Feline Hyperthyroidism Flashcards
What type of cancer is feline hyperthyroidism?
benign, mostly hyperplasia/ adenoma
What’s the likely cause(s) of feline hyperthyroidism?
multifactorial
- genetic mutation
- food
- environmental contamination
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
What’s the typical signalment for feline hyperthyroidism?
mid age to older (median 12.5yo)
no breed or sex predilection
What’s the most common c/s for feline hyperthyroidism?
- weight loss
- increased appetite
- hyperactivity
- v/d
- poor grooming
- PU/PD
- behavioural changes
What’s apathetic hyperthyroidism?
uncommon, when the cat is actually lethargic/ obtunded with poor appetite
Is “thyroid slip” indicative of hyperthyroidism?
no, can be noted in up to 70% of euthyroid cats
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
Does feline hyperthyroidism cause hypertension?
difficult to know. there is an association, but hard to prove causation
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
How does feline hyperthyroidism lead to PU/PD?
- possibly due to decrease aquaporin channels and increased tubular solute excretion
How does feline hyperthyroidism decrease creatinine?
- increases creatinine tubular secretion
- increases the GFR
- decreases muscle mass
BUT 25% of the cats are still azotemic
How does hyperthyroidism lead to vomiting?
unclear in cats
How does hyperthyroidism lead to diarrhea?
Thyrotoxicosis decrease GI transit time/ increases GI motility
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
What biochem changes are noted in feline hyperthyroidism?
- 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?
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?
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?
- 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?
- 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?
- 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?
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?
- 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?
- 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
