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SA DX & TX II > Exam 4 - Hypothyroidism > Flashcards

Exam 4 - Hypothyroidism Flashcards

1
Q

what is the 2 step process of how is thyroid hormone regulated?

A
  1. TRH is released from the hypothalmus - release is determined by complex processes within the brain & there is likely some negative feedback from TSH & T4
  2. 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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2
Q

why is thyroid hormone so important?

A

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

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3
Q

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

A

true

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4
Q

why do thyroid hormone levels drop when illness occurs?

A

they drop to conserve the body’s resources

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5
Q

how does thyroid hormone move around the body?

A

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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6
Q

how is T3 produced by other organs?

A

deiodination of T4 by other organs

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7
Q

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

A

true

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8
Q

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

A

hypothyroidism

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9
Q

why do we see clinical signs in hypothyroid dogs? what clinical signs are seen?

A

decreased levels of circulating T4 & T3 cause signs

decreased basal metabolic rate

skin & hair coat changes

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10
Q

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

A

true

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11
Q

what is the pathophysiology of lymphocytic thyroiditis?

A

autoantibodies are directed at thyroglobulin & thyroid hormone antigens activate the complement cascade & cell-mediated cytotoxic destruction of thyroid follicular cells

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12
Q

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

A

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

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13
Q

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

A

false - doesn’t indicate

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14
Q

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

A

true

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15
Q

what is the pathophysiology of idiopathic thyroid atrophy causing hypothyroidism in dogs? when do we see this occur?

A

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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16
Q

what are some examples of iatrogenic causes of hypothyroidism?

A

surgical removal of both thyroid glands

anti-thyroidal medication

radioactive iodine therapy

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17
Q

what is the mechanism of secondary hypothyroidism causing disease?

A

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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18
Q

what is tertiary hypothyroidism?

A

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

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19
Q

what is congenital hypothyroidism?

A

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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20
Q

what is the common signalment of animal affected by hypothyroidism?

A

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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21
Q

what are the more common clinical signs seen in animals with hypothyroidism?

A

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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22
Q

what are the less common clinical signs seen in animals with hypothyroidism?

A

derm issues - truncal alopecia, pyoderma, demodex, malassezia dermatitis, myxedema (tragic face), weakness, exercise intolerance, & behavioral changes

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23
Q

what are the uncommon common clinical signs seen in animals with hypothyroidism?

A

gall bladder mucocele, cold intolerance/hypothermia, bradycardia, facial nerve paralysis, megaesophagus, corneal lipid defects, repro disorders, myxedoma coma, & atherosclerosis

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24
Q

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

A

cbc - mild anemia, normocytic normochromic, non-regenerative

urinalysis - unremarkable

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25
Q

what lab findings are seen on a chemistry panel of animals with hypothyroidism?

A

hypercholesterolemia - can be dramatic, seen in 75% of cases

fasting hypertriglyceridemia

mild increase in ALP activity

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26
Q

what drugs may impact thyroid hormone test results?

A

glucocorticoids, sulfa drugs, phenobarbital, & NSAIDS

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27
Q

can you diagnose hypothyroidism based on a single total T4 test?

A

NOPE

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28
Q

what is important to remember about greyhounds when testing thyroid hormone levels?

A

euthyroid sighthounds have lower T4 levels than other breeds

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29
Q

T/F: non-thyroidal illness can impact thyroid hormone secretion which can make testing much more difficult to interpret

A

true

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30
Q

how do barbiturate drugs impact thyroid hormones?

A

decrease tT4 & fT4 - TSH may be normal or high

31
Q

how do glucocorticoids impact thyroid hormones?

A

decrease tT4 & fT4 - cortisol inhibits TRH/TSH release, protein binding may also be affected

32
Q

how do NSAIDS impact thyroid hormones?

A

decrease tT4 - effect appears to be mild

33
Q

how do sulfa drugs impact thyroid hormones?

A

decrease tT4 & fT4, increase TSH - direct impact on thyroid tissue, but recovery is expected within 3 weeks

34
Q

what does a total T4 panel measure?

A

both bound & free T4 - fT4 <1% of total

35
Q

why are total T4 & fT4 tests great for screening?

A

normal result eliminates the diagnosis of hypothyroidism

36
Q

why is one total T4 test not enough to diagnose hypothyroidism?

A

low value is not enough evidence - tT4 measurement is < reference range 20% of the time in healthy dogs!!!

37
Q

T/F: total T4 overlaps between euthyroid & hypothyroid dogs at the bottom end of the reference range

A

true

38
Q

how do anti-thyroid antibodies impact total T4 testing?

A

rare, but they can falsely elevate tT4 - have to measure fT4 by ED to identify this

39
Q

what happens to tT4 levels in non-thyroidal/euthyroid sick syndrome illnesses?

A

levels are very quickly driven down!!!!

40
Q

if your serum total T4 result from a dog is > 2.0 mcg/dl, what is the likelihood that the animal has hypothyroidism?

A

extremely unlikely!!!

41
Q

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?

A

unlikely

42
Q

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?

A

maybe!

43
Q

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?

A

possibly

44
Q

if your serum total T4 result from a dog is < 0.5 mcg/dl, what is the likelihood that the animal has hypothyroidism?

A

extremely likely!!!

45
Q

what will fT4 be in a dog with hypothyroidism?

A

subnormal

46
Q

what is the gold standard for measuring free T4?

A

equilibrium dialysis - slow & expensive

less impacted by non-thyroidal illness/euthyroid sick syndrome

47
Q

why is measuring TSH a good way to diagnose hypothyroidism in dogs?

A

almost every dog with primary hypothyroidism is expected to have high levels of TSH - some dogs may have normal levels

48
Q

T/F: TSH results are affected by concurrent disease, & medications

A

true

49
Q

if you have a healthy euthyroid dog, what are your expected results for thyroid tests?

A

normal tT4

normal fT4

normal TSH

10% of normal dogs can have an increased TSH

50
Q

if you have a dog with primary hypothyroidism, what are your expected results for thyroid tests?

A

low tT4

low fT4

increased TSH

25% of affected dogs can have a normal TSH

51
Q

if you have a dog with secondary hypothyroidism, what are your expected results for thyroid tests?

A

low tT4

low fT4

low TSH

52
Q

if you have a dog with euthyroid sick syndrome, what are your expected results for thyroid tests?

A

low tT4

normal fT4

normal TSH

free T4 will drop with extreme illness

53
Q

what is the most important autoantibody to measure for in a hypothyroid dog? why test for them?

A

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

54
Q

what do you expect to see on test results if you have a dog that was positive for anti-thyroid antibodies?

A

spurious increase in tT4 but no effect on fT4-ED

55
Q

why do we not commonly do TSH stimulation tests? how is it done?

A

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

56
Q

what is the purpose of using nuclear medicine scanning for diagnosing hypothyroidism?

A

provides information about iodine uptake & thyroid tissue function

57
Q

what is the standard therapy & dosing used for treating hypothyroidism? when does serum concentration peak?

A

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

58
Q

when should you see patient improvement after starting therapy for hypothyroidism?

A

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

59
Q

what monitoring should be done long term for dogs with hypothyroidism?

A

recheck them every 4-6 months

60
Q

when do you recheck a newly diagnosed hypothyroid dog?

A

check tT4 after 2-4 weeks of therapy ideally 4-6 hours post pilling

61
Q

what monitoring should be done for hypothyroid dogs?

A

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

62
Q

what clinical signs suggest an overdose of levothyroxine?

A

anxiety, polyphagia, weight loss, polyuria, & polydipsia

63
Q

why use brand name medication for treating hypothyroid dogs?

A

several generic versions have reduced hormonal content in contrast to those stated on the label

64
Q

what is euthyroid sick syndrome?

A

systemic illness that quickly affects thyroid status that is very complex & is centrally & peripherally mediated

65
Q

what components are included in euthyroid sick syndrome?

A

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

66
Q

what is step 1 of euthyroid sick syndrome? what is step 2?

A
  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
67
Q

T/F: spontaneous acquired hypothyroidism is rare in cats

A

true

68
Q

what clinical signs are associated with spontaneous acquired hypothyroidism in cats?

A

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

69
Q

what testing is done for feline hypothyroidism?

A

T4, fT4, +/- TSH

70
Q

what is congenital hypothyroidism in kittens? what are the clinical signs associated with it?

A

disproportionate dwarfism (evident by 2 months), mental retardation, constipation, poor hair coat with mostly undercoat & a few guard hairs, +/- palpable goiter

71
Q

what are the causes of congenital hypothyroidism in cats?

A

dyshormonogenesis

thyroid peroxidase deficiency

pituitary dysfunction

72
Q

how is congenital hypothyroidism diagnosed in kittens?

A

diagnosis is supported by radiographic findings of abnormal long bones & vertebral bodies

73
Q

which type of hypothyroidism is more common in cats?

A

congenital more common than acquired!!!

74
Q

what differential should always be considered in kittens with recurring constipation?

A

congenital hypothyroidism

SA DX & TX II (62 decks)

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