ISSN 2398-2969      

Hypothyroidism

icanis

Introduction

  • Cause: congenital or acquired metabolic condition.
  • Signs: alopecia, lethargy, seborrhea.
  • Diagnosis: laboratory tests, response to therapy (see Hypothyroidism diagnostic flowchart).
  • Treatment: thyroxine supplementation.
  • Prognosis: good but will require life-long therapy.
Print off the owner factsheet Hypothyroidism (Thyroid hormone deficiency) Hypothyroidism (Thyroid hormone deficiency) to give to your client.

Pathogenesis

Etiology

  • In normal animal, thyroid hormone production is under control of the hypothalamic-pituitary-thyroid axis Hypothalamic-pituitary-thyroid axis diagram.

Primary hypothyroidism

  • >95% of cases.
  • May be congenital (rare) or acquired.
  • Most common causes of acquired primary hypothyroidism are lymphocytic thyroiditis or idiopathic atrophy of the thyroid gland.
  • Thyroid tumors: primary or occasionally secondary, are rare causes of acquired primary hypothyroidism.
  • Lymphocytic thyroiditis: most common cause.
  • Probably auto-immune, causes diffuse infiltration of thyroid gland by lymphocytes, plasma cells and macrophages, resulting in progressive destruction of the follicles and secondary fibrosis.
  • Circulating antibodies to thyroglobulin present (found in >50% of dogs with hypothyroid dogs).
  • Idiopathic atrophy: probably a primary degenerative disorder, causes loss of thyroid parenchyma with replacement by adipose tissue.

Secondary hypothyroidism

  • <5% of cases.
  • Decreased secretion of thyroid stimulating hormone (TSH) from pituitary → secondary follicular atrophy of thyroid.
  • Cause: pituitary tumors, congenital malformation of pituitary gland in the German Shepherd dog (with pituitary dwarfism Congenital panhypopituitarism ).

Tertiary hypothyroidism

  • Poorly defined in the dog.
  • Deficient production or release of TRH.

Iodine deficiency

  • Very rare.

Pathophysiology

  • Destruction of normal thyroid tissue → negative feedback increasing TSH secretion → remaining functional tissue secreting T4 and T3 at proportionately higher rates.
  • When more than 75% of gland is destroyed then T4 and T3 secretion is reduced → effects nearly every body tissue → wide range of clinical signs.

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.
  • Graham P (2009) Canine hypothyroidism: diagnosis and therapy. In Practice 31 (2), 77-82 VetMedResource.
  • Franch J et al (2004) Management of leishmanial osteolytic lesions in a hypothyroid dog by partial tarsal arthrodesis.Vet Rec 155 (18), 559-562 PubMed.
  • Gieger T L, Hosgood G, Taboada J et al (2000) Thyroid function and serum hepatic enzyme activity in dogs after phenobarbital administration. JVIM 14 (3), 277-281 PubMed.
  • Dixon R M & Mooney C T (1999) Evaluation of serum free thyroxine and thyrotropin concentrations in the diagnosis of canine hypothyroidism. JSAP 40 (2), 72-78 PubMed.
  • Panciera D L (1999) Is it possible to diagnose hypothyroidism? JSAP 40 (4), 152-157 PubMed.
  • Hess R S & Ward C R (1998) Diabetes mellitus, hyperadrenocorticism, and hypothyroidism in a dog. JAAHA 34 (3), 204-207 PubMed.
  • Dixon R M, Graham P A et al (1997) Comparison of endogenous serum thyrotropin (cTSH) concentrations with bovine TSH response test. Results in euthyroid and hypothyroid dogs. JVIM 11, 121.
  • Ramsey I K (1997) Diagnosing canine hypothyroidism. In Practice 19 (7), 378-383 VetMedResource.
  • Scott-Moncrieff J C & Nelson R W (1997) Response of serum canine thyrotropin (cTSH) to stimulation by thyrotropin releasing hormone (TRH) in euthyroid dogs, hypothyroid dogs and euthyroid dogs with concurrent disease. JVIM 11, 121.
  • Frank L A (1996) Comparison of TRH to TSH stimulation for evaluating thyroid function in dogs. JAAHA 32 (6), 481-487 PubMed.
  • Hall I A, Campbell K I, Chambers M D, & Davis C N (1993) Effect of trimethoprim/sulfamethoxazole on thyroid function in dogs with pyoderma. JAVMA 202 (12), 1959-1962 PubMed.
  • Ferguson D C (1994) Update on diagnosis of canine hypothyroidism. Vet Clin North Am Sm Anim Pract 24 (3), 515-539 PubMed.
  • Pancieria D L (1990) Canine hypothyroidism. Part II. Thyroid function tests and treatment. Comp Cont Ed Pract Vet 12 (6), 843-858 VetMedResource.
  • Ferguson D C (1988) The effect of nonthyroidal factors on thyroid function test in dogs. Comp Cont Ed Pract Vet 10 (12), 1365-1377 VetMedResource.
  • Kemppainen R J, Thompson F N, Lorenz M D, Munnell J F & Chakraborty P K (1983) Effects of prednisone on thyroid and gonadal endocrine function in dogs. J Endocrinol 96 (3), 293-302 PubMed.

Other sources of information

  • Gaughan K R, Bruyette D S & Jordan F R (1996) Comparison of thyroid function testing in non-greyhound pet dogs and racing greyhounds (abstract). Proceedings of 14th ACVIM forum. pp 768.
  • Nelson R W & Feldman E C (1996) Hypothyroidism. In:Canine and Feline Endocrinology and Reproduction. 2nd edn. Philadelphia: W B Saunders. pp 68-117.
  • Muller G H et al (1995) Muller and Kirk's Small Animal Dermatology. 5th edn. Philadelphia: W B Saunders. pp 691-703 (Detailed dermatology text book for in-depth reading).
  • Refsal K R & Nachreiner R F (1995)Monitoring thyroid replacement therapy. In: Current Veterinary Therapy XII. Eds: R W Kirk & J D Bonagura. Philadelphia: W B Saunders (Guidance on monitoring therapy).
  • Ferguson D C, Kemppainen R J & Beale K M (1993) Hypothyroidism. St Petersburg: Daniels Pharmaceuticals Inc (Easy reading colour booklet).
  • Kemppainen R J & MacDonald J M (1993) Canine hypothyroidism. In: Current Veterinary Dermatology - the Science and Art of Therapy. Eds: C E Griffin, K W Kwochka & J M Macdonald. 1st edn. St Louis: Mosby Year Book. pp 265-272 (Well-presented dermatology text book).
  • Peterson M E & Ferguson D C (1990) Thyroid diseases. In: Textbook of Veterinary Internal Medicine, Vol 2. Ed: S J Ettinger. Philadelphia: W B Saunders. pp 1632-1675 (Detailed coverage of physiology of the thyroid gland and diagnostic tests for hypothyroidism).

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