Canine Hypothryoidism Flashcards
What is the most common etiology of canine hypothyroidism?
primary hypothyroidism caused by autoimmune lymphocytic thyroiditis* or idiopathic atrophy
(secondary/tertiary = pituitary dwarfism with decreased GH and TSH)
What is the most common signalment associated with canine hypothyroidism?
middle-aged Golden Retrievers, Dobermans, Boxers, Shelties, etc.
What are 4 signs of hypothyroidism?
decreased metabolic rate
- lethargy
- weakness (myopathy)
- obesity
- exercise intolerance
What are the 3 most common dermatologic findings associated with canine hypothyroidism?
- bilaterally symmetrical, hyperpigmented alopecia beginning in areas of friction (initially seen as excessive shedding)
- dry or oily seborrhea
- facial myxedema - tragic facial expression
Is pruritis a common finding in hypothyroidism?
no, unless there is a concurrent cutaneous infection
What 5 cardiovascular findings are seen with hypothyroidism? How do patients present?
- bradycardia
- weak pulses
- muffled heart sounds
- low-voltage ECG complexes
- reduced myocardial contractility
dogs usually do not present clinically with CV changes (but cats do!)
How is the nervous system affected by hypothyroidism?
peripheral neuropathy
- localized = facial, vestibular, front limb, larynx
- generalized
- megaesophagus
What 5 CNS abnormalities are seen with hypothyroidism?
- multifocal CNS signs
- hemiparesis
- central vestibular signs
- cerebellar hypermetria or ataxia
- multiple cranial nerve defecits - vestibular, facial, trigeminal
In what animals are reproductive abnormalities seen with hypothyroidism? What are 3 signs?
females with severe and prolonged hypothyroidism
- infertility
- increased periparturient pup mortality
- low birth weight
What is congenital hypothyroidism? What are 4 signs?
rare disorder in cats associated with a mutation in TSH receptors
- disproportionate dwarfism (open growth plates)
- mental dullness and lethargy
- broad skulls, macroglossia, delayed dental eruption
- ataxia, hypermetria
What are the 2 most common laboratory findings with hypothyroidism?
- hypercholesterolemia (must be fasted for sample!)
- mild nonregenerative anemia
What is the general flow of diagnosing hypothyroidism?
- appropriate clinical signs
- routine chemistry
- specific thyroid function tests
- response to treatment
What are the 4 thyroid function tests used to diagnose hypothyroidism?
- T4
- fT4
- TSH
- autoantibodies to thyroglobulin and thyroid hormones
What kind of test is serum total T4 for diagnosing hypothyroidism? What is required for diagnosis?
highly sensitive for hypothyroidism —> above low normal excludes hypothyroidism
low T4 AND classic signs
What commonly alters serum total T4 values?
T4 autoantibodies can cause false elevations
What is the most accurate method of measuring T4 in hypothyroid patients?
equilibrium dialysis for fT4
How does equilibrium fT4 compare to serum total T4? What disadvantages are associated?
- more sensitive and specific
- less affected by non-thyroidal factors
more expensive and less readily available
What is expected of serum TSH levels in hypothyroid patients?
decreased thyroid hormones causes an increase in TSH
What is serum TSH good for diagnosing? How does it compare to other tests? What is required for hypothyroidism diagnosis?
early hypothyroidism where TSH is elevated and T4 and fT4 are low normal
not very sensitive
elevated TSH AND decreased T4 or fT4
What breeds require special consideration in thyroid function tests?
Sighthounds - naturally low T4 and fT4
What 4 drugs affect thyroid function tests?
- corticosteroids - test accuracy
- sulfonamides - induces hypothyroidism
- phenobarbital - mimics hypothyroidism
- clomipramine - test accuracy
What is the most common affect of illness on thyroid function tests? What are 2 other possibilities? What does this result in?
decreased T4
- fT4 decreased (less frequent)
- elevated TSH
MIMICS HYPOTHYROIDISM - delay thyroid testing until illness is resolved
How is thyroglobulin used to diagnose hypothyroidism?
naturally present in colloid and contains T4 and T3 —> antibodies against it are markers for autoimmune thyroiditis that may progress to hypothyroidism
- not clinical = monitor, no treatment
What do autoantibodies to T4 or T3 indicate? What is the significance of their presence?
autoimmune thyroiditis —> NOT hypothyroidism or hyperthyroidism
interfere with T4/T3 assays, causing false elevations that bring to hypothyroid or euthyroid range
What test is not affected by T4/T3 autoantibodies?
fT4 equilibrium dialysis
What are the most sensitive tests for diagnosing hypothyroidism? What is required for diagnosis?
- T4 and fT4 = 90-98%
- TSH = less sensitive = elevated in 65-75%
low T4/fT4 AND elevated TSH for most accurate diagnosis
How is hypothyroidism treated?
Levothyroxine at an initial dose of 0.02-0.8 mg/kg QD
How should patients respond to Levothyroxine treatment?
usually rapid - should have increased activity, improved attitude, and weight loss in 1-2 weeks
(dermatologic signs require months to resolve)
What therapeutic monitoring is recommended post-pill testing with Levothyroxine? What should be done if there is poor clinical response?
recheck in 6-8 weeks, 4-6 hours after dose given that day
ensure adequacy of dosage, compliance, and diagnosis
What is indicative of over-supplementation of Levothyroxine when treating hypothyroidism? What should be done if this occurs?
SIGNS OF HYPERTHYROIDISM!
- PU/PD
- panting
- hyperactivity
- polyphagia
stop treatment for 2-3 days then re-institute with 50-75% of original dose
