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Hypothyroidism

ffelis

Introduction

  • Very rare spontaneous condition in adult cats.
  • Congenital form does occur.
  • Acquired form is usually iatrogenic following definitive therapy for hyperthyroidism, or due to excessive dose of anti-thyroid medications (reversible).
  • Cause: congenital, spontaneous acquired (various mechanisms) or iatrogenic.
  • Diagnosis: clinical signs, low serum thyroxine, elevated canine thyroid stimulating hormone (cTSH), suppressed dynamic hormone response tests.
  • Treatment: supplementation with thyroxine.
  • Prognosis: good for adult onset forms, guarded for congenital.

Pathogenesis

Etiology

  • Congenital:
    • Abnormal thyroid gland development.
    • Iodine deficiency or failure of thyroid gland to respond to thyroid stimulating hormone (TSH) Thyroid stimulating hormone.
  • Acquired:
    • Failure of TSH production (head trauma, single case report).
    • Prevention of thyroid hormone synthesis by excessive dose of anti-thyroid medication for hyperthyroid patient.
  • Destruction of thyroid tissue:
    • Surgical removal.
    • Radioiodine therapy for hyperthyroidism Hyperthyroidism: radio-iodine treatment  Thyroid gland: radio-iodine isolation cage .
    • Thyroiditis, hyperplastic goiter, thyroid atropy (spontaneous forms).
    • Not always clear in spontaneous acquired form.

Pathophysiology

  • Inadequate production of thyroxine for a variety of reasons.
  • Inadequate production of TSH.

Spontaneous form

  • One reported case had lymphocytic thyroiditis (believed to be immune-mediated thyroid destruction) cf canine hypothyroidism. This has also been recognized in a litter of kittens. Other case reports describe absence of thyroid tissue and diffuse hyperplastic goiter respectively.

Congenital form

  • Thyroid agenesis.
  • Dyshormonogenesis.
    • Defects in thyroid peroxidase enzyme  →  poor assimilation of iodine into hormone.
  • Congenital inability of thyroid gland to respond to thyrotropin (TSH).

Iatrogenic

  • Surgical removal of thyroid tissue with bilateral thyroidectomy Thyroidectomy.
  • Radioiodine ablation of all thyroid tissue (necessary for treatment of thyroid carcinoma, undesirable for thyroid adenoma).
  • Excessive dose of carbimazole Carbimazole or methimazole Methimazole administered to hyperthyroid patient.

Timecourse

  • Congenitally affected kittens are normal for first few weeks of life.
  • Iatrogenic: following treatment of hyperthyroidism.
  • Spontaneous acquired: insidious onset in older animals.

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.
  • Galgano M, Spalla I, Callegari C et al (2014) Primary hypothyroidism and thryoid goiter in an adult cat. JVIM 28 (2), 682-686 PubMed.
  • Peterson M E (2013) Feline focus: Diagnostic testing for feline thyroid disease: hypothyroidism. Compend Contin Educ Vet 35 (8), E4 PubMed.
  • Mellanby R J, Jeffery N D, Gopal M S et al (2005) Secondary hypothyroidism following head trauma in a cat. J Feline Med Surg 7 (2), 135-139 PubMed.
  • Mooney C (1998) Unusual endocrine disorders in the cat. In Practice 20 (7), 345-349 VetMedResource.
  • Rand J S, Levine J, Best S J et al (1993) Spontaneous adult onset hypothyroidism in a cat. JVIM (5), 272-276 PubMed.
  • Jones B R, Gruffydd-Jones T J, Sparkes A H et al (1992) Preliminary studies on congenital hypothyroidism in a family of Abyssinian cats. Vet Rec 131 (7), 145-148 PubMed.

Other sources of information

  • Peterson M E (2016) Diagnosis and management of iatrogenic hypothyroidism. In: August's Consultations in Feline Internal Medicine. 7th edn. Susan E Little (ed). Elsevier, pp 260-269.
  • Jones B R (1998) Feline hypothyroidism. In: Manual of Small Animal Endocrinology, 2nd Ed. Eds: A Torrance and C Mooney. Cheltenham: BSAVA. pp 199-202.

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