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SA Medicine Endocrine > Hyperthyroidism > Flashcards

Hyperthyroidism Flashcards

1
Q

Definition of hyperthyroidism

A
  • excessive production and secretion of T4 and/or T3 by the thyroid gland
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2
Q

Hyperthyroid species predilection

A
  • most common endocrine disorder in cats

- One of the most common geriatric diseases in cats

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3
Q

What is the most common pathology behind hyperthyroidism in cats?

A
  • Adenomatous hyperplasia
  • Adenoma
  • Benign
  • 95-98% of hyperthyroid cats
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4
Q

More common to have unilateral or bilateral disease in hyperthyroid?

A
  • Bilateral in ~70%
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5
Q

How common is thyroid carcinoma in cats with hyperthyroid?

A
  • <2-5%
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6
Q

Prevalence of hyperthyroid?

A
  • Steadily increasing since around 1980
  • Cause for changes unknown
  • Reason for increased prevalence unknown
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7
Q

Proposed etiologies for hyperthyroid

A
  • Genetic
  • Nutritional
  • Environmental
  • Immune-mediated
  • Infectious
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8
Q

Nutritional etiology possibilities for hyperthyroid

A
  • Wide variability in iodine consumptions
  • Selenium deficiency
  • Commercial diets, especially canned food (liver, fish, giblet)
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9
Q

Environmental etiology possibilities for hyperthyroid

A
  • Cat food cans (BPA)

- Water pollutants

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10
Q

Age of cats with hyperthyroid

A
  • Older
  • Mean 13 years
  • less than 5% <10 years
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11
Q

Clinical signs of cats with hyperthyroid

A
  • Weight loss
  • Polyphagia
  • PU/PD***
  • Hyperactivity
  • Vomiting
  • “Apathetic” ~10%
  • Anorexia, depression
  • May not be noticeable early in the disease
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12
Q

Physical exam findings of cats with hyperthyroid

A
  • Thin
  • Palpable thyroid (“slip”)
  • Hyperactive
  • Poor hair coat
  • Dehydration
  • Poorly behaved
  • Possible cervical ventroflexion due to muscle weakness
  • Cardiac changes in ~50%
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13
Q

Cardiac changes in hyperthyroid cats

A
  • Increase in Beta receptors
  • Tachycardia (>220 bpm)
  • Murmur
  • Gallop rhythm
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14
Q

Differentials for PU/PD

A
  • Diabetes mellitus

- Renal disease (often concurrent)

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15
Q

Differentials for cardiac disease

A
  • Hypertrophic cardiomyopathy
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16
Q

DfDx for Weight loss/GI disease

A
  • Exocrine pancreatic insufficiency
  • Inflammatory bowel disease
  • GI lymphosarcoma
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17
Q

Diagnosis of hyperthyroid

A
  • History
  • Clinical signs
  • PE
  • Minimum Database findings
  • These are suggestive, but definitive diagnosis based on thyroid levels
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18
Q

CBC changes hyperthyroid

A
  • Increased HCT (dehydration)
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19
Q

Chemistry changes hyperthyroid

A
  • Azotemia (dehydration or concurrent renal disease; 15-20% of treated cats will develop renal insufficiency)
  • Increased ALT** (usually mild to moderate; metabolic rate increases the rate of turnover of hepatocytes; most common and most striking abnormality
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20
Q

Urinalysis changes hyperthyroid

A
  • Isosthenuria common

- If you see it around 1.030

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21
Q

What is the screening test of choice for hyperthyroid?

A
  • Total T4

- High sensitivity and specificity for hyperthyroidism if increased

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22
Q

What to do if you have a cat that appears hyperthyroid and has CBC/Chem/UA changes of hyperthyroid but normal TT4?

A
  • Occurs in 10% of more hyperthyroid cats
  • First step is to repeat a total T4 at a different time or different lab on a different day
  • Free T4
  • T3 suppression test
  • Scintigraphy if all else fails
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23
Q

Why can total T4 be normal in a patient with hyperthyroid?

A
  • Daily fluctuation

- Non-thyroidal illness or euthyroid sick

24
Q

Repeating T4

A
  • Look for daily fluctuation

- Either at a different time point or using a different lab method

25
Q

Free T4 by Equilibrium Dialysis

A
  • More sensitive than total T4 (96%) but less specific than total T4 (i.e. more false positives)
  • Will still be elevated in small portion of euthyroid sick cats
  • Use in combination with total T4
  • More useful in strong suspects with normal T4
26
Q

T3 suppression test when to use?

A
  • As a last resort after TT4 and FT4
27
Q

Theory of T3 suppression

A
  • T3 should inhibit TSH production
  • Decreased TSH –> decreased T4
  • Following T3 administration, T4 should be <50% baseline in normal cats
  • Minimal suppression in hyperthyroid cats
28
Q

T3 suppression test procedure

A
  • Baseline tT4 and tT3
  • Administer 25 mg of T3 PO q8hr for total 7 doses
  • Measure tT4 and tT3 again on the morning of day 3
  • tT3 measurement to ensure medication was given (and received)
29
Q

Nuclear scintigraphy

A
  • Radioactive isotope administered
  • Taken up similarly to iodine
  • Hyperthyroid cats have an increased uptake
  • Gamma emission then quantified
  • Confirms hyperthyroidism
  • Can determine if unilateral vs bilateral (surgical planning)
  • Surgery not an option of intrathoracic
  • Helps differentiate adenoma from carcinoma (metastasis)
30
Q

Normal nuclear scinitgraphy

A
  • Iodine in salivary glands and thyroid lobes
31
Q

Blood pressure in cats with hyperthyroid

A
  • > 180 mmHg

- Also associated with cardiac disease and renal disease

32
Q

End organ damage with hypertension

A
  • Brain
  • Eyes
  • Heart
  • Kidneys
33
Q

Hypertensive fundus

A
  • Tortuous retinal arteries and venules

- Intraretinal hemorrhage

34
Q

Echocardiogram on a cat with hyperthyroid

A
  • “Thyrotoxic” cardiomyopathy
  • Similar to HCM
  • Usually resolves with therapy
35
Q

Additional diagnostics to perform on a hyperthyroid cats

A
  • Blood pressure, fundic exam, and echo
36
Q

Treatment categories for hyperthyroid

A
  • Medical
  • Surgical
  • Radioactive iodine
  • Nutritional
37
Q

Medical management for hyperthyroid primary treatment

A
  • Methimazole primarily
38
Q

Methimazole mechanism

A
  • Inhibition of thyroid peroxidase to block thyroid hormone synthesis
  • Lag in effect because it doesn’t affect synthesized hormone
  • Oral or transdermal formulations
39
Q

Advantages of methimazole

A
  • Inexpensive
  • No anesthesia or surgery
  • No hospitalization
  • Reversible
40
Q

Disadvantages of methimazole

A
  • Side effects
  • Frequent blood monitoring
  • Daily medication
  • NOT permanent
41
Q

Reversibility of side effects of methimazole

A
  • MOST are reversible with discontinuation IF caught in time
42
Q

Methimazole side effects

A
  • GI upset (10%, less with transdermal)
  • Neutropenia/Thrombocytopenia
  • Facial excoriation +/- erythema and pruritus
  • Hepatotoxicity (necrosis, degeneration)
  • Renal decompensation
43
Q

“Renal decompensation” with methimazole

A
  • Possible with ANY treatment modality
  • Know that cats ALREADY have renal disease
  • Hyperthyroidism increases the GFR, treatment decreases GFR and thus unmasks renal disease
  • Perform a methimazole “trial” prior to surgery or radiation
  • Recheck renal values and USG when T4 has normalized
44
Q

What would you use to treat hypertension in a patient with hyperthyroid?

A
  • Amlodipine
45
Q

What would you use to treat sympathetic overdrive (tachycardia, arrhythmias, hyperactivity, and aggression) caused by hyperthyroid?

A
  • Tachycardia, arrhythmias, hyperactivity, aggression
  • Beta blockers
  • Propanolol and atenolol
46
Q

Advantages of thyroidectomy for hyperthyroid treatment

A
  • Relatively inexpensive

- Usually permanent

47
Q

Disadvantages of thyroidectomy for hyperthyroid treatment

A
  • Anesthesia
  • Hypoparathyroidism
  • Ectopic tissue
  • Permanent
  • Recurrent laryngeal nerve damage
  • Risk of hypothyroid especially with bilateral removal
48
Q

I-131

A
  • I-131 concentrated in hyperfunctional thyroid cells
  • Beta particles destroy local hyperactive tissue
  • Normal tissue should be preserved
49
Q

How long after I-131 will plasma T4 become normal?

A
  • Within 3 months in 95%
50
Q

Advantages of I-131

A
  • Usually effective
  • No pills
  • No anesthesia or surgery
  • Rapid correction
  • Ectopic tissue
  • Carcinomas (high dose)
  • Permanent
51
Q

Disadvantages of I-131

A
  • Facilities
  • Radiation protocol
  • At least 1 week in hospital
  • Expensive
  • Iatrogenic hypothyroidism (5%)
  • PERMANENT
  • Renal disease might be unmasked at the end
52
Q

Dietary management for hyperthyroid

A
  • Severely iodine restricted diet
  • Inhibits ability to form thyroid hormone
  • Must be VERY low
  • Must be the ONLY DIET consumed by the cat (can’t eat spiders or mice)
53
Q

Advantages of Hill’s y/d

A
  • Promising
  • Good survival
  • Can be used in azotemic cats
  • Simple if one-cat household
54
Q

Unknowns about Hill’s y/d

A
  • Cats with hyperthyroid complications?
  • Other cats in household?
  • Is the control adequate for all cats?
  • I-131 after diet?
55
Q

Prognosis for hyperthyroid

A
  • Variable but generally good

- Affected by age at diagnosis, physical condition, and concurrent disease

SA Medicine Endocrine (8 decks)

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