Hypothyroidism in Cats (Felis) | Vetlexicon
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Hypothyroidism

ISSN 2398-2950

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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.

Presenting signs

  • Congenital form:
    • Poor growth in kittens.
    • Mental dullness.
    • Constipation.
  • Acquired forms:
    • Lethargy.
    • Weight gain despite normal/reduced appetite.
    • Greasy haircoat.
    • Increased dander.

Age predisposition

  • Few weeks (signs usually noticed at re-homing) for congenital form.
  • Iatrogenic in older cats (as a result of treatment for hyperthyroidism Hyperthyroidism).
  • Young adult in spontaneous form.

Breed/Species predisposition

  • Abyssinian Abyssinian and DSH for development of congenital form.

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

Presenting problems

Client history

  • Congenital form:
    • Poor growth in kittens.
    • Lethargy, mental dullness.
    • Constipation Constipation.
  • Acquired form:
    • Lethargy, weight gain, seborrhea, poor appetite (spontaneous form, occasionally in iatrogenic cases).
    • Signs may be lacking altogether (iatrogenic cases).

Clinical signs

Congenital

  • Stunted kittens.
  • Disproportionate dwarfism.
  • Depression.
  • Weight loss in kittens.
  • Cretinism in kittens.
  • Dental eruption delayed (up to 18 months).
  • Pinnal alopecia.
  • Goiter.
  • Bradycardia in kittens.
  • Seborrhea and coat matting.
  • Soft fluffy coat in kittens.

Adult - spontaneous

  • Depression.
  • Seborrhea and coat matting.
  • Alopecia/thinning haircoat.
  • Secondary skin infections.
  • Myxedema.
  • Hypothermia Hypothermia.
  • Bradycardia.

Adult - iatrogenic

  • Weight gain.
  • Seborrhea.
  • Lethargy.
  • Signs may be lacking altogether.

Diagnostic investigation

Hematology

Biochemistry

Hormone assay

  • Serum thyroxine(T4) Thyroxine assay:
    • Low serum thyroxine concentrations.
    • Newborn kittens have T4 50% of mothers.
    • By 4 weeks should be 50-70 nmol/l.
Beware non-thyroidal illness causing suppression of serum thyroxine levels artificially.
  • Serum canine thyroid stimulating hormone (cTSH):
    • Feline TSH cross reacts with the canine assay.
    • Combination of low TT4 and raised cTSH is diagnostic for hypothyroidism.
    • However reference ranges are not fully established.
  • TSH response test (although clinical grade TSH is difficult to obtain):
    • No rise in basal thyroxine concentrations 6 hours after TSH administration.
  • TRH response test Thyrotropin releasing hormone (TRH) stimulation test (although causes transient side effects and thyroxine response not as dramatic as that to TSH):
    • Low basal thyroxine levels which fail to stimulate 4 hours after TRH administration.

Radiography

  • Evidence of disproportionate dwarfism.
  • Delayed closure of ossification centers in long bones, and short vertebral bodies with congenital forms.

Confirmation of diagnosis

Discriminatory diagnostic features

  • History (of treated hyperthyroidism).
  • Signs.

Definitive diagnostic features

  • Thyroxine and cTSH assays.
  • Hormone response tests.

Gross autopsy findings

  • Goiter in some congenital cases and one spontaneous acquired case.
  • ?Atrophy of thyroid tissue in acquired forms.

Cutaneous changes

  • Superficial and follicular orthokeratotic hyperkeratosis.
  • Telogenization of hair follicles.
  • Myxedema.

Thyroid gland

  • Hypoplastic thyroid tissue.
  • Lymphocytic infiltration (lymphocytic thyroiditis).
  • Interstitial fibrosis and vacuolated colloid.

Differential diagnosis

Congenital form

Adult onset form

Iatrogenic form

  • Temporary hypothyroidism; spontaneous recovery of thyroid function can be seen in many cases over 1-6 months following radioactive iodine treatment or surgical thyroidectomy.

Treatment

Initial symptomatic treatment

  • Thyroxine Levothyroxine supplementation (10-20 microg/kg/day) initially - titrated to effect.
  • Iatrogenic: reduce dose of carbimazole or methimazole if azotemic or if combination of low TT4 and raised TSH has been demonstrated, even in the absence of overt signs.

Monitoring

  • Clinical signs for response to treatment.
  • Thyroxine assay Thyroxine assay to alter dosage of thyroxine if necessary.
  • cTSH does not always decrease as would be expected.
  • Monitor for resolution of azotemia particulary in iatrogenic form.

Prevention

Outcomes

Prognosis

  • Excellent for control once supplementation started in adult cats.
  • Guarded for congenital forms (most kittens with TSH resistance are dead by 18 weeks of age).
  • Some kittens with dyshormongenesis develop goiter and signs become less noticeable with age.

Expected response to treatment

  • Improving demeanor (usually within a week of supplementation starting).
  • Weight loss in adult cats.
  • Improving coat condition (may take several months to manifest).

Reasons for treatment failure

  • Incorrect diagnosis.
  • Inadequate supplementation or poor monitoring of treatment.
  • Some congenital cases fail to respond adequately.

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.