Lecture 12 3/27/25 Flashcards
What are the characteristics of canine hypothyroidism prevalence?
-common, but still only seen in less than 1% of dogs
-commonly diagnosed and misdiagnosed
What is tertiary hypothyroidism?
dysfunction of the hypothalamus leading to TRH deficiency
What is secondary hypothyroidism?
failure of the pituitary gland leading to decreased TSH
What is primary hypothyroidism?
dysfunction of the thyroid gland itself
Which type of hypothyroidism is most common?
primary hypothyroidism
What are the characteristics of congenital hypothyroidism?
-rare
-typically causes early death and may go undiagnosed
-defect can be anywhere in hypothalamus-pituitary-thyroid axis
What are some examples of congenital hypothyroidism in which the animals survive?
-pituitary dwarfs
-congenital secondary hypothyroidism in giant schnauzers
-congenital primary hypothyroidism with goiter in terrier breeds
What are the possible causes of primary hypothyroidism?
-lymphocytic thyroiditis
-thyroid atrophy
-neoplasia
-congenital
-drugs
-surgery
Which causes of primary hypothyroidism account for 95% of cases?
lymphocytic thyroiditis and thyroid atrophy
What are the characteristics of lymphocytic thyroiditis?
-diffuse cellular infiltration
-immune-mediated
-clinical signs occur with > 80% destruction
-slowly progressive over 1 to 3 years
-suspected that genetics, environment, and/or infection can be initiative
What are the characteristics of thyroid atrophy?
-thyroid parenchyma is replaced by adipose tissue with no inflammatory cells
-idiopathic
-suspected to be the end-stage of lymphocytic thyroiditis
What are the characteristics of neoplastic destruction of the thyroid?
-infiltrative tumor arises from thyroid gland or via metastasis
-most thyroid tumors are unilateral
-hypothyroidism secondary to thyroid destruction occurs in less than 10% of thyroid tumor cases
Which drug types can cause animals to develop clinical hypothyroidism disease?
-anti-thyroid drugs
-sulfa drugs
What is the approach to treating hypothyroidism caused by drugs?
-withdrawal of the offending drug
-supplementation with thyroid hormones as needed
What are the characteristics of iodine involvement in hypothyroidism development?
-iodine deficiency can cause hypothyroidism, but is rare
-iodine excess can inhibit iodine uptake and thyroid hormone secretion
What are the iatrogenic causes of hypothyroidism?
-surgery, especially thyroidectomy
-radioiodine therapy
What is the typical signalment for hypothyroidism dogs?
-typically middle-age, but can be any age
-varied age of onset based on breed
-no recognized gender predilection
-more common in neutered animals
-mid to large breed dogs
Which dog breeds have a predisposition for hypothyroidism?
-golden retrievers
-doberman pinschers
-labrador retrievers
-cocker spaniels
What are the characteristics of hypothyroidism clinical presentation?
-slow, progressive onset of signs
-signs not evident until > 80% of thyroid function is lost
-subtlety of signs varies with age, breed, and type of coat
What percent of hypothyroidism dogs present with dermatologic signs?
around 85%
What are the dermatologic signs of hypothyroidism?
-bilateral truncal alopecia
-rat tail
-loss of guard hairs/”puppy” coat
-seborrhea
-chronic otitis
-hyperpigmentation
-secondary pyoderma
What are the characteristics of myxedema?
-severe hypothyroidism
-excess mucopolysaccharides and hyaluronic acid accumulate in dermis and bind water
-results in “tragic facial expression”
What are the clinical signs of myxedema coma?
-obtunded mental state
-hypothermia
-hypotension
-bradycardia
-hypoventilation
What are the metabolic signs of hypothyroidism?
-decline in metabolic rate
-lethargy
-overweight/obese
-weakness
-cold intolerance
-cardiovascular signs/bradycardia
-NO PU/PD
What are the neurological abnormalities that can occur in hypothyroidism?
-polyneuropathy
-focal neuropathy
-cervical spondylomyelopathy
-megaesophagus
-CNS signs
What are the cardiovascular abnormalities that can occur in hypothyroidism?
-sinus bradycardia
-decreased fractional shortening/contractility changes
What are the reproductive abnormalities that can occur in hypothyroidism?
female:
-infertility
-shortened estrus
-prolonged estrual bleeding
-prolonged anestrus
-galactorrhea
male:
-infertility
-testicular atrophy
What are the ocular abnormalities that can occur in hypothyroidism?
-corneal lipid deposits
-KCS
-anterior uveitis
-lipemic aqueous humor
What hematology abnormality is seen in 35% of hypothyroidism dogs?
mild to moderate normocytic, normochromic, non-regenerative anemia
What serum chemistry abnormalities can be seen in hypothyroidism dogs?
-mildly increased liver enzymes (ALT, AST, ALP)
-mildly increased creatinine kinase
-fasting hypercholesterolemia
-fasting hypertriglyceridemia
What does the hyperlipidemia associated with hypothyroidism put dogs at risk for?
-atherosclerosis
-increased ALP
-proteinuria
-pancreatitis
-gallbladder sludge
-gallbladder mucocele
What must be considered in the diagnostic approach to hypothyroidism patients?
-patients should have clinical signs and/or CBC/chem changes to consider testing
-must have something to monitor
-should only pursue testing once they are free of other illnesses
-animals empirically treated for hypothyroidism with thyroxine should be off supplementation for at least 1 week prior to testing
What are the characteristics of T4?
-main hormone secreted
-normal range of 1.0 to 4.0
What are the characteristics of T3?
-most active
-T4 is converted to T3 intracellularly
What are the characteristics of total T3 measurement?
-very little T3 is secreted by thyroid gland
-cannot distinguish normal vs. hypothyroid vs. euthyroid sick based on T3 measurement
-minimal value in measuring
What are the characteristics of free T3 measurement?
-diagnostic value is unknown
-may be low in very advanced hypothyroidism
what are the characteristics of total T4 measurement?
-measures total amount of circulating thyroxine
-tends to be lower with increasing age
-greater than 90-95% of hypothyroid dogs have low T4
-can be affected by T4 autoantibodies or illness
What are the characteristics of euthyroid sick syndrome?
-occurs with nonthyroidal illness
-total T4 is low despite patient NOT having hypothyroidism
-T4 improves once non-thyroidal illness resolves
-therapy not required
-should retest after non-thyroidal illness resolves
Which breeds can have a T4 measurement below the reference interval in health?
-greyhound
-sloughi
-scottish deerhound
-alaskan sled dog
-basenji
Which dog breeds have their own breed-specific T4 reference intervals?
-alaskan malamute
-collie
-golden retriever
-keeshond
-samoyed
-siberian husky
-greyhound
-sloughi
Which drug classes can cause a low total T4 measurement?
-glucocorticoids
-phenobarbital
-sulfa drugs** (causes clinical signs)
-tricyclic antidepressants
When should a total T4 be run?
-otherwise healthy patient with hypothyroidism signs
-ruling out hypothyroidism
-alone or in combo with TSH to monitor thyroid supplementation
What are the characteristics of free T4?
-measures non-protein bound thyroxine fraction
-measured via equilibrium dialysis
-single best test; best combo of sensitivity, specificity, and accuracy
-can be low with drugs, illness, and/or breed
What are the characteristics of TSH measurement?
-expected to be increased in dogs with hypothyroidism
-in reality, some hypothyroid dogs have normal TSH and some normal dogs have high TSH
-high TSH with concurrently low TT4 or fT4 is very supportive of primary hypothyroidism dx
What are the characteristics of thyroglobulin autoantibodies?
-suggest presence of autoimmune thyroiditis
-do not predict current thyroid status
-could be a marker for future hypothyroidism
-may be affected by recent vx
Which combination of tests provides the best sensitivity for hypothyroidism?
TT4 + fT4 + TSH
Which combination of tests provides the best specificity for hypothyroidism?
fT4 + TSH
Which combination of tests is best for determining if drugs or non-thyroidal illness are causing hypothyroidism?
fT4 or TT4 + TSH + autoantibody testing
What are the characteristics of hypothyroidism testing?
-having at least 2 tests that support a hypothyroid diagnosis provides > 98% of correct diagnosis
-tests should be used to confirm clinical suspicion
What are other possible diagnostic tests for hypothyroidism?
-TSH stim.
-TRH stim.
-scintigraphy
-ultrasound
-trial therapy
What are the characteristics of hypothyroidism treatment?
-supplementation with sodium levothyroxine/L-thyroxine
-illegal to use non-FDA approved products; stikc with Thyro-Tabs or ThyroKare
-good bioavailability
-treatment is lifelong
What are the steps to take in patients with reduced dose considerations for thyroid supplementation (i.e. heart dz, diabetes, cushing’s)?
-start with 50% dose to avoid rapid changes in metabolic rate
-slowly increase dose to tolerable levels
-in the event of myxedema coma, give IV levothyroxine
What is the prognosis for hypothyroidism?
-excellent for acquired hypothyroidism
-guarded for myxedema coma
Which clinical signs should be monitored for improvement in hypothyroid dogs?
-mentation and activity: improvement within days
-anemia: improvement within weeks
-neuro/derm/cardio signs: improvement within months
How should clinical reassessment of hypothyroid patients occur?
-reassess clinical signs
-measure T4 and TSH 4 to 8 weeks after beginning treatment
-goal is to achieve normal T4 and TSH readings with control of clinical signs
What can cause hypothyroidism treatment to fail?
-poor owner compliance
-incorrect dose
-generic thyroid drugs
-decreased bioavailability
-poor absorption from GI tract (concurrent GI dz)
-misdiagnosis
What are the characteristics of feline hypothyroidism?
-very rare
-typically iatrogenic; following hyperthyroidism treatment
-syndrome resolves with time
What are the clinical signs of feline hypothyroidism?
-lethargy
-obesity
-derm. signs
-cardiovascular signs
-dwarfism
-azotemia
How is feline hypothyroidism diagnosed?
TT4 or fT4 measurement
When is levothyroxine treatment indicated in cats?
-clinical signs present
-azotemia
-hypothyroidism does not resolve within 3 to 6 months
What is the prognosis for feline hypothyroidism?
-good prognosis for primary hypothyroidism with appropriate therapy
-guarded prognosis for kittens with congenital hypothyroidism due to skeletal and neurological abnormalities
