Feline Hyperthyroidism Flashcards by Vivian Fan

What type of cancer is feline hyperthyroidism?

benign, mostly hyperplasia/ adenoma

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

multifactorial
- genetic mutation
- food
- environmental contamination

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How many lobes are usually affected in feline hyperthyroidism?

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

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What’s the typical signalment for feline hyperthyroidism?

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

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

weight loss
increased appetite
hyperactivity
v/d
poor grooming
PU/PD
behavioural changes

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What’s apathetic hyperthyroidism?

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

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Is “thyroid slip” indicative of hyperthyroidism?

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

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What cardiovascular changes are noted in cats with hyperthyroidism?

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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Does feline hyperthyroidism cause hypertension?

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

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How does feline hyperthyroidism lead to increased GFR?

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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How does feline hyperthyroidism lead to PU/PD?

possibly due to decrease aquaporin channels and increased tubular solute excretion

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How does feline hyperthyroidism decrease creatinine?

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

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How does hyperthyroidism lead to vomiting?

unclear in cats

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How does hyperthyroidism lead to diarrhea?

Thyrotoxicosis decrease GI transit time/ increases GI motility

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How does thyrotoxicosis influence CBC?

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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What biochem changes are noted in feline hyperthyroidism?

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

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

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

What’s occult hyperthyroidism?

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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

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

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diabetes mellitus
CKD
congestive heart failure
IBD
hepatic disease
neoplasia
inflammatory airway disease

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

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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

How reliable is TSH testing in feline hyperthyroidism?

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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)

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

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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

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

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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

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

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

What are some main options to treat feline hyperthyroidsim?

- thioureylene anti-thyroid drugs - radioiodine - surgery - dietary elimination of iodine

How does anti-thyroid drugs work?

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

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

at or below mid reference range

When is clinical improvement noted in cats on methimazole?

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

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

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

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

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

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

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

What are some side effects associated with I131?

typically well tolerated - post treatment renal insufficiency - hypothyroidism

What are some side effects of surgical thyroidectomy?

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

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

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

How can post treatment renal insufficiency be predicted?

- 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

How is post treatment renal insufficiency managed?

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

Feline Hyperthyroidism Flashcards

(36 cards)