canine hypothyroidism

Question 1

etiology for hypothyroidism

Answer 1

• > 95% have primary hypothyroidism
  
  • Lymphoplasmacytic thyroiditis
  • Idiopathic thyroid atrophy
  • Associated with high TSH
  • Immune-mediated pathogenesis supported by antibodies against thyroglobulin, T4, T3
• 75% of gland must be destroyed for clinical signs
• Takes 1-3 years for thyroid to be destroyed
• LP thyroiditis inherited in Beagle, Borzoi

Question 2

congenital hypothyroidism

Answer 2

• Rarecause of primary hypothyroidism
• Failure of thyroid gland to develop: “thyroid dysgenesis”

Question 3

Thyroid neoplasia

Answer 3

• NOTa common cause of primary hypothyroidism
• 60% of dogs with thyroid neoplasia are euthyroid
• 30% have low serum T4
• ≤ 10% are hyperthyroid (as seen in cats)

Question 4

most common cause of hypothyroidism

Answer 4

• < 5% have secondary hypothyroidism
• Serum TSH low or undetectable (not helpful though)
• Primary pituitary disease (e.g. pituitary dwarfism)
• Suppression of TSH secretion by exogenous or endogenous glucocorticoids (most common cause of secondary hypothyroidism)
• Congenital TSH deficiency in Giant Schnauzers (autosomal recessive)

Question 5

what is the main cause of canine hypothyroidism

Answer 5

• Bottom line
  
  • > 95% of the hypothyroid dogs you encounter will have primary disease due to lymphoplasmacytic thyroiditis or idiopathic atrophy

Question 6

signalment for hypothyroidism

Answer 6

• Middle-aged to older dogs (usually 2-6 years of age)
• No sex predilection
• Any breed (including mixed breeds) but especially:
  
  • Golden retriever, Doberman pinscher, Labrador retriever, Cocker spaniel
• Less common in German shepherds and mixed breeds
• Rare in toy breeds (congenital in toy fox terriers)

Question 7

clinical findings for hypothyroidism

Answer 7

• Lethargy, inactivity, mental dullness (35%)
• Weight gain despite no changes in food intake (48%)
• Cold intolerance
• Hyperkeratosis (dry, scaling skin) (33%)
• Seborrhea (16%)
• Cutaneous signs verycommon
• Classic endocrine alopecia (25%)
  
  • “Non-pruritic bilaterally symmetric truncal alopecia sparing head and extremities”
• Thin hair coat (25%)
• “Rat” tail (12%)
• Loss of hair at sites of excessive wear (11%)
• Hyperpigmentation (15%)
• Otitis externa (13%)
• Superficial bacterial skin infections (12%)
  
  • Folliculitis, pyoderma
• Reproductive signs
  
  • Failure to cycle in female dogs (4%)
  • Possible relationship to infertility in female dogs
• Tragic facial expression due to myxedema (8%)

Question 8

neuromuscular signs in hypothyroid dogs

Answer 8

• Weakness (12%)
• Seizures (4%)
• Facial nerve paralysis (4%)
• Vestibular signs (3%)
• Dysphagia/megaesophagus (3%)
• Laryngeal paralysis (3%)

Question 9

this test is a good screening test for hypothyroidism

Answer 9

• serum total t4
• Useful screening test to rule OUT hypothyroidism
  Non-thyroidal illness and several drugs can cause low total T4

Question 10

• Single most accurate test of thyroid function

(If you had to pick just one!)

Answer 10

Equilibrium dialysis (method of choice)
Very high sensitivity (98%) and specificity (93%)
Not affected by circulating anti-thyroid antibodies
Less often affected by non-thyroidal illness

Question 11

disadvantages of serum TSH in dx hypothyroidism

Answer 11

• Normal: 0-0.6 ng/ml
• Can’t distinguish low from normal
• Used alone, has low specificity and low sensitivity
• Should be used together with total T4 or free T4 by equilibrium dialysis
• Low total or free T4 with high TSH supports primary hypothyroidism

Question 12

advantages of thyroid pertechnetate in dx hypothyroidism

Answer 12

• Thyroid pertechnetate (99mTcO4) scintigraphy
• Clearly differentiates primary hypothyroidism (little or no 99mTcO4 uptake) from non-thyroidal illness (normal 99mTcO4 uptake)

Question 13

disad. of thyroidThyroid Scintigraphy

Answer 13

nt practical

Question 14

disadv. of US in dx hypothyroidism

Answer 14

• Is nota “function” test
• Notcommonly used

Question 15

non thyroidal causes of hypothyroidism

Answer 15

• Decreased serum thyroid hormone concentrations in euthyroid dogs in response to concurrent non-thyroidal illness
• Caused by decreased TSH secretion, decreased T4 synthesis, decreased thyroid-binding globulin, inhibition of 5’-deiodination of T4 to T3
• Body’s attempt to decrease cellular metabolism during illness

Question 16

tx for hypothyroidism

Answer 16

• Levothyroxine (T4)
• Use only products known to be effective in dogs
  
  • Dnt use human products
• GI absorption of T4 is low in dogs compared to humans
• Considerable dog-to-dog variability in GI absorption

Question 17

resons for failure of hypothyroid tx

Answer 17

• Consider treatment failure if no response after 8 weeks
• Most common reason for “failure to respond”
• Inaccurate diagnosis (low total T4 was due to non-thyroidal illness or drugs)

Question 18

common causes of tx failure in hypothyroidism

Answer 18

• Poor owner compliance (make sure dog is really getting the medication)
• Outdated or ineffective medication
• Inappropriate dose or frequency of administration
• Poor GI absorption
• Rapid metabolism and excretion (e.g. concurrent phenobarbital administration)

Question 19

DISCUSS USE OF T3 IN THE TX OF HYPOTHYROIDISM

Answer 19

• There is NO known indication for the use of T3 (liothyronine) in the treatment of hypothyroidism
• T4 to T3 conversion defects have not been shown to exist

Question 20

COMPLICATIONS OF HYPOTHYROIDISM

Answer 20

• Myxedema Coma
  
  • Profound weakness, decreased consciousness
  • Hypothermia, edema, bradycardia, hypotension, hypoventilation
  • Typical lab findings of hypothyroidism plus hypoxemia, hypercarbia, hyponatremia, hypoglycemia
  • Low to undetectable serum T4
  • Rx: IV T4, 5 μg/kg q12h

Question 21

RESPONSE TO TX FOR HYPOTHYROIDISM

Answer 21

Increased mental alertness with 1-2 weeks
Decrease in body weight toward normal beginning after 2 months
Hair re-growth beginning after 1 month but may take several months

Question 22

PX FOR HYPOTHYROIDISM

Answer 22

• Requires life-long treatment but prognosis is excellent
• With appropriate treatment, hypothyroid dogs should have normal life expectancy