Pathophysiology of the Thyroid Gland Flashcards
what is the primary effect of thyroid hormone?
increase metabolic rate and promote fetal growth and development
what does thyroid hormone do with erythropoiesis?
increases it
increases erythropoietin secretion
facilitates response to erythropoietin
what is the thyroglossal duct?
tract that thyroid tissues migrates down from the floor of the pharynx
failure of the thyroglossal duct to involute leads to _________________________
a thyroglossal cyst
who is congenital hypothyroidism especially common in?
small ruminants
what is the most common thyroid disease in horses and small ruminants?
congenital hypothyroidism
what type of goiter does congenital hypothyroidism cause?
diffuse hyperplastic goiter
nonneoplastic
noninflammatory
______________________ of a pregnant animal can cause congenital goiter
iodine deficiency
can iodine toxicity cause congenital hypothyroidism?
yes
what do high nitrate levels cause with the thyroid?
nonfunctional hypertrophy of the thyroid: inhibits ability to utilize available iodide
how might a pregnant mare get nitrate toxicity?
consumes plants or contaminated water
common grasses accumulate nitrate
nitrates accumulate with _________________________________
fertilization
drought conditions
some herbicides
are goitrogenic substances more likely to affect the fetus or the mother?
fetus
what does familial dyshormonogenetic goiter cause?
enzyme defect in thyroid hormone synthesis: impaired biosynthesis of thyroglobulin
what are the possible findings with congenital hypothyroidism?
rough haircoat, alopecia
hypothermia
weakness
stillbirths
may die within a few days of birth
true/false: congenital hypothyroidism can also present with CNS lesions
true
poor suckling, incoordination, poor righting reflexes
___________ is the most common thyroid disease of birds
goiter
what species of birds are goiters seen in predominantly?
pigeons
canaries
cockatiels
budgerigars (budgies)
what are some causes of goiter in birds?
iodine-deficient seed diet
goitrogenic substances
acute septicemia
exposure to organophosphates or chlorinated bisphenols
what are the clinical signs of goiter in birds often from?
mass effect: pressure, fluid buildup
hormone deficiency
what clinical signs are associated with hormone deficiency from goiter in birds?
immunodeficiency
depression and lethargy
abnormal skin and feathers
reproductive problems
what are the most common causes of canine hypothyroidism?
lymphocytic thyroiditis
idiopathic thyroid atrophy
what happens in lymphocytic thyroiditis?
slowly progressive destruction and fibrosis
what happens in idiopathic thyroid atrophy?
thyroid replaces with adipose tissue
degenerate follicular cells in colloid
no inflammation, antibody tests negative
neoplastic destruction is an ____________ cause of canine hypothyroidism
uncommon
what are some causes of neoplastic destruction of the thyroid gland that leads to canine hypothyroidism?
thyroid carcinoma
metastatic disease
is secondary hypothyroidism due to pituitary disease common?
no- uncommon
german shepherds with cystic craniobuccal (Rathke’s) pouch are deficient in ___________________ and ___________________
thyroid hormone
growth hormone
what is pituitary thyrotroph suppression a common cause of?
secondary hypothyroidism
what are some drugs that cause secondary hypothyroidism?
phenobarbital
glucocorticoids and NSAIDs
sulfonamides
thyroid supplements
what is the most common cause of feline hypothyroidism?
iatrogenic hypothyroidism
how can we cause iatrogenic hypothyroidism in cats?
radioactive iodine treatment
antithyroid drugs
surgical thyroidectomy
why is it difficult to produce hypothyroidism in dogs?
accessory tissue
what are the clinical signs of canine hypothyroidism?
decreased metabolic rate
dermatologic manifestations
other less common signs
why do hair coat manifestations occur in canine hypothyroidism?
hair follicles enter the telogen phase prematurely
what clinical signs of hypothyroidism are associated with decreased metabolic rate?
lethargy and mental dullness
exercise intolerance/unwillingness to exercise
tendency to gain weight with no change in food intake
obesity seen in 40-50% of cases
what are the laboratory abnormalities associated with canine hypothyroidism?
hypercholesterolemia
hypertriglyceridemia less common
nonregenerative anemia
what is myxedema?
dermal accumulation of glycosaminoglycans and hyaluronic acid
accumulates water and thickens skin: tragic facial expression
what are some less common signs of canine hypothyroidism?
neuromuscular
reproductive
cardiovascular
what are some cardiovascular signs of canine hypothyroidism?
abnormal rhythms or bradycardia
occasional atherosclerosis with hyperlipidemia
what are the available tests for canine hypothyroidism?
total T4 and T3
free T4
endogenous TSH
function tests
autoantibody assays
who can you perform an endogenous TSH test on?
dogs and cats only
what are some things that affect thyroid hormones?
age, breed, reproductive status
non-thyroid illness
changes in hormone-protein binding can affect _______________
total T4
how can changes in hormone-protein binding affect total T4?
negative feedback on TSH from increased fT4 will result in lower total T4
fT4 is more rapidly cleared
true/false: some drugs can cause displacement of T4 from proteins
true: cortisol
how sensitive is free T4 by equilibrium dialysis?
sensitive: 80-98%
decreased free T4 is strongly suggestive of primary hypothyroidism but _______________________________
not a definitive diagnosis
what will the value of a free T4 direct assay be increased by?
anti-T4 antibody
what does increased TSH suggest with endogenous canine TSH?
hypothyroidism
why is endogenous canine TSH not a good screening test for primary hypothyroidism?
20-40% of hypothyroid dogs have normal TSH
true/false: autoantibodies may provide prognostic information but are not diagnostic for hypothyroidism
true
how common is adult primary hypothyroidism in equines?
rare
what can secondary hypothyroidism in equines be due to?
pituitary pars intermedia dysfunction
drugs
nonthyroidal illness
food deprivation
zinc, copper, iodine, endophyte-infected fescue
how common is feline hyperthyroidism?
very common in older cats
what is the etiology of feline hyperthyroidism?
multinodular hyperplasia or benign follicular adenomas
true/false: if you can feel a thyroid gland in a cat (thyroid slip), the cat is hyperthyroid
false: not all cats with nodules are hyperthyroid
what are the clinical signs of feline hyperthyroidism?
weight loss
polyphagia
hyperactivity
change in behavior
coat changes
vomiting
pu/pd
cardiac disturbances
what are some cardiac abnormalities associated with feline hyperthyroidism?
increased heart rate and contractility
left ventricular concentric hypertrophy
what are some laboratory abnormalities associated with feline hyperthyroidism?
erythrocytosis +/- macrocytosis
stress leukogram
elevated liver enzymes
azotemia
hyperphosphatemia
how many cats with feline hyperthyroidism have azotemia?
25%
can you use total T3 to diagnose feline hyperthyroidism?
not reliable
what is the single best test to diagnose feline hyperthyroidism?
total T4
why would you perform a T3 suppression test?
measure T3 to asses compliance or occult disease
if you have a cat that matches feline hyperthyroidism but the TT4 is low or normal, what should you do?
repeat TT4 in a week or two
fT4
T3 suppression test
thyroid scintigraphy
in thyroid neoplasma, nonfunctional adenomas (benign) are common in ____________________________
older cats, horses, and ponies
what type of thyroid neoplasm are dogs more likely to get?
nonfunctional thyroid carcinomas
what does calcitonin from C cells (parafollicular cells) do?
acts to decrease calcium concentration
what does calcitonin do to renal tubular reabsorption?
decreases it of calcium and phosphorous
thyroid C cell neoplasms are most commonly functional in ___________
bulls
what does thyroid hormone do to increase metabolic rate and promote fetal growth and development?
peripheral regulation by T3 and T4
thermogenesis: O2 consumption and ATP hydrolysis
fetal: neurological and skeletal development
neonatal growth
what is circulating T4 and T3 bound to?
thyroxine-binding globulin
transthyretin
other carrier proteins
what does thyroid hormone do with lipids?
stimulates synthesis
increases plasma fatty acids and cholesterol synthesis
stimulates degradation/excretion of lipids
cholesterol and triglycerides are _____________ related to hormone level
inversely
what does thyroid hormone do with carbohydrate metabolism?
enhances insulin action
increases gluconeogenesis and glycogenolysis
increases GI tract carbohydrate absorption
what does thyroid hormone do with the cardiovascular system?
increases heart rate, contractility, and output
vasodilation: enhances perfusion
what is a thyroglossal cyst filled with?
fluid or mucus
who is a thyroglossal cyst most commonly found in?
dogs and pigs
what is the most common thyroid disease in horses and ruminants?
congenital hypothyroidism
what happens in congenital hypothyroidism?
neonatal disease: problem in mother
decreased T3/T4 levels
increased TSH stimulates gland hyperplasia
what can cause congenital hypothyroidism?
iodine deficiency
iodine toxicity
nitrate toxicity
goitrogenic cubstances
familial dysshormonogenetic goiter
what happens in iodine toxicity that leads to congenital hypothyroidism?
decreased thyroid hormone production and secretion
inhibits iodine uptake
inhibits proteolysis of thyroglobulin
accumulation in colloid but no release into blood
what do soybeans contain that inhibits thyroperoxidase?
genistein
what do cabbages contain that inhibit iodine binding and thyroperoxidase?
isothiocyanates
who has familial dyshormonogenetic goiter been reported in?
sheep: corriedale, merino, romney
cattle: afrikander
goats: saanen dwarf
pigs
what does a goiter look like?
neck tissues thickened and edematous
bilaterally enlarged lobes
lobes firm and dark red
true/false: mild signs may improve when thyroid gland matures and produces its own hormones for congenital hypothyroidism
true
those born with low T4 levels at birth may _________________ to supplementation
those born with normal T4 levels at birth may ___________________ to supplementation
respond
not respond
what does the pressure on the heart and major vessels, crop, and esophagus cause in avian goiter?
sudden death
loss of voice, weight loss
wheezing, difficulty swallowing
can avian goiter be palpated?
may or may not
what can the clinical signs of avian goiter be due to?
mass effects
hormone deficiency
what reproduction problems are caused by avian goiter?
decreased hatchability
increased embryo mortality
what autoantibodies are increased in immune-mediated lymphocytic thyroiditis?
thyroglobulin Ab
colloid antigen Ab
thyroperoxidase Ab
what can cause lymphocytic thyroiditis?
immune-mediated
genetic link
triggered by infection?
defect in suppressor T cells?
what is advanced lymphocytic thyroiditis characterized by?
replacement of thyroid follicle with adipose tissue
is thyroid carcinoma functionally active?
not usually
who is congenital pituitary disease common in? can lead to secondary hypothyroidism
german shepherds: cystic craniobuccal pouch
giant schnauzers and boxers: TSH deficiency
what is pituitary thyrotroph suppression often due to?
nonthyroidal illness or drugs
any severe disease
hyperadrenocorticism
phenobarbital, sulfas, NSAIDs, steroids
who is myxedema seen in?
severe, longstanding cases of canine hypothyroidism
what function tests do we have for canine hypothyroidism?
TSH stimulation test
T3 suppression test
what does non-thyroidal illness cause regarding thyroid?
decreased albumin
impaired protein binding
decreased cellular uptake of fT4
increased liver metabolism
decreased TSH and/or TRH secretion
what makes total T4 a good screening test for hypothyroidism?
sensitivity 90-95%
hypothyroidism unlikely if T4 within normal limits
is a low TT4 concentration enough to diagnose primary hypothyroidism?
no
who is total T3 useful for?
sighthounds: TT4 and fT4 lower than most, but TT3 is not
what does decreased fT4 indicate in a free T4 equilibrium dialysis?
strongly suggestive but not definitive diagnosis for canine hypothyroidism
what will the value of a free T4 direct assay be increased by?
anti-T4 antibody
is endogenous canine TSH a good screening test for primary hypothyroidism?
no
very low TT4 with appropriate clinical ssigns and laboratory results ____________ diagnostic
may be
how is low TT4 usually confirmed?
fT4 and/or TSH
who is at decreased risk for feline hyperthyroidism?
siamese
himalayan
what is the etiology/cause of feline hyperthyroidism?
genetic
decreased expression of G proteins
overexpression of c-ras oncogenes
environmental factors
what are the cardiac signs of feline hyperthyroidism?
tachycardia
systolic murmur
gallop rhythms
dyspnea due to pleural effusion
how is free T4 useful in feline hyperthyroidism?
more sensitive than total T4 in cats with subclinical disease
occasionally normal with concurrent non thyroid illness
occasionally high with non thyroid illness
