ISSN 2398-2950      

Hyperthyroidism

ffelis

Introduction

  • Most common endocrine disease of cats.
  • First reported in 1979.
  • Cause: functional thyroid adenoma, ademomatous hyperplasia or rarely thyroid carcinoma  Thyroid gland: neoplasia    →   excessive concentrations of thyroxine (T4) Thyroxine assay and tri-iodothyronine (T3) Endocrinology: tri-iodothyronine (T3).
  • Signs: weight loss, polyphagia; lethargy, vomiting, diarrhea, hyperactivity, vocalization, inappetence less commonly.
  • Diagnosis: laboratory demonstration of elevated T4.
  • Treatment: carbimazole Carbimazole, methimazole Methimazole, radioactive iodine, surgical removal of tumor.
  • Prognosis: good with treatment and careful aftercare.
    Print off the Owner factsheet on hyperthyroidism  Hyperthyroidism - disease and treatment to give to your client.

Pathogenesis

Etiology

  • Functional thyroid adenomas/carcinoma in one or both thyroid glands   →   elevated [T4] and [T3]   →   increased metabolic rate and heat production   →   weight loss and increased appetite.
  • Elevated [T4] and [T3]   →   increased sympathetic stimulation   →   tachycardia and hypertrophic cardiomyopathy Heart: hypertrophic cardiomyopathy and behavioral changes.
  • Benign adenomatous change in 98% cases - functional malignant carcinoma is rare (<3% cases).

Pathophysiology

  • In normal thyroid hormone production is under control of hypothalamic-pituitary-thyroid axis   Endocrinology: hypopituitary thyroid axis - diagram .
  • Increased [T4] affects many body systems.
  • Development of disease poorly understood and likely multifactorial. Several theories proposed:
    • Some thyroid follicular cells have higher growth potential   →     focal hyperplasia and adenoma development.
    • Circulating factors, eg iodine levels in diet or immunoglobulins stimulate thyroid development.
    • Environmental goitrogens   →   thyroid pathology.
    • Oncogene mutations.
  • Epidemiological risk factors:
    • Use of a litter box.
    • >50% wet food in the diet.
    • Diet including fish, liver or giblets.
    • Exposure to canned foods.
    • Increasing age.
    • Non-pure breed.

Timecourse

  • Months to years.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Carney H C, Ward C R, Bailey S J et al (2016) 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg 18 (5), 400-416 PubMed.
  • Harvey A M, Hibbert A , Barrett E L et al (2009) Scintigraphic findings in 120 hyperthyroid cats. J Fel Med Surg 11 (2), 96-106 PubMed.
  • Hibbert A, Gruffyd-Jones T, Barrett E L et al (2009) Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Fel Med Surg 11 (2), 116-124 PubMed.
  • Wakeling J, Everard A, Brodbelt D et al (2009) Risk factors for feline hyperthyroidism in the UK. J Feline Med Surg 50 (8), 406-414 PubMed.
  • Shiel R E, Mooney C T (2007) Testing for hyperthyroidism in cat. Vet Clin North Am Small Anim Pract​ 37 (4), 671-691 PubMed.
  • Ward C R (2007) Feline thyroid storm. Vet Clin North Am Small Anim Pract​ 37 (4), 745-754 PubMed.
  • Milner R J, Channell C D, Levy J K et al (2006) Survival times for cats with hyperthyroidism treated with iodine 131, methimazole, or both: 167 cases (1996-2003). JAVMA 228 (4), 559-563 PubMed.
  • Shales C (2005) Feline hyperthyroidism: A review of the literature. UK Vet 10 (5), 27-33.
  • Hoffmann G, Marks S L, Taboada J et al (2003) Transdermal methimazole treatment in cats with hyperthyroidism. J Feline Med Surg (2), 77-82 PubMed.
  • Mooney C T (2002) Pathogenesis of feline hyperthyroidism. J Feline Med Surg (3), 167-169 PubMed.
  • Norsworthy G D, Adams V J, McElhaney M R et al (2002) Relationship between semi-quantitative thyroid palpation and total thyroxine concentration in cats with and without hyperthyroidism. J Feline Med Surg (3), 139-143 PubMed.
  • Mooney C T (2001) Feline hyperthyroidism. Diagnostics and therapeutics. Vet Clin North Am Small Anim Pract 31 (5), 963-983 PubMed.
  • Slater M R, Geller S & Rogers K (2001) Long-term health and predictors of survival for hyperthyroid cats treated with iodine 131. JVIM 15 (1), 47-51 PubMed.
  • Bucknell D G (2000) Feline hyperthyroidism - spectrum of clinical presentations and response to carbimazole therapy. Aust Vet J 78 (7), 462-465 PubMed.
  • Martin K M, Rossing M A & Ryland L M et al (2000) Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. JAVMA 217 (6), 853-856 PubMed.
  • Flanders J A (1999) Surgical options for the treatment of hyperthyroidism in the cat. J Feline Med and Surg (3), 127-34 PubMed.
  • Kass P H, Peterson M E, Levy J et al (1999) Evaluation of environmental, nutritional and host factors in cats with hyperthyroidism. JVIM 13 (4), 323-329 PubMed.
  • Peterson M E, Melian C & Nichols C E (1998) Measurement of serum concentrations of total and free T4 in hyperthyroid cats and cats with nonthyroidal disease. JVIM 12, 211.
  • Broussard J D, Peterson M E, Fox P R (1995) Changes in clinical and laboratory findings in cat with hyperthyroidism from 1983 to 1993. JAVMA 206 (3), 302-5 PubMed.
  • Peterson M E & Becker D V (1995) Radioiodine treatment of 524 cats with hyperthyroidism. JAVMA 207 (11), 1422-1428 PubMed.
  • Peterson M E, Kinzter P P, Cavanagh P G et al (1983) Feline hyperthyroidism - Pre-treatment clinical and laboratory evaluation of 131 cases. JAVMA 183 (1), 103-110 PubMed.

Other sources of information

  • Stephens M, O'Neill D G, Church D B & Brodbelt D C (2013) Feline hyperthyroidism among UK veterinary practices: prevalence, risk factors and spatial distribution. In: Scientific Proceedings, British Small Animal Veterinary Association Congress,Birmingham, April 4-7, 2013, pp 596-597.
  • Mooney C M (2009) Hyperthyroidism. In: Ettinger S J, Feldman E C (eds) Textbook of Veterinary Internal Medicine; diseases of the Dog and Cat. 6th edn. Philadelphia, W B Saunders Co, pp 1761-1779.
  • Feldman E C & Nelson R W (2004) Feline hyperthyroidism (thyrotoxicosis). In: Feldman E C, Nelson R W (eds) Canine and Feline Endocrinology and Reproduction. 3rd edn, St Louis, Elsevier. pp 153-215.

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