Exam 4 - Hypothyroidism Flashcards
what is the 2 step process of how is thyroid hormone regulated?
- TRH is released from the hypothalmus - release is determined by complex processes within the brain & there is likely some negative feedback from TSH & T4
- TSH is released from the thyrotrophs in the pituitary - released in response to TRH & is inhibited by T4 (classic negative feedback loop)
why is thyroid hormone so important?
drives metabolism & energy use - crucial for fetal/juvenile development, drive calorigenesis, boost protein/enzyme synthesis, etc
T/F: every organ in the body is influenced by thyroid hormone in some way
true
why do thyroid hormone levels drop when illness occurs?
they drop to conserve the body’s resources
how does thyroid hormone move around the body?
attached to transport protein - 99%
less than 1% is unbound/free - only the free portion is able to enter cells (fT4 & fT3) with entry mediated by transporter proteins
how is T3 produced by other organs?
deiodination of T4 by other organs
T/F: the thyroid glands are the only source of T4, and they release large amounts of T4 & lesser amounts of T3 (triiodothyronine)
true
what is the most commonly diagnosed endocrine disease in dogs that is almost to a point of being overdiagnosed?
hypothyroidism
why do we see clinical signs in hypothyroid dogs? what clinical signs are seen?
decreased levels of circulating T4 & T3 cause signs
decreased basal metabolic rate
skin & hair coat changes
T/F: in dogs, primary hypothyroidism accounts for 95% of call cases
true
what is the pathophysiology of lymphocytic thyroiditis?
autoantibodies are directed at thyroglobulin & thyroid hormone antigens activate the complement cascade & cell-mediated cytotoxic destruction of thyroid follicular cells
when are clinical signs of hypothyroidism due to lymphocytic thyroiditis evident?
only seen when > 75% of the gland has been destoyed
T/F: presence of autoantibodies in hypothyroid dogs indicates the level of thyroid function/dysfunction
false - doesn’t indicate
T/F: breeders are now screening for thyroid autoantibodies as part of the OFA certification before breeding due to certain breeds of dogs having a higher prevalence of disease suggesting a familial tendency
true
what is the pathophysiology of idiopathic thyroid atrophy causing hypothyroidism in dogs? when do we see this occur?
non-inflammatory replacement with adipose & fibrous tissue - may be a primary degenerative disorder or end stage lymphocytic thyroiditis
mean age of diagnosis is higher in these dogs compared to lymphocytic thyroiditis
what are some examples of iatrogenic causes of hypothyroidism?
surgical removal of both thyroid glands
anti-thyroidal medication
radioactive iodine therapy
what is the mechanism of secondary hypothyroidism causing disease?
problem specific to the anterior pituitary gland resulting in decreased TSH levels
TSH secretion is inadequate & the thyroid gland becomes atrophied
due to malformation, cystic process, neoplasia, surgery, & thyrotroph cell suppression (euthyroid sick syndrome/non-thyroidal illness & drugs)
what is tertiary hypothyroidism?
problem specific to the hypothalamus resulting in depressed TRH levels due to malformation or destruction - rare
what is congenital hypothyroidism?
due to iodine deficiency, thyroid dysgenesis, dyshormonogenesis, & faulty thyroid peroxidase activity
cretinism - severe form of hypothyroidism in puppies causing retarded growth (disproportionate dwarf) & mental development
what is the common signalment of animal affected by hypothyroidism?
no gender predisposition
clinical signs in middle aged dogs, 4-6+ years (not expected in dogs younger than 2)
goldens, dobermans, irish setters, danes, & beagles
what are the more common clinical signs seen in animals with hypothyroidism?
lethargy/general blah
weight gain without increase in intake
dermatological issues - dull hair coat, brittle/coarse hair, poor hair growth after clipping, alopecia on the bridge of the nose, tail, pressure points with affected areas potentially being hyperpigmented, & comedones/seborrhea
what are the less common clinical signs seen in animals with hypothyroidism?
derm issues - truncal alopecia, pyoderma, demodex, malassezia dermatitis, myxedema (tragic face), weakness, exercise intolerance, & behavioral changes
what are the uncommon common clinical signs seen in animals with hypothyroidism?
gall bladder mucocele, cold intolerance/hypothermia, bradycardia, facial nerve paralysis, megaesophagus, corneal lipid defects, repro disorders, myxedoma coma, & atherosclerosis
what lab findings are seen on cbc & urine analysis of animals with hypothyroidism?
cbc - mild anemia, normocytic normochromic, non-regenerative
urinalysis - unremarkable
what lab findings are seen on a chemistry panel of animals with hypothyroidism?
hypercholesterolemia - can be dramatic, seen in 75% of cases
fasting hypertriglyceridemia
mild increase in ALP activity
what drugs may impact thyroid hormone test results?
glucocorticoids, sulfa drugs, phenobarbital, & NSAIDS
can you diagnose hypothyroidism based on a single total T4 test?
NOPE
what is important to remember about greyhounds when testing thyroid hormone levels?
euthyroid sighthounds have lower T4 levels than other breeds
T/F: non-thyroidal illness can impact thyroid hormone secretion which can make testing much more difficult to interpret
true
how do barbiturate drugs impact thyroid hormones?
decrease tT4 & fT4 - TSH may be normal or high
how do glucocorticoids impact thyroid hormones?
decrease tT4 & fT4 - cortisol inhibits TRH/TSH release, protein binding may also be affected
how do NSAIDS impact thyroid hormones?
decrease tT4 - effect appears to be mild
how do sulfa drugs impact thyroid hormones?
decrease tT4 & fT4, increase TSH - direct impact on thyroid tissue, but recovery is expected within 3 weeks
what does a total T4 panel measure?
both bound & free T4 - fT4 <1% of total
why are total T4 & fT4 tests great for screening?
normal result eliminates the diagnosis of hypothyroidism
why is one total T4 test not enough to diagnose hypothyroidism?
low value is not enough evidence - tT4 measurement is < reference range 20% of the time in healthy dogs!!!
T/F: total T4 overlaps between euthyroid & hypothyroid dogs at the bottom end of the reference range
true
how do anti-thyroid antibodies impact total T4 testing?
rare, but they can falsely elevate tT4 - have to measure fT4 by ED to identify this
what happens to tT4 levels in non-thyroidal/euthyroid sick syndrome illnesses?
levels are very quickly driven down!!!!
if your serum total T4 result from a dog is > 2.0 mcg/dl, what is the likelihood that the animal has hypothyroidism?
extremely unlikely!!!
if your serum total T4 result from a dog is 1.5-2.0 mcg/dl, what is the likelihood that the animal has hypothyroidism?
unlikely
if your serum total T4 result from a dog is 1.0-1.5 mcg/dl, what is the likelihood that the animal has hypothyroidism?
maybe!
if your serum total T4 result from a dog is 0.5-1.0 mcg/dl, what is the likelihood that the animal has hypothyroidism?
possibly
if your serum total T4 result from a dog is < 0.5 mcg/dl, what is the likelihood that the animal has hypothyroidism?
extremely likely!!!
what will fT4 be in a dog with hypothyroidism?
subnormal
what is the gold standard for measuring free T4?
equilibrium dialysis - slow & expensive
less impacted by non-thyroidal illness/euthyroid sick syndrome
why is measuring TSH a good way to diagnose hypothyroidism in dogs?
almost every dog with primary hypothyroidism is expected to have high levels of TSH - some dogs may have normal levels
T/F: TSH results are affected by concurrent disease, & medications
true
if you have a healthy euthyroid dog, what are your expected results for thyroid tests?
normal tT4
normal fT4
normal TSH
10% of normal dogs can have an increased TSH
if you have a dog with primary hypothyroidism, what are your expected results for thyroid tests?
low tT4
low fT4
increased TSH
25% of affected dogs can have a normal TSH
if you have a dog with secondary hypothyroidism, what are your expected results for thyroid tests?
low tT4
low fT4
low TSH
if you have a dog with euthyroid sick syndrome, what are your expected results for thyroid tests?
low tT4
normal fT4
normal TSH
free T4 will drop with extreme illness
what is the most important autoantibody to measure for in a hypothyroid dog? why test for them?
autoantibodies against thyroglobulin - antithyroglobulin antibodies are present in up to 60% of dogs with hypothyroidism & 3% of euthyroid dogs, & the presence suggests lymphocytic thyroiditis but doesn’t predict eventual clinical hypothyroidism (useful for breeders)
T4 & T3 less important
what do you expect to see on test results if you have a dog that was positive for anti-thyroid antibodies?
spurious increase in tT4 but no effect on fT4-ED
why do we not commonly do TSH stimulation tests? how is it done?
rarely done due to issues obtaining reagents
collect a baseline T4 & administer TSH
collect post sample 4-6 hours later - normal dogs will have a substantial increase in tT4
what is the purpose of using nuclear medicine scanning for diagnosing hypothyroidism?
provides information about iodine uptake & thyroid tissue function
what is the standard therapy & dosing used for treating hypothyroidism? when does serum concentration peak?
levothyroxine - 0.02 mg/kg po bid - better absorption on an empty stomach but can give with food as long as protocol is consistent
4-6 hours after dose is given
when should you see patient improvement after starting therapy for hypothyroidism?
improvement in attitude within 2 weeks - clin path changes take several weeks
gallbladder mucocele may resolve in a few months - ursodiol & low fat diet
skin issues take month
what monitoring should be done long term for dogs with hypothyroidism?
recheck them every 4-6 months
when do you recheck a newly diagnosed hypothyroid dog?
check tT4 after 2-4 weeks of therapy ideally 4-6 hours post pilling
what monitoring should be done for hypothyroid dogs?
adjust dose as needed to keep tT4 at the upper end of the reference range or even just above
recheck every 4-6 months
some dogs can be managed SID - improves compliance
what clinical signs suggest an overdose of levothyroxine?
anxiety, polyphagia, weight loss, polyuria, & polydipsia
why use brand name medication for treating hypothyroid dogs?
several generic versions have reduced hormonal content in contrast to those stated on the label
what is euthyroid sick syndrome?
systemic illness that quickly affects thyroid status that is very complex & is centrally & peripherally mediated
what components are included in euthyroid sick syndrome?
decrease in TSH production
decrease in thyroid hormone production
changes in binding to carrier proteins & transport
changes in metabolism of thyroid hormones within cells
changes in receptor activity
changes in iodothyronine deiodinase activities
what is step 1 of euthyroid sick syndrome? what is step 2?
- decrease in total T4 with fT4 being normal initially - fT4 ED is more reliable under these circumstances
- decrease in both total & free T4-ED, which is predictive of a poor outcome in dogs with non-thyroidal illness
T/F: spontaneous acquired hypothyroidism is rare in cats
true
what clinical signs are associated with spontaneous acquired hypothyroidism in cats?
clinical signs similar to dogs, but not identical
derm changes but no alopecia, lethargy that can be so profound they are hyporexic, weight gain, anemia, hypercholesterolemia
what testing is done for feline hypothyroidism?
T4, fT4, +/- TSH
what is congenital hypothyroidism in kittens? what are the clinical signs associated with it?
disproportionate dwarfism (evident by 2 months), mental retardation, constipation, poor hair coat with mostly undercoat & a few guard hairs, +/- palpable goiter
what are the causes of congenital hypothyroidism in cats?
dyshormonogenesis
thyroid peroxidase deficiency
pituitary dysfunction
how is congenital hypothyroidism diagnosed in kittens?
diagnosis is supported by radiographic findings of abnormal long bones & vertebral bodies
which type of hypothyroidism is more common in cats?
congenital more common than acquired!!!
what differential should always be considered in kittens with recurring constipation?
congenital hypothyroidism
