Hypothyroidism

By | November 16, 2015
  • the commonest endocrine disorder of the dog
  • the acquired naturally occurring condition has not been documented in the cat although the extremely rare congenital form has been described
  • the metabolically active thyroid hormones are L-thyroxine (T4) and L-3,5,3-triiodothyronine (T3)

Aetiology

Primary

  • congenital agenesis (rare)
  • non-functional thyroid tumour (rare)
  • lymphocytic thyroiditis; the most important cause (approximately 90%), an autoimmune disorder which results in thyroid destruction
  • idiopathic thyroid necrosis and atrophy may represent the end stage of lymphocytic thyroiditis

Secondary

  • Less common than primary1 causes (less than 5%)
  • TSH deficiency leading to inadequate stimulation of the thyroid gland with subsequent reduction in the production of thyroid hormone
  • congenital hypopituitarism (usually in association with GH deficiency)
  • pituitary neoplasia (may present with other signs, e.g. diabetes insipidus)

Tertiary

  • thyrotropin-releasing hormone (TRH) deficiency results in deficiency of TSH and reduction in thyroid hormone production
  • due to hypothalamic lesions — extremely rare
  • other equally rare causes of hypothyroidism include: iodine deficiency
  • defects in thyroid hormone synthesis (dyshormonogenesis)
  • antithyroid hormone antibodies
  • abnormality in plasma iodine binding or T4 to T3 conversion

Clinical features

History

  • Careful history-taking is particularly important in suspect hypothyroidism cases. The disease has been called ‘the great impersonator’. There may be:
  • a gradual onset in lethargy and depression
  • thermophilia — heat-seeking and unwillingness to go out in cold weather
  • gradual onset of poor exercise tolerance
  • obesity — this is variable, some dogs may be of normal weight or thin
  • slow regrowth of hair after clipping

Physical signs

These are extremely variable and any one or more of the following may be present:

Cutaneous

  • cool skin
  • bilaterally symmetrical alopecia
  • hyperpigmentation, thickening of the skin and scaling seborrhoea
  • secondary pyoderma (probably associated with depressed immune system function)
  • pruritus is absent unless there is a secondary pyoderma or seborrhoea
  • dull coat, hairs easily epilated
  • puffy skin (myxoedema)
  • easy bruising
  • hypertrichosis
  • comedones

Gastrointestinal

  • occasional vomiting, diarrhoea or constipation
  • none of these are common

Cardiovascular

  • there may be bradycardia
  • weak apex beat, low voltage on ECG recordings
  • cardiac arrhythmias

Reproductive

  • irregular oestrus cycles or anoestrus
  • decreased libido
  • testicular atrophy
  • gynaecomastia and galactorrhoea (rarely)

Ocular

  • blepharoptosis
  • corneal lipidosis
  • keratoconjunctivitis sicca and corneal ulceration
  • retinopathy

Neuromuscular

  • facial and laryngeal paralysis
  • myopathy, e.g. of the temporal and masseter muscles

Diagnosis

  • history, physical examination, laboratory investigation

Laboratory findings

Non-specific

  • normocytic, normochromic, non-regenerative anaemia is seen in approximately 25% of cases
  • hypercholesterolaemia occurs in approximately 33% of cases (sample the fasting dog only)
  • serum creatinine phosphokinase (CPK) is often elevated
  • skin biopsy: may see hyperkeratosis, epidermal atrophy, melanosis, follicular atrophy and increased dermal thickening. These signs are consistent with an endocrinopathy and not specific for hypothyroidism

Specific

circulating thyroid hormone concentrations. Total T4 (TT4) is measured by radioimmunoassay. The normal TT4 range is 13—52 nmol/l

extremely low levels of TT4 (< 7 nmol/l) together with compatible clinical signs support the diagnosis of hypothyroidism

There is considerable overlap between TT4 levels in some normal dogs and in dogs with hyperthyroidism. Some drugs, such as phenobarbitone, diphenylhydrantoin and glucocorticoids, depress thyroid hormone levels. Chronic systemic diseases also depress thyroid hormone levels (‘euthyroid sick syndrome’). To overcome these problems further diagnostic tests are required

TSH response test

blood is collected into a plain tube

5 IU of bovine TSH are given intravenously

a second blood sample is collected 4 hours later

allow to clot, centrifuge and separate serum

send both samples to a laboratory accustomed to canine testing and request measurement of TT4

Interpretation: Normal dogs will at least double the basal TT4 and fall within or exceed the normal post-TSH TT4 range for the laboratory. Hypothyroid dogs may show an increase in TT4 levels but they will not reach the normal post-TSH TT4 level.

Further refinements to both the non-dynamic and dynamic function tests have been suggested recently (Larsson 1988). Using the equation:

k = 0.7 x free thyroxine (FT4) concentration (nmol/l) -cholesterol concentration (mmol/1)

hypothyroid dogs are diagnosed by having a k value less than -4, whereas euthyroid dogs have a k value of greater than +1. The TSH test accuracy was improved by using the equation:

k = 0.7 x basal TT4 concentration (nmol/l) + increase in TT4 concentration after TSH (nmol/l)

Hypothyroid dogs have a k value of less than 15, whereas euthyroid dogs have a k value of greater than 30.

thyroid biopsy. Not commonly employed in practice, but will distinguish between euthyroidism, primary and secondary hypothyroidism, lymphocytic thyroiditis and idiopathic necrosis and atrophy

Treatment

  • the drug of choice is thyroxine sodium (Eltroxin, Glaxo) at a dose of 10-20 ug/kg b.i.d. Start therapy at the lower dose and only proceed to the higher dose if the dog fails to respond after a few months. Caution should be exercised with dogs suffering from cardiovascular disease by introducing the drug gradually
  • overdosage is difficult to achieve, and side-effects are therefore rare; if encountered they are principally panting, anxiety, restlessness, tachycardia, polyphagia, polyuria and diarrhoea
  • the half-life of T4 is 24 hours and peak plasma levels occur 4-12 hours after administration
  • most authorities suggest the use of T3 in those cases that fail to improve on T4. However, Thoday () states that he has never seen a dog with hypothyroidism respond to T3 having failed to respond to T4 and the author agrees with this statement
  • it is important to ensure that an adequate dose is given and for a sufficient length of time. If lethargy was a notable symptom, this is often the first sign to disappear, dogs regaining vitality within a few weeks. Other signs, particularly alopecia, may take months to improve. It is recommended that a minimum of three months of treatment is given prior to making clinical assessments. Treatment, once established as beneficial to the dog, is for life

 

Selections from the book: “Skin Diseases in the Dog and Cat”. D. I. Grant, BVetMed (1991)