Hyperthyroidism Flashcards
1
Q
describe hyperthyroidism
A
- much MUCH more common in cats than dogs
-cats (primary hyperthyroidism):
–bilateral adenomatous hyperplasia (70% of cases
–carcinoma (<3%)
–ectopic thyroid tissue: anywhere between base of tongue and base of heart
-dogs:
–iatrogenic (over-supplementation, diet)
-misdiagnosis (thyroid hormone autoantibodies)
–functional thyroid carcinoma: uncommon form of a rare tumor in dogs
- spontaneous hyperparathyroidism in dogs:
-thyroid tumors represent <2% of all tumors in dogs
-most are non functional, some are destructive and actually result in hypothyroid and a FEW are functional and cause hyperthyroid
-most will have a palpable cervical mass!
2
Q
describe clinical presentation of hyperthyroid in cats
A
- older cats
-most >8yr at diagnosis
-mean age 13 years - protected breeds (at lower risk but can still be affected)
-siamese
-himalayan
3
Q
describe hyperthyroid physiology
A
- increased basal metabolic rate
-increased O2 consumption and heat generation - cardiovascular changes:
-increased beta adrenergic receptors/catecholamines
-increase heart rate/strength
-increased cardiac output
-increased systolic blood pressure - renal changes:
-increased renal blood flow
-increased GFR
-increased RAAS activity
-increased renal sodium absorption
-decreased ability to concentrate urine - hematologic changes: increased erythrocytosis
5 GI changes: increased otility
4
Q
describe common owner reported clinical signs of hyperthyroid
A
- weight loss
- ravenous appetite
- unkempt dull coats
- V/D
- behavior change: agitation restlessness, aggressiveness
- PU/PD
5
Q
describe less common owner-reported clinical signs of hyperthyroid
A
- apathetic hyperthyroidism
-opposite signs to standard disease (anorexia, lethargy)
-more likely in advanced cases or those with severe concomitant diseases like CHF or neoplasia - end-organ injury from systemic arterial hypertension
-ocular: acute blindness from retinal detachment, hyphema
-CNS: seizures, acute CNS signs (vestibular, etc.)
HYPERTHYROIDISM IS A LEADING CAUSE OF HYPERTENSION IN CATS
6
Q
describe physical exam of a hyperthyroid cat
A
- hair coat changes
- low BCS/MCS
-some hyperthyroid cats are overweight tho so don’t let BCS rule out! - thyroid slip:
-thyroid palpation: palpate from larynx to thoracic inlet
-enlarged gland will sink down the neck - cardiac abnormalities:
-tachycardia
-murmur
-gallop sound - thickened/ropey intestines
- hypertensive retinopathy on fundic exam
-vessel tortuosity, retinal hemorrhage, retinal detachment
7
Q
describe thyroid storm
A
- rare, severe, life-threatening manifestaiton
- clinical syndrome: acute, exaggerated manifestations of thyrotoxicosis
- precipitated by aggressive thyroid palpation, thyroid surgery, radioactive iodine treatment, stress
- signs:
-hyperthermia (>104)
-CNS dysfunction: seizures, coma, extreme agitation, altered mentation
-neuromuscular dysfunction: LMN weakness, cervical ventroflexion
-cardiovascualr dysfunction: tachycardia, arrhythmias, CHF, systemic arterial hypertension
-GI dysfunction: V/D
8
Q
describe clinicopathologic findings of hyperthyroidism
A
CBC: erythrocytosis
biochem!!:
1. ELEVATED ALT
2. elevated ALP
3. azotemia (10-15%)
4. +/- hyperphosphatemia
urinalysis:
1. variable USG
2. +/- proteinuria, ketonuria
9
Q
describe hyperthyroidism and CKD
A
- frequent comorbidities: both are common diseases of older cats
-CKD can challenge diagnosis of hyperthyroidism in some cats (non-thyroidal illness syndrome can lower TT4 into normal range)
-hyperthyroidism can challenge diagnosis and staging of CKD - hyperthyroidism masks/hides kidney disease for 2 reasons:
-muscle wasting: less muscle mass = less creatinine generated so lower blood creatinine concentrations (over-estimation of true GFR); SDMA can be used to circumvent this
-hyperthyroidism increases renal blood flow GFR, which damages nephrons and accelerates progression of CKD
- hyperthyroidism harms the kidneys:
-extra-renal increase in GFR is not a reason to not treat hyperthyroidism in cats with CKD!
-is just a reason to be careful when you do
-goal is to achieve euthyroid and avoid hypothyroid status with treatment
10
Q
describe serum creatinine in hyperthyroid cats
A
- serum creatinine will be lower due to increased GFR and decreased muscle mass
-so is a poor indicator of CKD - if a hyperthyroid cat is persistently azotemic with no extra-renal factors = azotemic CKD (can be quite advanced)
- if a hyperthyroid cat is non-azotemic, it may or may not have CKD
-you are unlikely to unmask severe renal disease after treating hyperthyroidism in these cases
-unmasking CKD is more likely in cats with low USG (but not perfect predictor)
-abdominal imaging can also show you kidney changes!!
11
Q
what to expect after treatment of hyperthyroid in cats with CKD?
A
- as we decrease T4:
-decrease cardiac output
-normalize systemic vascular resistance
-decrease RAAS activity - all of which decrease GFR and
-decrease muscle metabolism and increase muscle mass (over months) - will increase serum creatinine concentration, proportional to increase in T4
- but glomerular hypertension and hyperfiltration improve!
12
Q
what are other diagnostic considerations in hyperthyroidism
A
- heart:
-hyperthyroidism causes thyrotoxic cardiomyopathy (proportional to severity/chronicity)
-ECG changes: hypertrophic cardiomyopathy-phenotype (reversible with treatment)
-thoracic rads can rule out CHF - liver:
-enzyme elevation from hyperthyroidism can be severe - GI:
-diarrhea and weight loss mimic enteropathy, so rule out hyperthyroidism in cats with these signs! - blood pressure:
-HYPERTHYROIDISM IS A LEADING CAUSE OF SYSTEMIC ARTERIAL HYPERTENSION IN CATS
-you should measure BP and do a fundic exam in ALL hyperthyroid cats (treated or untreated)
13
Q
describe hormone assessment of hyperthyroidism
A
- hypothalamus TRH: high
- pituitary gland, TSH: low/suppressed
- T4 secretion: high
14
Q
describe diagnosis of hyperthyroid
A
- TT4: screening
-can be used as a diagnostic test
-if clin signs syggest and TT4 s elevated, hyperthyroidism!
-if high-normal and clin suspicion is high, pursue confirmatory testing
-perform annually in older cats - confirmatory testing:
-TT4: if elevated = hyperthyroid
-if high-normal and clinical suspicion is high, and since euthyroid sick can suppress TT4 into reference interval, submit TT4 and fT4 and TSH:
–if high fT4 and low TSH = hyperthyroid likely
-retesting in a few weeks is also always an option - but remember!
-fT4 can be elevated in random samples from some healthy cats
-never use fT4 alone for screening
-only assess fT4 along with TT4 +/- TSH
-always look at the whole clinical picture - TSH considerations:
-historic assay is for canine TSH, meant to detect high TSH in hypothyroidism so is unable to detect TSH in all cats and can give false undetectable result
-use a feline-optimized assay if you can - a low TSH can suggest early hyperthyroidism; usually develops within 1-2 years
-normal TT4 + undetected TSH: check if used canine assay, use a feline assay if you can
-if previously normal TSH now undetectable = developing hyperthyroidism likely
15
Q
describe hyperthyroid treatment
A
2 choices
- medical management (lifelong)
-methimazole
-iodine restricted diet - definitive treatment (hopefully one and done)
-radioactive iodine (I-131)
-thyroidectomy: less common now, high morbidity
16
Q
describe an iodine restricted diet
A
- ills y/d
-most diets caontain at least 0.46ppm of iodine, hills contains less than 0.2
-response rate is greater than 80% - absolutely the ONLY thing the cat can eat!
-be careful with supplements, meds, water sources
-not for outdoor cats
-palatability can limit use in some cats
-disease control usually within 4-8 weeks (the higher the T4 the longer it can take, up to 6 months)
-do not use concomitantly with methimazole (higher risk of hyperthyroidism)
17
Q
describe methimazole
A
- blocks thyroid peroxidase, inhibiting T3/T4 synthesis
- does not destroy abnormal tissue
- progressive hyperplasia can require progressive dose increase
- adverse reactions:
-direct GI upset: if see, transition to transdermal/compounded formulation
-iatrogenic hypothyroidism: if see, decrease dose +/- drug holiday
–clinical: (low TT4, high TSH, clinical signs) may require supplementation with levothyroxine
–subclinical (normal TT4, high TSH, no clin signs)
-facial excoriations: if see STOP IMMEDIATELY
-hepatotoxicity
-blood dyscrasias: thrombocytopenia, neutropenia
-myasthenia gravis
18
Q
describe methimazole transdermal formulation
A
- when to use:
-fractious cats
-clients cannot administer oral meds
-GI upset on oral meds - differences from oral:
-usually takes higher dose to control disease (but still start as same dose you would use PO)
-takes longer to gain control
19
Q
describe methimazole monitoring
A
- recheck TT4 and TSH
-oral: 2 weeks after start/changing treatment and periodically
-transdermal: 3-4 weeks after start/changing treatment and periodically
-high TSH suggests hypothyroidism (either overt (low TT4) or subclinical (normal T4))
–iatrogenic hypothyroid worsens azotemia and decreases survival in these cats - therapeutic goal:
-TT4 mid to low normal with normal TSH - monitoring for adverse effects/changes in clinical abnormalitiers
-concurrently perform CBC and chem for first 1-2 months
-always measure BP even after treatment!!
20
Q
describe radioactive iodine (I-131)
A
- destroys hyepractive thyroid tissue while sparing suppressed normal tissue
- healthy suppressed thyroid cells should not take up I-131
- a pre treatment undetectable TSH is helpful to confirm this! - hyperfunctional thyroid cells take up I-131
- I-131 emits beta radiation, killing cells within 1-2 min
-should spare most normal cells
-these normal cells “wake up” to produce T4 in 1-3 months - treatment of choice for most hyperthyroid cats
-best for physiologically stable that can be safely maintained in isolation for a few days
-treats: thyroid adenoma, thyroid carcinoma, ectopic thyroid tissue - one-time SQ injection; isolation for approx 5 days
-$2200-2600: but cheaper than lifelong methimazole and associated monitoring
-success: 90-95%, less than 5% need a second injection
21
Q
describe pre-referral considerations for radioactive I-131 treatment
A
- ideal candidate is systemically stable to enter isolation: we cannot touch them!!
- BP, CBC, chem, urinalysis, +/- imaging
- high TT4 with undetectable TSH makes it safer
-normal tissue is less likely to take up I-131 - must be off methimazole for 1-2 weeks prior
-off hill’s y/d for 2-4 weeks prior - methimazole trials are not usually needed before I-131 BUT
-should be considered in untreated cats with:
–azotemia in the later IRIS CKD stage 2 or greater; will cat decompensate in isolation?
–not needed for cats in IRIS stage 1 or early stage 2, azotemia may worsen but will likely not decompensate in isolation
22
Q
describe post I-131 monitoring
A
- recheck TT4 and TSH
-1, 3, 6, 12 months
-most controlled within 1 month but full response can take 3-6 months
- <5% will develop clinical hypothyroidism
-worsens kidney funciton and decreases survival
-requires T4 supplementation
-usually resolves within 1-3 months - monitor for changes in clinical abnormalities
-concurrently perform BP and assess kidney function + any other abnormalities present pre-treatment
23
Q
describe antihypertensive therapy for hyperthyroid
A
- persistently hypertensive, hyperthyroid cats need antihypertensive therapy
-telmisartan or amlodipine - treatment of hypothyroid might not resolve hypertension
- hypertensive target organ damage is occurring while you wait for euthyroidism
-hypertension is a silent killer!
