Exam 4 - Hyperthyroidism Flashcards by Madeleine Swindell

what is the most common endocrine disease in cats? what signalment of cats is commonly affected?

hyperthryoidism

usually geriatric cats - very rare if occurring in cats <7 years of age

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what clinical signs are associated with increased metabolic rate from hyperthyroidism in cats?

weight loss

hunger

hyperactivity & restlessness

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what clinical signs are associated with increased sensitivity to catecholamines from hyperthyroidism in cats?

increased number of beta adrenergic receptors

increased receptor sensitivity

increased free catecholamine levels

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what are the 2 underlying causes of feline hyperthyroidism? which is more common?

benign adenoma or adenomatous hyperplasia - very common
malignant carcinoma - less than 2% of cats, may feel like benign lesions, overtly invasive with attachment to surrounding tissues, & may have multiple masses in cervical region or distant mets

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what are the proposed causes of feline hyperthyroidism?

tendency for nodular hyperplasia & follicular cysts occur with age

genetic predisposition

variations in dietary iodine content

soy-based isoflavones in the diet

thyrotoxic substances in the environment - lining of pop top cat food cans & fire retardants

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what is the rule of 7?

add a T4 to your diagnostic work up plan for any cat 7 years or older

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what signalment of cat is most commonly affected by hyperthyroidism?

most cats are > 10 years

has been seen in cats as young as 3

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what are the most common clinical signs associated with hyperthyroidism in cats?

weight loss - very consistent & predictable finding, may be dramatic in advanced cases with a cachetic appearance, low muscle condition score, normal cats may have a normal weight if they were obese before

polyphagia - pronounced in early disease course

hyperactivity/night time wandering/disrupted sleep patterns - increased vocalization, anxiousness, & irritability

pu/pd

vomiting - often occurs after eating, due to deranged gastric motility

diarrhea/increased fecal volume - due to intestinal hypermotility & malabsorption

hair coat changes - unkempt, greasy, dry, scaly

respiratory changes - panting, secondary to cardiac compromise

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what may be seen on physical exam that supports a suspicion for hyperthyroidism in cats?

tachycardia, > 200 bpm, agitated, poor BCS, loss of muscle mass

auscultation - murmur or gallop sound due to thyrotoxic cardiomyopathy

retinal exam - may see hypertensive retinopathy, so if you see this, be suspicious of concurrent ckd

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how do you do cervical palpation in a cat when trying to diagnose hyperthyroidism?

normal thyroid lobes are not palpable!!!

palpate along the trachea ventral to the larynx & down to the thoracic inlet - keep your fingers loose & feel for the slip as the nodule shoots by

30% unilateral & 70% bilateral

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what is seen on a cbc from a hyperthyroid cat?

hematocrit is usually robust due to the effect of T4 on erythropoiesis

anemia suggests concurrent disease

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what is seen on a urinalysis from a hyperthyroid cat?

increased renal blood flow can drop the USG

most cats maintain a USG > 1.035

very poorly concentrated urine is uncommon

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what is seen on a biochemistry panel from a hyperthyroid cat?

liver enzymes, ALT/ALP increased in 85% of cats

bun & creatinine should be within reference range in uncomplicated patients, but an increase in the ratio can occur due to polyphagia & muscle wasting

cholesterol is usually within reference range

glucose usually normal, but fructosamine may be lower due to increased protein turnover

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how can hyperthyroidism mask ckd in cats? how can you counteract this?

creatinine is lowered by increased renal blood flow & gfr

creatinine also lowered by the loss of muscle mass

SDMA - better assessment of renal status, elevated SDMA strongly predicts underlying CKD, but a normal SDMA doesn’t rule this out

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what is the screening test of choice for hyperthyroidism in cats? how do you interpret results?

serum thyroxine - total T4, cheap & quick turn around with very few false positives

increased value in an older cat with appropriate signs confirms the diagnosis

some cats may have results in the upper half of the reference range (very early phase of the disease, or can be from concurrent non-thyroidal illnesses such as ckd, ibd, dm, & neoplasia)

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what are the problems with specificity in free T4 testing in cats?

false positives! should never be used as an initial screening test!!

only use in cats with suspicious clinical signs & equivocal tT4 results - 10% of sick cats may have elevated fT4 levels as a manifestation of atypical euthyroid syndrome (these cats have a low or borderline fT4)

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what is the purpose of using a fT4 test in a cat?

more consistently increased than tT4 in early hyperthyroidism or with concurrent disease

should be higher than reference range in 98% of cats with hyperthyroidism

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when would a T3 suppression test be used? how is it done? why do we not do it as often now?

used in cats with equivocal tT4 & fT4 results

measurement of tT4 before & after 7 oral q8hr doses of T3 - T4 should be suppressed in normal cats but not in cats with hyperthyroidism!!!

rarely done now because it is hard to pill a cat 3 times a day

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how long must a cat be off of methimazole before thyroid scintigraphy?

2 weeks prior to scanning

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why is thyroid scintigraphy used for cats with hyperthyroidism?

useful step in planning treatment - provides a lot of information such as normal/abnormal, size of abnormal lobes, unilateral vs bilateral, benign vs malignant, etc

usually required before radiation therapy & helps to identify malignant lesions

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how is endogenous serum thyroid stimulating hormone measured as a way to diagnose hyperthyroidism in cats?

very low or undetectable TSH level with high normal tT4 & mildly elevated fT4 is consistent with hyperthyroidism - newly developed feline TSH assays

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when would you take thoracic radiographs for a cat with hyperthyroidism?

indicated for detection of cardiomegaly, metastatic thyroid disease, or unrelated concurrent diseases that may affect treatment options

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when would you pursue echocardiography for a cat with hyperthyroidism?

indicated for patients with cardiomegaly, arrhythmias, & gallop sounds

evidence of heart failure

when would you take systolic blood pressure measurements for a cat with hyperthyroidism?

degree of hypertension is generally mild

overt evidence of target organ damage is uncommon

what is the mechanism of action of methimazole?

inhibits the synthesis of hormones by blocking oxidation of iodine & coupling of iodothyronines to form T3 & T4 but DOES NOT block the iodine pump!!!

what are the side effects associated with methimazole?

anorexia, vomiting, neutropenia, thrombocytopenia, hepatotoxicity, & facial pruritus

what is the dosing used for methimazole in cats with hyperthyroidism?

starting does of 2.5 mg/kg PO every 12 hours which can later be decreased to 1.25 mg/kg PO for cats with very early or subclinical hyperthyroidism can use transdermal products if cat is hard to medicate - may mitigate vomiting but not other side effects

what monitoring should be done for a cat on methimazole?

monitor tT4, CBC, chem (for liver & kidneys) every 14 days for 3 visits - check for hematologic side effects in ALT or creatinine - increase dose gradually until tT4 levels are in the lower half of the reference range & clinical signs have normalized

T/F: long term use of methimazole is associated with progressive increase in gland size with an apparent risk of malignant transformation

true

T/F: tT4 monitoring in cats can be done anytime after their morning dose of methimazole

true

how is diet change used as a treatment for hyperthyroidism in cats?

hill's y/d diet - < 32 ppm iodine content, so there is insufficient iodine available for incorporation into new thyroid hormone leading to decreased thyroid hormone output & normalization of clinical signs

what are the challenges with using diet change as a way to treat hyperthyroidism in cats?

must be fed as the sole diet - no treats or other food should not be fed to other cats relatively low in protein compared to other standard cat diets not always palatable care in transition from methimazole to prevent an acute drop in tT4

what is a thyroid storm? what causes it?

rare condition - refers to an acute exacerbation of clinical signs of thyrotoxicosis along with varying degrees of organ decompensation (acute CNS, gi, & cardiac compromise) due to increased cellular response to T4 along with an abrupt increase in hormone levels

T/F: thyroid storms can be attenuated by beta blockers

true

what are some of the triggers that have been identified for causing thyroid storms?

infection, thyroid surgery, abrupt discontinuation of anti-thyroid drugs, & aggressive palpation of the thyroid gland

what is iatrogenic hypothyroidism in cats? what causes it?

concern following definitive therapy - iodine or thyroidectomy thyroid levels will drop quickly in most cats but then will return to normal - ectopic tissue becomes active in surgery cases atrophied areas of affected glands are spared during iodine & slowly regain activity

how do you investigate iatrogenic hypothyroidism in cats? what is the concern of this condition?

if T4 is subnormal 6 months post definitive treatment - measure fTSH or cTSH level to determine the body's need for thyroid hormone with an elevated TSH indicating hypothyroidism sustained hypothyroidism will cause a decline in renal function

how is iatrogenic hypothyroidism in cats treated?

begin supplementation with levothyroxine at 0.05-0.1 mg/kg BID or SID

what are the disadvantages of radioactive iodine treatment for hyperthyroidism?

only available at specialty centers requires 7-10 day period of strict isolation after treatment which may not suitable for anxious cats or ones with concurrent diseases like DM because they can't be treated small risk of post treatment hypothyroidism - requires life long supplementation & can worsen pre-existing ckd if concerned about renal function - establish euthyroid state prior to radiation & check creatinine

what are the advantages of using radioactive iodine treatment for hyperthyroidism in cats?

safe, simple, & effective in carefully selected patients rapid return to euthyroid state (days or weeks) side effects & complications are rare

how is radioactive iodine used for treating hyperthyroidism in cats?

radioactive iodine is concentrated in the thyroid cells & targets those with hyperplasia by emitting gamma & beta particles that kill the cells low dose treatment is used for benign disease - 95% of patients have permanent resolution of clinical signs high dose treatment used for malignant disease - 75% response

T/F: radioactive iodine treatment is widely regarded as the optimal treatment for hyperthyroidism in cats

true

why is anesthesia a concern for surgery for treating hyperthyroidism?

cats with hyperthyroidism are very sensitive to sedatives/anesthesia anesthesia induced arrhythmias are common

what pre-op assessment & stabilization should be done prior to surgery for treating hyperthyroidism in a cat?

scintigraphy should be performed to determine if the disease is unilateral or bilateral, benign or malignant, and if ectopic tissues are present patients should be euthyroid at the time of surgery - use methimazole

why is hypocalcemia a common post-surgical complication after thyroidectomy?

usually transient but may be permanent if all parathyroid tissue has been removed or destroyed signs include weakness, muscle fasciculations, & seizures occurring hours to days after surgery

other than hypocalcemia, what are common post-surgical complications seen in cats after a thyroidectomy?

horner's syndrome due to vagal nerve damage hypothyroidism - usually not a longterm problem due to the persistence of small amounts of functional thyroid tissue may worsen renal function in cats with early ckd

why is a bilateral thyroidectomy recommended for a hyperthyroid cat only if one side is enlarged?

high incidence of disease in the contralateral side within 12 months!!! 2 stage approach can be performed with a one month gap between with every effort made to identify & preserve the parathyroidnglands

what are your 4 options for treating hyperthyroidism?

1. medical management - methimazole, twice daily dosing 2. diet change - iodine deficient diet 3. surgical removal 4. radioactive iodine treatment

what are the pros & cons of using methimazole for treating hyperthyroid cats?

pros - reversible, dose titrated, affordable, & readily available option cons - twice daily dosing, not curative, requires a lot of monitoring, & side effects are common

what are the pros & cons of using an iodine deficient diet for treating hyperthyroid cats?

pros - reversible, easy to do, little monitoring, affordable, & readily available option cons - no treats, not curative, efficacy is questionable, multi cat households pose a problem, & not a super palatable diet

what are the pros & cons of using surgery for treating hyperthyroid cats?

pros - curative, one time treatment, little monitoring, & benefit of histopathology cons - high cost, not as available, complicated surgery, & invasive procedure

what are the pros & cons of using radioactive iodine treatment for treating hyperthyroid cats?

pros - curative, one time treatment, little monitoring, & few risks cons - high cost, not readily available, requires isolation period, & risk of hypothyroidism

what are the 2 main causes of canine hyperthyroidism?

1. functional thyroid carcinoma - most are non-functional 2. diet-induced - raw food diets that contain tissues from the neck

how is canine hyperthyroidism diagnosed?

increased total T4 low TSH + cervical mass in dogs with a carcinoma need to confirm type with thyroid scintigraphy

T/F: historically, canine hyperthyroidism was very rare but is now becoming more prevalent

true