis: Saunders Elsevier. pp:583–609
  17. Metzger FL & Rebar AH (2012) Clinical pathology interpretation in geriatric veterinary patients. Vet Clin North Am Small Anim Pract 42(4):615-629
  18. Itoh T et al (2007) Functional thyroid gland adenoma in a dog treated with surgical excision alone. J Vet Med Sci 69(1):61-63
  19. Campos M et al (2012) Recombinant human thyrotropin in veterinary medicine: current use and future perspectives. J Vet Intern Med 26(4):853-862
  20. Thrall DE & LaRue SM (1995) Palliative radiation therapy. Semin Vet Med Surg (Small Anim) 10:205–208
  21. Ogilvie GK et al (1989) Phase II evaluation of doxorubicin for treatment of various canine neoplasms. J Am Vet Med Assoc 195:1580–1583
  22. Fineman LS et al (1998) Cisplatin chemotherapy for treatment of thyroid carcinoma in dogs: 13 cases. J Am Anim Hosp Assoc 34:109–112
  23. Ogilvie GK et al (1991) Efficacy of mitoxantrone against various neoplasms in dogs. J Am Vet Med Assoc 198:1618–1621
  24. Hammer AS et al (1994) Treatment of tumor- bearing dogs with actinomycin D. J Vet Intern Med 8:236–239