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Internal Medicine/ Ettinger > Canine Hypothyroidism > Flashcards

Canine Hypothyroidism Flashcards

1
Q

The majority of T4 is bound to which protein while in circulation?

A

thyroxine binding protein

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2
Q

Most of the steps involved in thyroid hormone synthesis is catalyzed by which enzyme?

A

thyroid peroxidase

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3
Q

How is thyroid hormone controlled?

A

hypothalamus –> TRH –> anterior pituitary –> TSH –> thyroid gland –> iodine trapping, T4/T3 release
excess circulating free T4 and T3 = negative feedback loop –> anterior pituitary and hypothalamus

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4
Q

What are some major functions for thyroid homrones?

A
  • metabolism
  • growth
  • CNS development
  • tissue turnover
  • (+) inotrope/ chronotrop
  • cholesterol synthesis & metabolism
  • stimulates erythropoiesis
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5
Q

What are the 3 types of thyroid disease?

A
  • primary = thyroid gland
  • secondary = pituitary
  • tertiary = hypothalamus
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6
Q

What’s the most common form of hypothyroidism?

A

primary, acquired –> lymphocytic thyroiditis and thyroid atrophy

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7
Q

Which breeds with high prevalence of thyroglobulin autoantibodies (TgAAs)?

A
  • Golden Retriever
  • English Setter
  • Rhodesian Ridgeback
  • Cocker Spaniel
  • Boxer
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8
Q

How is lymphocytic thyroiditis diagnosed?

A
  • biopsy = gold standard
  • can also be inferred with thyroglobulin autoantibodies (TgAAs)
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9
Q

What’s the typical signalment for canine hypothyroidism?

A
  • 7yo
  • predisposed purebreds may have an earlier onset
  • intact males and spayed females
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10
Q

What are the most common c/s of canine hypothyroidism?

A
  • dermatological and metabolic concerns
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11
Q

What are the metabolic concerns with hypothyroid dogs?

A
  • obese/ weight gain
  • decreased metabolism
  • shivering, heat seeking
  • lethargy, mental dullness
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12
Q

What are some dermatological abnormalities associated with canine hypothyroidism?

A
  • poor hair coat
  • alopecia (truncal, rat tail, bilateral symmetrical)
  • non-pruritic
  • dry, scaly skin; seborrhea, otitis, hyperpigmentation, secondary bacterial infection
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13
Q

What’s the cause of the “tragic facial expression”?

A

Myxedema - accumulation of hyaluronic acid in the dermis

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14
Q

What are some of the ECG changes noted in canine hypothyroidism?

A
  • low voltage R wave
  • inverted T wave
  • bradycardia
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15
Q

What are some echocardiogram changes noted in canine hypothyroidism?

A
  • decreased fractional shortening
  • mild increased L ventricle end systole diameter
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16
Q

How does thyroid hormone affect the heart>

A

it increases the heart’s sensitivity to adrenergic stimulation –> (+) inotrope

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17
Q

Are hypothyroid dogs often clinical for the cardiovascular changes?

A

no

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18
Q

What are some neuromuscular changes noted in canine hypothyroidism?

A
  • no direct causal effect, BUT
  • central vestibular disease –> suspect related to atherosclerosis –> brain infarction or metabolic derrangement
  • fatal myxedema coma has been reported
  • may be difficult to ddx between neuropathy vs myopathy
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19
Q

What eye abnormality is noted in canine hypothyroidism?

A

arcus lipoides (esp in German Shepherd Dogs)

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20
Q

What reproductive changes are noted in canine hypothyroidism?

A
  • short term: fertility is normal, but prolonged parturition and lowered pup survival
  • long term: decreased fertility, and increased peri-parturient mortality , lower birth rate
  • male repro indices are not affected
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21
Q

How is congenital canine hypothyroidism different than pituitary dwarfism?

A

hypothyroid = disproportional dwarfism

22
Q

What’s autoimmune polyendocrine syndrome type 1 (APS-1) in people?

A

Triad of autoimmune hypothyroidism, Addison’s, and mucocutaneous candidiasis –> rare, autosomal recessive

23
Q

What’s autoimmune polyendocrine syndrome type 2 (APS-2) in people?

A

2 or more of the following:
- Addison’s
- Grave’s
- autoimmune thyroiditis
- Type 1A diabetes mellitus
- primary hypogonadism
- myasthenia gravis
- Celiac

24
Q

Similar to APS-2 is seen in dogs. Particularly with which ones?

A

Addison’s - if poor response to mineralocorticoids, persistent hyponatremia, etc
- diabetes mellitus also common to have in addition to hypothyroidism

25
Q

What’s potential MOA of low thyroid induced diabetes mellitus?

A

transdifferentiating of somatotropic pituitary cells to thyrosomatotropes –> excessive growth hormone = highly diabetogenic

26
Q

What are the most common blood work changes noted in canine hypothyroidism?

A
  • anemia
  • hypercholesterolemia
  • hypertriglyceridemia
  • increased CK
  • decrease fructosamine?
27
Q

How is the hypothyroid anemia characterized as?

A

mild, none regenerative, normocytic, normochromic
- physiological anemia, due to lack of erythropoietin stimulation and lack of stimulatory effect of thyroid hormone on the bone marrow

28
Q

How is the platelet count in hypothyroid dogs?

A

increased; thrombopoiesis and erythropoiesis have an inverse relationship

29
Q

What’s MOA for hypercholesterolemia with hypothyroidism?

A

Thyroid hormone is involved in synthesis, mobilization and degradation of cholesterol. Hypothyroidism has profound effect of the degradation.

30
Q

Why does hypothyroidism increase CK?

A
  • decreased metabolism or excretion
  • myopathy
31
Q

Why does hypothyroidism cause increased fructosamine?

A
  • decrease protein turnover
32
Q

Why does hypothyroidism lead to mild liver enzyme elevation?

A
  • specifically in ALP and GGT –> lipid deposition
33
Q

How is free T4 tested?

A

Free T4 by equilibrium dialysis

34
Q

What are some storage/ interference with thyroid testing?

A
  • free T4 significantly increased with stored at room temp for a long time
  • severe hyperlipemia may falsely increase free T4 by equilibrium dialysis
35
Q

How is TSH tested?

A

Using species specific hormone assay

36
Q

Which breeds have low thyroid hormones?

A

sighthounds

37
Q
A
38
Q

How does sulfonamides induce hypothyroidism?

A

it reversibly inhibits thyroxin peroxidase

39
Q

What are some drugs that can lead to hypothyroidism?

A
  • glucocorticoids
  • phenobarbital
  • aspirin
  • ketoprofen
  • carprofen
  • clomipramine
  • sulfonamides
40
Q

What’s the diagnostic utility of total T3?

A

typically not used due to low sensitivity and specificity.
BUT, can be useful in Greyhounds when they have low total T4 but normal total T3

41
Q

What’s the diagnostic utility of total T4?

A
  • highly sensitive but low specificity
  • T4 autoantibodies can falsely increase the result
  • should never be used alone for diagnosis of hypothyroidism
42
Q

Which is the single most accurate test for hypothyroidism?

A

free T4

43
Q

What’s the diagnostic utility of total cTSH?

A
  • moderate sensitivity
  • but highly specific, not really affected by non-thyroid illness
44
Q

What’s the diagnostic utility of TgAA?

A
  • increased = lymphocytic thyroiditis
  • has no info in thyroid function
  • an increase indirectly supports hypothyroidism
45
Q

What are the utilities of provocative tests for canine hypothyroidism?

A
  • human recombinant TSH, pretty good test
  • TRH, not as reliable as the others
46
Q

Which imaging modality is useful for diagnosing hypothyroidism in general?

A

ultrasound
hypothyroid thyroid gland = smaller, heterogenous echogenicity, irregular outline, more rounded

47
Q

When is technetiusm-99M scans useful?

A

for Greyhounds
- or investigating central hypothyroidism
- ddx between dysgenesis from dyshormonogensis in congenital hypothyroidism

48
Q

How is hypothyroidism treated?

A

with endogenous T4 supplementation

49
Q

How long does it take for c/s to resolve once T4 supplements have been given?

A
  • within days: metabolic changes, ex. lethargy, dullness
  • first few months: 10% weight loss
  • 2-3 months: improvement in dermatological signs
  • first 3 months: serum cholesterol and triglycerides decrease rapidly, RBC increases gradually
  • up to 6months: neurological signs
50
Q

How to treat myxedema coma?

A

IV T4, improvement should be noted in 30hours

51
Q
A

Internal Medicine/ Ettinger (93 decks)

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