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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
Free T4 by Equilibrium Dialysis
- 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
T3 suppression test when to use?
- As a last resort after TT4 and FT4
27
Theory of T3 suppression
- 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
T3 suppression test procedure
- 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
Nuclear scintigraphy
- 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
Normal nuclear scinitgraphy
- Iodine in salivary glands and thyroid lobes
31
Blood pressure in cats with hyperthyroid
- >180 mmHg | - Also associated with cardiac disease and renal disease
32
End organ damage with hypertension
- Brain - Eyes - Heart - Kidneys
33
Hypertensive fundus
- Tortuous retinal arteries and venules | - Intraretinal hemorrhage
34
Echocardiogram on a cat with hyperthyroid
- "Thyrotoxic" cardiomyopathy - Similar to HCM - Usually resolves with therapy
35
Additional diagnostics to perform on a hyperthyroid cats
- Blood pressure, fundic exam, and echo
36
Treatment categories for hyperthyroid
- Medical - Surgical - Radioactive iodine - Nutritional
37
Medical management for hyperthyroid primary treatment
- Methimazole primarily
38
Methimazole mechanism
- Inhibition of thyroid peroxidase to block thyroid hormone synthesis - Lag in effect because it doesn't affect synthesized hormone - Oral or transdermal formulations
39
Advantages of methimazole
- Inexpensive - No anesthesia or surgery - No hospitalization - Reversible
40
Disadvantages of methimazole
- Side effects - Frequent blood monitoring - Daily medication - NOT permanent
41
Reversibility of side effects of methimazole
- MOST are reversible with discontinuation IF caught in time
42
Methimazole side effects
- GI upset (10%, less with transdermal) - Neutropenia/Thrombocytopenia - Facial excoriation +/- erythema and pruritus - Hepatotoxicity (necrosis, degeneration) - Renal decompensation
43
"Renal decompensation" with methimazole
- 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
What would you use to treat hypertension in a patient with hyperthyroid?
- Amlodipine
45
What would you use to treat sympathetic overdrive (tachycardia, arrhythmias, hyperactivity, and aggression) caused by hyperthyroid?
- Tachycardia, arrhythmias, hyperactivity, aggression - Beta blockers - Propanolol and atenolol
46
Advantages of thyroidectomy for hyperthyroid treatment
- Relatively inexpensive | - Usually permanent
47
Disadvantages of thyroidectomy for hyperthyroid treatment
- Anesthesia - Hypoparathyroidism - Ectopic tissue - Permanent - Recurrent laryngeal nerve damage - Risk of hypothyroid especially with bilateral removal
48
I-131
- I-131 concentrated in hyperfunctional thyroid cells - Beta particles destroy local hyperactive tissue - Normal tissue should be preserved
49
How long after I-131 will plasma T4 become normal?
- Within 3 months in 95%
50
Advantages of I-131
- Usually effective - No pills - No anesthesia or surgery - Rapid correction - Ectopic tissue - Carcinomas (high dose) - Permanent
51
Disadvantages of I-131
- Facilities - Radiation protocol - At least 1 week in hospital - Expensive - Iatrogenic hypothyroidism (5%) - PERMANENT - Renal disease might be unmasked at the end
52
Dietary management for hyperthyroid
- 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
Advantages of Hill's y/d
- Promising - Good survival - Can be used in azotemic cats - Simple if one-cat household
54
Unknowns about Hill's y/d
- Cats with hyperthyroid complications? - Other cats in household? - Is the control adequate for all cats? - I-131 after diet?
55
Prognosis for hyperthyroid
- Variable but generally good | - Affected by age at diagnosis, physical condition, and concurrent disease

SA Medicine Endocrine (8 decks)

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