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Internal Medicine/ Ettinger > Feline Hyperthyroidism > Flashcards

Feline Hyperthyroidism Flashcards

1
Q

What type of cancer is feline hyperthyroidism?

A

benign, mostly hyperplasia/ adenoma

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2
Q

What’s the likely cause(s) of feline hyperthyroidism?

A

multifactorial
- genetic mutation
- food
- environmental contamination

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3
Q

How many lobes are usually affected in feline hyperthyroidism?

A

Bilateral asymmetrical = most common
- ectopic disease = uncommon, about 4%
- adenocarcinoma = rare, about 1-2%, but could be under-reported

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4
Q

What’s the typical signalment for feline hyperthyroidism?

A

mid age to older (median 12.5yo)
no breed or sex predilection

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5
Q

What’s the most common c/s for feline hyperthyroidism?

A
  • weight loss
  • increased appetite
  • hyperactivity
  • v/d
  • poor grooming
  • PU/PD
  • behavioural changes
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6
Q

What’s apathetic hyperthyroidism?

A

uncommon, when the cat is actually lethargic/ obtunded with poor appetite

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7
Q

Is “thyroid slip” indicative of hyperthyroidism?

A

no, can be noted in up to 70% of euthyroid cats

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8
Q

What cardiovascular changes are noted in cats with hyperthyroidism?

A
  • heart murmur
  • tachycardia
  • gallop rhythm
  • thyroid hormones (mainly T3) = positive chronotropic effects –> short AV conduction times, stimulates beta adrenergic receptors
  • both T3 and T4 exert positive inotropic effects by altering ion channel activities and enhanced cardiac myosin isoenzymes
  • increased expression of myocardial proteins = myocardial hypertrophy
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9
Q

Does feline hyperthyroidism cause hypertension?

A

difficult to know. there is an association, but hard to prove causation

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10
Q

How does feline hyperthyroidism lead to increased GFR?

A
  • thyroid hormones increases cardiac output and decreases vascular resistance = increased GFR
  • in renal cortex, increased thyroid hormone –> increase NO synthase activity –> increased vasodilation, # of beta adrenergic receptors, and decrease vascular resistance –> increased GFR
  • up-regulates Cl- channel in the proximal tubule and Loop of Henle –> less Cl- sensed in the macula densa in the distal tubule –> increases tubuloglomerular feedback –> increased GFR
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11
Q

How does feline hyperthyroidism lead to PU/PD?

A
  • possibly due to decrease aquaporin channels and increased tubular solute excretion
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12
Q

How does feline hyperthyroidism decrease creatinine?

A
  • increases creatinine tubular secretion
  • increases the GFR
  • decreases muscle mass
    BUT 25% of the cats are still azotemic
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13
Q

How does hyperthyroidism lead to vomiting?

A

unclear in cats

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14
Q

How does hyperthyroidism lead to diarrhea?

A

Thyrotoxicosis decrease GI transit time/ increases GI motility

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15
Q

How does thyrotoxicosis influence CBC?

A

thyroid hormone increases oxygen demand: thyroid hormone-erythropoietin-induced erythrocytosis
- RBC may be elevated or high normal, which would be unusual in patients with chronic disease

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16
Q

What biochem changes are noted in feline hyperthyroidism?

A
  • increases in ALT (80%) and ALP (50%)
  • due to metabolic hepatic stress, passive congestion, direct toxicity, etc.
  • also azotemia
17
Q

What’s the most common way to diagnose hyperthyroidism in cats?

A

total T4
- T3 = not good due to too much overlap between normal and hyperthyroid cats
- TSH not widely used, but may be useful

18
Q

What’s occult hyperthyroidism?

A

In patients highly suspected to have hyperthyroidism based on c/s but tT4 is normal… could be due to:
- normal fluctuation of tT4, which can fall back into the normal range
- non-thyroidal illness decrease thyroid level

19
Q

What are some non-thyroidal illness that can decrease thyroid level in the cat?

A
  • diabetes mellitus
  • CKD
  • congestive heart failure
  • IBD
  • hepatic disease
  • neoplasia
  • inflammatory airway disease
20
Q

Free T4 has high sensitivity in cats with hyperthyroidism. Why isn’t it used as a first line diagnostic test?

A
  • low specificity and poor positive predictive value
  • as much as 12% of non-thyroidal illness cats with have increased fT4
  • expensive to run, assay methodology is controversial
21
Q

How reliable is TSH testing in feline hyperthyroidism?

A
  • for subclinical feline hyperthyroidism, patient can still have normal tT4/fT4 with a low TSH.
  • reliable assay is not widely available
  • if using cTSH, difficult to pick up low TSH, but may be useful in picking up increased TSH (iatrogenic hypothyroidism)
22
Q

When is T3 suppression test done? How does it work?

A

T3 suppression test can be done to differentiate cats with occult hyperthyroidism vs normal
- pre-T3 blood sample, T3 supplements (q8h x 7 doses), post-T3 blood sample
- testing for T3 and T4
- if hyperthyroid, there will be blunted or no change in T4
- if euthyroid, there will be a decrease in T4

23
Q

What’s the utility of TRH in diagnosing feline hyperthyroidism?

A
  • TRH should increase T4 by >60% in euthyroid cats, but <50% in hyperthyroid cats
  • however, there is no commercial product available, and side effects are noted in cats given TRH –> severe cholinergic and CNS mediated reactions
24
Q

What’s the utility of thyroid scintigraphy in diagnosing feline hyperthyroidism?

A
  • very sensitive in diagnosing feline hyperthyroidism
  • may be useful for presurgical screening (laterality, ectopic, intrathoracic etc.)
  • limited by availability
25
Q

What are some main options to treat feline hyperthyroidsim?

A
  • thioureylene anti-thyroid drugs
  • radioiodine
  • surgery
  • dietary elimination of iodine
26
Q

How does anti-thyroid drugs work?

A

ex. methimazole, carbimazole
- works by inhibiting thyroperoxidase, which then inhibits iodination of tyrosyl residues in the thyroglobulin, and inhibit coupling of tyrosyl residues in T3 or T4.
- therefore, less T3, T4 made

27
Q

What’s the aim for tT4 level for cats on methimazole?

A

at or below mid reference range

28
Q

When is clinical improvement noted in cats on methimazole?

A

oral: 3-4 weeks
transdermal: 4 weeks, response is less consistent compared to oral

29
Q

What are some severe, life threatening side effects of methimazole in cats?

A

typically noted in the first 3 months
- hepatotoxicity
- thrombocytopenia
- bleeding diathesis
- agranulocytosis/ neutropenia

30
Q

What are some common, mild side effects of anti-thyroid medications?

A
  • GI: nausea, vomiting, diarrhea, lethargy (more common with carbimazole)
  • mild leukocyte changes: lymphopenia, eosinophilia
  • facial excoriations
31
Q

What should be done if mild side effects from anti-thyroid medications are noted?

A
  • CBC changes should be monitored but usually resolve with time. No need to discontinue treatment
  • facial excoriations will require discontinuation of the medication
32
Q

What are some side effects associated with I131?

A

typically well tolerated
- post treatment renal insufficiency
- hypothyroidism

33
Q

What are some side effects of surgical thyroidectomy?

A
  • hypocalcemia (6%, usually transient, seen in 2-3d post surgery)
  • death (3%)
34
Q

How well does diet modification work as treatment for feline hyperthyroidism?

A

iodine restricted diet (hill y/d)
- limited data
- T4 normal in about 4 weeks (68% of cats; 75% by 8 weeks), but still in upper range of normal
- questionable whether that level of suppression is enough to improve clinical signs
- but, these cats also don’t develop azotemia
- no other diet/ treat/ prey can be ingested

35
Q

How can post treatment renal insufficiency be predicted?

A
  • difficult
  • USG does not predict the risk of post treatment renal insufficiency
  • can try using reversible treatment (ex. medications) first to see how the kidneys functions, but no study to prove its benefit
36
Q

How is post treatment renal insufficiency managed?

A

studies have shown that hypothyroidism and azotemia can shorten survival time compared to euthyroid and azotemic patients –> therefore, can supplement thyroxine

Internal Medicine/ Ettinger (93 decks)

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