Disorders of the Thyroid Gland Flashcards

Dr. Swan

1
Q

Briefly look at the physiology of the hypothalamic-pituitary-thyroid axis

A
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2
Q

The majority of T3 and T4 are _____

A

protein-bound

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3
Q

What are the effects of T3 and T4 in circulation?

A

have a positive effect of development, growth, and metabolism on organs and tissues throughout the body

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4
Q

What 2 cell types are the thyroid gland composed of?

A
  1. thyroid follicular epithelial cells
  2. thyroid C-cells
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5
Q

What are the functions of thyroid follicular epithelial cells?

A
  • produce colloid (thryoglobulin) storage form
  • convert thyroglobulin by iodinating to form T3/T4 to release into circulation
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6
Q

What is circulating T3 and T4 called? Small or large amount?

A

fT3 and fT4 (free)

small

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7
Q

What is the function of thyroid c-cells?

A

produce calcitonin

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8
Q

What is canine hypothyroidism characterized by?

A

decreased circulating T4

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9
Q

What is often normal in dogs with hypothyroidism?

A

T3

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10
Q

What are 3 primary hypothyroidism causes?

A

immune-mediated destruction of thyroid follicular epithelial cells (lymphocytic thyroiditis)

severe thyroid follicular atrophy - probably end-stage immune-mediated thyroiditis

neoplastic destruction

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11
Q

What are the T4 levels in a dog with hypothyroidism? TSH?

A

total T4 (tT4): decreased
free T4 (fT4): decreased
TSH: increased

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12
Q

What is canine secondary hypothyroidism?

A

decreased TSH
pituitary-related malformation

compression, destruction, or atrophy of thyrotrophs from pituitary adenomas

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13
Q

What is canine tertiary hypothyroidism?

A

decreased TRH (from hypothalamus)

very rare

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14
Q

What are clinical signs of canine hypothyroidism?

A
  • weight gain
  • heat-seeking behavior or cold intolerance
  • lethargy
  • dermatopathy (alopecia surrounding base of tail)
  • persistent or reoccurring infections
  • vestibular
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15
Q

What are CBC findings of canine hypothyroidism?

A

normocytic normochromic non-regenerative anemia

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16
Q

What are chemistry findings of canine hypothyroidism?

A

hypercholesterolemia
hypertriglyceridemia

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17
Q

What is the screening assay for canine hypothyroidism?

A

total T4 (tT4)

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18
Q

Regarding canine hypothyroidism, a minimum panel of what should be performed?

A

tT4, fT4, TSH

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19
Q

What assay can be used to diagnose 50% of canine hypothyroidism cases?

A

thyroglobulin autoantibodies (TgAA)

lymphocytic thyroiditis - 50% of cases

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20
Q

What is total t4?

A

measurement of both bound and free T4 hormone within circulation

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21
Q

If tT4 is in the upper half of the reference interval, is hypothyroidism unlikely or likely?

A

unlikely

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22
Q

What can cross-react with the tT4 result and falsely elevate it?

A

TgAA

23
Q

What is non-thyroidal illness syndrome?

A

systemic illnesses suppress tT4 and mimics hypothyroidism

24
Q

A patient was tested for T3, T4, free T4, TSH, and TgAA. What is a likely diagnosis?

A

hypothyroidism due to idiopathic thyroid follicular atrophy

25
Q

A patient was tested for T3, T4, free T4, TSH, and TgAA. What is a likely diagnosis?

A

normal

26
Q

A patient was tested for T3, T4, free T4, TSH, and TgAA. What is a likely diagnosis?

A

hypothyroidism due to immune-mediated thyroiditis where tT4 is WRI due to antibodies interfering with tT4 assay falsely elevating level

27
Q

A patient was tested for T3, T4, free T4, TSH, and TgAA. What is a likely diagnosis?

A

low tT4 level potentially breed-related or Non-Thyroidal Illness Syndrome

28
Q

A patient was tested for T3, T4, free T4, TSH, and TgAA. What is a likely diagnosis?

A

hypothyroidism - consider pituitary-related disorder

29
Q

What are the two forms of feline hypothyroidism?

A
  • congenital
  • iatrogenic or adult-onset disease
30
Q

What are clinical signs associated with feline hypothyroidism - congenital disease?

A

slow growth rate
gingival overgrowth
- short limbs
- broad head
- etc

31
Q

What are clinical signs associated with feline hypothyroidism - iatrogenic or adult-onset?

A

lethargy, weight-gain, obesity, seborrhea

32
Q

How do you diagnose feline hypothyroidism?

A

low serum T4 - without evidence of Non-Thyroidal Illness Syndrome

33
Q

What is feline hyperthyroidism characterized by?

A

increased circulating T3/T4 hormones

34
Q

What are the forms of feline hyperparathyroidism?

A

primary hyperthyroidism
secondary hyperthyroidism - pituitary

35
Q

What happens with primary hyperthyroidism - feline?

A

neoplasia: thyroid adenomas or adenomatous hyperplasia: unilateral (70%) or bilateral

thyroid carcinoma
neoplasia in ectopic thyroid tissue (3-5%)

36
Q

What happens with secondary hyperthyroidism - feline?

A

functional pituitary thryoitroph adenoma
- bilateral symmetrical hypertrophy of thyroid glands would be expected

37
Q

What are clinical signs associated with feline hyperthyroidism?

A
  • weight loss
  • polyphagia
  • increased activity
38
Q

What are physical exam findings of feline hyperthyroidism?

A
  • palpable thyroid enlargement
  • tachycardia +/- murmur
  • unkempt haircoat
  • ventroflexion of head
39
Q

Why do some cats with hyperthyroidism have murmurs?

A

they get myocardial hypertrophy as a result of excessive T3/T4

40
Q

In feline hyperthyroidism, geriatric cats will often have multiple comorbidites, the most common which is

A

concurrent renal disease

41
Q

What are CBC findings for feline hyperthyroidism?

A

erythrocytosis
macrocytosis
increased Heinz bodies

42
Q

What does a chemistry panel look like with feline hyperthyroidism?

A
  • increased ALT
  • increased ALP
43
Q

If a feline with hyperthyroidism has azotemia on chemistry, is is most often due to _____. How do they interact with each other?

A

concurrent chronic kidney disease

hyperthyroidism increases GFR (due to hypertension) which can improve azotemia. After you treat the cat for hyperthyroidism, can have risk of cat becoming azotemic

44
Q

What is the screening assay for hyperthyroidism?

A

total T4 (tT4)

45
Q

Which T4 is less affected by NTIS?

A

fT4

46
Q

A cat was tested for T4 and free T4 with this result. What is a likely diagnosis?

A

hyperthyroidism

47
Q

A cat was tested for T4 and free T4 with this result. What is a likely diagnosis?

A

hyperthyroid cat but tT4 is in the gray zone

fT4 used to diagnose due to tT4 being in grey zone

48
Q

A cat was tested for T4 and free T4 with this result. What is a likely diagnosis?

A

normal

49
Q

What is primary hyperthyroidism - canine?

A

thyroid follicular neoplasia - 25% are functional in dogs

CARCINOMA 90%

ectopic thyroid is relatively common

50
Q

What is secondary hyperthyroidism - canine?

A

pituitary thyrotroph adenoma —> oversecretion of TSH, uncommon

51
Q

Most canine thyroid tumors are non-functional, therefore dogs present for signs associated with a _______, rather than hyperthyroidism

A

mass effect - tumor grows

highly likely a carcinoma

52
Q

What are clinical signs of canine hyperthyroidism?

A
53
Q

What are clinical signs of thyroid mass - hyperthyroidism in a canine?

A

visible mass in neck - almost always carcinoma