Small Animal > Thyroid Disorders > Flashcards

Thyroid Disorders Flashcards

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1
Q

true or false: most t4 & t3 is protein bound

A

True! and only unbound thyroid hormone is biologically active

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2
Q

is T3 or T4 more biologically active?

A

T3 is more active than T4

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3
Q

2 mechanisms of hypothyroidism

A

congenital - RARE
acquired - lymphocytic thyroiditis [auto-immune] (50%) or idiopathic thyroid atrophy (50%)

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4
Q

what happens in lymphocytic thyroiditis

A

the thyroid tissue gets destroyed so there is no tissue to produce T3/T4
TRH and TSH are extremely high, but no thyroid to respond

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5
Q

signalment of hypothyroidism

A

middle aged dogs (mean = 7 years) but purebreds may develop signs younger
sterilized dogs are the highest risk

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6
Q

what is the prevalence of hypothyroidism

A

0.2-0.8% of dog population (probably towards the lower end)

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7
Q

breeds at risk of hypothyroidism

A

any dog can develop, but tends to be more common in purebreds
golden retriever!!!! Doberman pinscher, beagle, daschund, minature schnauzer, Great Dane, mini poodle, etc

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8
Q

hypothyroidism clinical signs

A

falls into metabolic or dermatologic
metabolic: lethargy, weight gain, cold and exercise intolerance
dermatologic (80-93%): alopecia and poor hair coat CLASSIC, hyperkeratosis and hyperpigmentation, infections: pyoderma/demodicosis/otitis externa (can present like allergies)

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9
Q

clinical signs of hypothyroidism that aren’t metabolic or dermatologic (less common)

A

neurological: probably - peripheral polyneuropathy, central vestibular disease and maybe laryngeal paralysis, facial nerve paralysis, megaesophagus

muscular: myopathy

ocular: corneal lipid deposits

cardiac: may reduce systolic function - but probably not a big deal

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10
Q

what is myxedema coma

A

even more rare - life threatening complication of hypothyroidism usually associated with precipitating factors (surgery, cardiac disease, sepsis, etc)

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11
Q

myxedema coma hallmark

A

non-pitting edema formation - mucopolysaccharide accumulation under skin via reduced inhibition of GAG formation and reduced clearance

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12
Q

patients with myxedema coma present with

A

hypothermia (no shivering), bradycardia, weak/anorectic, and collapsed

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13
Q

how does congenital hypothyroidism present?

A

any of the other clinical signs
significantly stunted growth and MSK abnormalities (wide skulls, short limbs, macroglossia)
delayed dental development
+/- goiter

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14
Q

what will you see on CBC/Chem with hypothyroidism??

A

CBC: often mild, non-regenerative anemia (50%) +/- thrombocytosis

Chem: hypercholesterolemia!!! (75%), hypertriglyceridemia (88%) !!! Possible mild ALP and ALT increases (results from storing lipids not used) but if moderate or marked, look for another reason

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15
Q

if you don’t have which findings on Chem its likely not true hypothyroidism

A

if you don’t have an increase in lipids (hypercholesterolemia and hypertriglyceridemia )

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16
Q

options for thyroid hormone testing

A

Total T4
Free T4
TSH

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17
Q

should you choose the patients you test carefully for thyroid disorders?

A

yes! patient is stable + has clinical signs + elevated lipids on chem

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18
Q

Total T4 is very sensitive (Sn: 89-100%) and mildly specific (73-82%). What does that mean?

A

If disease is there, likely to catch it (few false negatives)
but lots of false positive due to Sp not being as high

Total T4 is an excellent screening test and not very expensive!!!

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19
Q

Free T4 is very specific (93-94%) and sensitive (80-98%). What does that mean?

A

Very few false positives and false negatives.

Very strong confirmatory test, but expensive.

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20
Q

TSH is sensitive (58-87%) and VERY specific (82-100%). What does that mean?

A

Few false positives.
Strong confirmatory test, but expensive.

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21
Q

what is the usual approach to hypothyroidism testing?

A

suspect based on clinical signs and history –> test TT4

if TT4 is low or low normal T4 - use FT4 +/- TSH - low FT4 or high TSH - HYPOTHYROIDISM!!

if TT4 is normal, reconsider diagnosis.

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22
Q

which test by itself has the best combo of Sn/Sp?

A

FT4 but it is very $$$$ and takes 5-7 days to come back

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23
Q

should you test for thyroid probs in series or parallel?

A

SERIES –> only proceed to the next test if you receive a positive result ie TT4 low then test for FT4

(in parallel would be testing both at the same time and interpreting)

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24
Q

Testing pitfalls - hypothyroidism (4)

A

non thyroid illness will VERY commonly reduce TT4
TT4 and FT4 decline with age
Breed - TT4 values below normal common in greyhounds/whippets/salukis
Medications that affect T4 - phenobarbital, glucocorticoids, sulfas, clomipramine, aspirin

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25
how to treat hypothyroidsm? what should the initial dose NOT exceed?
levothyroxine at a dose of 0.02 mg/kg PO q12h initial dose should NOT be above 0.8 mg/dog
26
Myxedma coma - how to treat
5 mcg /kg IV q12h - hard because not a lot of injectable levothyroxine
27
how to monitor hypothyroid patients
the rule of "4-6" check TT4 in 4-6 weeks check 4-6 hours post pill
28
if you treat hypothyroidism in an animal that doesn't need it, what happens?
clinical signs of hyperthyroid --> heart disease, vascular incidents, agitation, weight loss
29
what is the pathophysiology of hyperthyroidism?
benign neoplasm - functional adenomatous hyperplasia OR functional adenoma -often bilateral (70%) carcinomas VERY rate
30
what is the most common endocrinopathy of the cat?
hyperthyroidism
31
signalment - hyperthyroidism
mostly a disease of senior cats (mean age = 13 years) common condition - 6-10% of cats > 9 years old no clear sex predilection DSH/DLH are at increased risk relative to purebreds
32
clinical signs - hyperthyroidism
weight loss - 92% polyphagia - 55% Palpable thyroid nodule - 98% muscle wasting - 77% less common signs: vomiting, hyperactivity, PU/PD, dental dz, low BCS, tachycardia, heart murmur, diarrhea
33
what causes cardiac disease in hyperthyroid cats
increased chronotropic / inotropic effects and increased myosin - net effect is myocardial hypertrophy (HCM phenotype but not HCM per se) may progress to congestive heart failure if untreated
34
is hyperthyroidism cardiac disease reversible?
mostly reversible with treatment of hyperthyroidism
35
hyperthyroidism is commonly associated with ___________?
hypertension (10-30%) identify and treat so retinas don't tear!!! may develop at any time even during treatment
36
if blood pressure is still high despite treatment for hyperthyroidism, what should you do?
start on blood pressure meds!!
37
CKD and hyperthyroidism often occur together in senior cats. Which one hides the other?
hyperthyroidism masks kidney disease because it increases GFR and therefore filtration of azotemic compounds which artificially reduces renal parameters on chem panel hyperthyroidism and hypertension can also cause CKD
38
treatment of hyperthyroidism unmasks kidney disease because
the return to normal T4 status reduced GFR revealing true renal values
39
how does hyperthyroidism cause CKD
hypertension can cause target organ damage
40
what is apathetic hyperthyroidism ?
depression, weight loss, lethargy and anorexia!!! happens in roughly 10% hyperthyroid cats
41
what is a thyroid storm
life threatening, fulminant increase in thyroid hormone can be caused by: damage to thyroid from radioactive iodine, aggressive palpation, but very uncommon now
42
hyperthyroidism on CBC/Chem/UA
CBC - erythrocytosis +/- microcytosis Chem - Increased ALT (80-90%- not huge increases, increased ALP (50%), Azotemia (concurrent CKD? Prerenal?) UA - isosthenuria common and 4% have positive culture but don't tx unless clinical signs
43
how to test for hyperthyroidism
total T4 Free T4
44
can you diagnose hyperthyroidism in cats with TT4?
yes! highly sensitive and specific for advanced disease but can miss 10% of all hyperthyroid cats and 40% with mild disease
45
what is the rule of TT4 testing in cats?
if TT4 is "high normal" you may still have hyperthyroidism
46
Free T4 in cats - when to use
only necessary if TT4 results are equivocal. 98% accurate but very expensive if O can't pay for this, retest TT4 in 2-3 weeks
47
testing approach to hyperthyroidism
TT4 if high, hyperthyroidism! TT4 - high-normal - run Free T4 - if Free T4 high - Hyperthyroidism! if Free T4 normal - reconsider diagnosis if TT4 is normal or low - reconsider diagnosis
48
hyperthyroidism treatment choices (4)
methimazole iodine restriction diet radioactive iodine surgery in order of commonality
49
methimazole mechanism of action
concentrates in the thyroid and inhibits thyroid peroxidase, therefore decreasing T4 production
50
dose of methimazole
1.25-2.5 mg/cat BID PO - start at higher end, especially if given transdermal
51
transdermal formulations of methimazole are common - what are the downsides?
unpredictable bioavailability and longer time to onset of effect
52
how to monitor methimazole
3 weeks after starting, perform a CBC/Chem/TT4 as kidney dz may be present and methimazole can cause disease such as IMHA/IMPA monitor every 3-6 months after
53
methimazole adverse effects - common in first 1-3 months
anorexia, vomiting, lethargy (25% of cats) --> you're safe to continue eosinophilia, leukopenia, lymphocytosis --> monitor! thrombocytopenia, IMHA, severe leukopenia - STOP hepatotoxicity - STOP facial excoriations (idiosyncratic dermatitis - will not get better during Tx) - STOP
54
are GI adverse events more common for oral or transdermal formulations of methimazole
oral! GI effects less common for transdermal. However, all other side effects are more common with transdermal administration
55
Low iodine diet - how does it work?
iodine is critical in T4 production. Restriction = reduced T4 less expensive, very simple but not as good long term (euthyroid in 72% of cases by day 90 however lots of cases will be regulated by a month in ish) safer though
56
how to monitor while on low iodine diet
same as methimazole! 4 weeks after starting therapy, hen every 3-6 weeks
57
what is an important consideration for low iodine diets?
only recommended in single cat households - they can't steal ANYTHING else because if they do their diet is ruined :(
58
pros of radioactive iodine
very effective and safe! well tolerated and arguably the best solution as it's curative!
59
how does radioactive iodine (I-131) work
given by IV or SQ injection and it concentrates in the thyroid and is locally desctructive 95-98% euthyroid within one dose
60
downsides of radioactive iodine
expensive upfront - $5-6000 special rooms, permits, handling and waste disposal requirements 30% of patients become hypothyroid and its permanent so may have to give levothyroxine for life cats have to be stable and healthy before they enter therapy because there are restricted access for diagnostics and handling during 3-7 days of isolation
61
do we ever do surgery for hyperthyroidism?!
not anymore. Usually pretreated with anti-thyroid drugs prior to procedure to stabilize metabolic effects with a low complication rate and permanent cure. >90% of cats are permanently hypothyroid if lesion was bilateral.
62
ECC thyroid treatments
beta-blockers (Atenolol) treats tachycardia, arrhythmias, improves hypertension, and is especially useful in a thyroid storm amlodipine - drug of choice for hypertension in cats
63
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