Canine hypothyroidism Flashcards
Outline types of hypothyroidism?
In dogs the culprit is nearly always the thyroid gland itself. Vast majority are primary hypothyroidism.
What is used as a diagnostic test for hypothyroidism?
T4 is used as diagnostic test.
What is the pathophysiology of primary hypothyroidism?
1ry (primary) hypothyroidism: 95% cases pathology in thyroid gland
Adult onset:
lymphocytic thyroiditis
- assumed to be an immune mediated disease
- infiltration of thyroid gland by lymphocytes, plasma cells and macrophages
- end stage-> replacement by fibrous tissue
idiopathic thyroid atrophy (less common)
- minimal inflammation or fibrosis
- end stage-> replacement by adipose tissue (fatty replacement of normal glandular tissue)
- Very rare to need to take a thyroid biopsy unless we are thinking a neoplasia is present
Thyroid carcinoma (rare in cats and dogs)
- hypothyroidism if >75% of the gland is destroyed
- very rarely- tumour cells can produce T4 causing hyperthyroidism
Iatrogenic (rare in dogs, can occur in cats bilateral thyroidectomy or I131 but usually transient)
What is congenital primary hypothyroidism?
Very uncommon in dogs
Caused by:
- dietary deficiency of iodine
- dyshormoneogenesis (eg an iodine organification defect)
- thyroid aplasia, hypoplasia or dysgenesis
Describe the pathophysiology of secondary hypothyroidism?
2ry hypothyroidism (↓TSH)
Disease at the level of the pituitary
- pituitary hypoplasia or cyst- RARE
- autosomal recessive inherited disorder in GSDs
- combined deficiency of GH, TSH and prolactin
- pituitary thyrotrophic cells fail-RELEVANT
- ↓ TSH results in thyroid atrophy
- suppression by glucocorticoids
- “euthyroid sick syndrome” a concurrent disease that supresses T4 production probably some mechanism effecting pituitary gland and effecting TSH.
- destruction of pituitary-RARE
- neoplasia
What is the pathophysiology of tertiary (3rd) hypothyroidism?
3ry hypothyroidism (↓TRH)
Disease at the level of the hypothalamus
- Reported in people
- Only 1 case report in a dog which had a pituitary mass and pituitary dependent hyperadrenocorticism. This had expanded to involve the hypothalamus.
- TRH stimulation tests were ultimately diagnostic
** Dogs with hyperadrenocorticism are MUCH more likely to have suppressed T4 due to excess glucocorticoid
How does the nose of a hypothyroid dog appear?
Dark shiny nose dogs/alopecia
What is the signalment of a dog with primary canine hypothyroidism?
- Middle aged to older dogs
- mean age 7 yrs
- Males and females, neutered and entire (not gender or neutered status specific)
- Breed predisposition
- beware of false impression that breeds that are just popular actually have more disease
- familial tendency
- may have earlier onset of signs in some breeds
Slowly progressive, gradual onset disease (similar to cushings becomes more obvious as time goes on but in early stages easy to miss the disease)
Draw a table of increased breed disposition likelihood of hypothyroidism?
What are the clinical signs of hypothyroidism?
Signs due to ↓ metabolic rate
- lethargy or weakness
- reported in 76% of cases (owners often attribute to age)
- mental dullness
- exercise intolerance
- reported in 24% of cases
- weight gain or obesity
- reported in 44% of cases
- cold intolerance/heat seeking behaviour
- Bradycardia (not a dramatic bradycardia)
What are the dermatological signs of hypothyroidism?
- alopecia or failure to regrow hair after clipping
- seborrhoea
- pyoderma
- ceruminous otitis
- 2ry Malassezia +/or demodex can be seen
- hyperkeratosis
- hyperpigmentation
- comedones
- bruising?
- poor wound healing?
What causes the alopecia seen in hypothyroid dogs?
Alopecia or failure to regrow hair after clipping (become worse over time):
- failure to initiate anagen phase of hair growth
- bilaterally symmetrical (trunk)
- areas of wear and tear (trunk, ventral abdomen & tail, dorsal nose)
- head & extremities often spared
What cause the pyoderma sometimes seen in hypothyroidism?
pyoderma: 16% of dogs will have secondary pyoderma because lack of thyroid hormones effects t cell function in the skin and the normal commensals can colonise. So pruritis in these dogs can be confusing:
- ↓ thyroid hormones suppress immune response, impair T cell function and reduce circulating lymphocytes
- focal, multifocal or generalised
- can cause marked pruritus ie confusing
Generally we assume endocrine skin disease is a non pruritic cause of alopecia but if secondary pyoderma occurs then pruritus can be a key feature.
How do skin changes in hypothyroidism differ from hyperadrenocorticism?
Hyperkeratosis that might be more of a hallmark for hypothyroidism compared to cushing dogs which have thin skin.
Skin changes may relate to thyroid hormone deficiency having a negative impact on protein synthesis, mitotic activity and oxygen consumption. This results in epidermal atrophy, sebaceous gland atrophy and keratinisation defects.
What coat changes may be seen with hypothyroidism?
- dry brittle hair
- easily epilated hair
- dull and/or faded colour
- loss of undercoat -> coarse coat
- loss of 1ry guard hairs -> “puppy like” coat
- hair retention occurs in some dogs rather than become alopaecic
- appearance can be variable
What is the pathophysiology with the ‘tragic facial expression’ seen with severe hypothyroidism?
Mucopolysacharide (hyaluronic acid) deposits in the dermis bind water (called myxodema) causing:
- thickened and puffy skin
- Over the face and forehead
- facial skin folds appear
- upper eyelids droop
- rare cases develop myxoedema coma
What is the rare event pictured assocaited with hypothyroidism?
Aortic thrombus seen: rare manifestation of hypothyroidism
rare event: atherosclerosis associated with hypercholesterolaemia (cannot metabolise fats very well)
Discuss the clinical index of suspicion?
What is congenital hypothyroidism and what are the signs?
- Mental retardation and stunted growth key features
- Disproportionate body size – large, broad heads, protruding tongues, square trunks and short limbs
- Dull and lethargic
- Persistence of puppy haircoat, alopecia
- Delayed dental eruption
- Can be primary or secondary
Draw a cartoon of a typical hypothyroid dog?
