Exam 4 - Hypothyroidism Flashcards by Madeleine Swindell

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)

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why is thyroid hormone so important?

drives metabolism & energy use - crucial for fetal/juvenile development, drive calorigenesis, boost protein/enzyme synthesis, etc

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T/F: every organ in the body is influenced by thyroid hormone in some way

true

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why do thyroid hormone levels drop when illness occurs?

they drop to conserve the body’s resources

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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

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how is T3 produced by other organs?

deiodination of T4 by other organs

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T/F: the thyroid glands are the only source of T4, and they release large amounts of T4 & lesser amounts of T3 (triiodothyronine)

true

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what is the most commonly diagnosed endocrine disease in dogs that is almost to a point of being overdiagnosed?

hypothyroidism

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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

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T/F: in dogs, primary hypothyroidism accounts for 95% of call cases

true

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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

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when are clinical signs of hypothyroidism due to lymphocytic thyroiditis evident?

only seen when > 75% of the gland has been destoyed

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T/F: presence of autoantibodies in hypothyroid dogs indicates the level of thyroid function/dysfunction

false - doesn’t indicate

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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

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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

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what are some examples of iatrogenic causes of hypothyroidism?

surgical removal of both thyroid glands

anti-thyroidal medication

radioactive iodine therapy

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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)

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what is tertiary hypothyroidism?

problem specific to the hypothalamus resulting in depressed TRH levels due to malformation or destruction - rare

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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

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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

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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

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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

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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

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what lab findings are seen on cbc & urine analysis of animals with hypothyroidism?

cbc - mild anemia, normocytic normochromic, non-regenerative

urinalysis - unremarkable

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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?

1. decrease in total T4 with fT4 being normal initially - fT4 ED is more reliable under these circumstances 2. 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