Exam 4 - Hyperthyroidism Flashcards
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
what clinical signs are associated with increased metabolic rate from hyperthyroidism in cats?
weight loss
hunger
hyperactivity & restlessness
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
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
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
what is the rule of 7?
add a T4 to your diagnostic work up plan for any cat 7 years or older
what signalment of cat is most commonly affected by hyperthyroidism?
most cats are > 10 years
has been seen in cats as young as 3
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
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
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
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
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
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
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
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)
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)
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
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
how long must a cat be off of methimazole before thyroid scintigraphy?
2 weeks prior to scanning
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
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
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
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?
- medical management - methimazole, twice daily dosing
- diet change - iodine deficient diet
- surgical removal
- 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?
- functional thyroid carcinoma - most are non-functional
- 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
