Hyperthyroidism Flashcards
Definition of hyperthyroidism
- excessive production and secretion of T4 and/or T3 by the thyroid gland
Hyperthyroid species predilection
- most common endocrine disorder in cats
- One of the most common geriatric diseases in cats
What is the most common pathology behind hyperthyroidism in cats?
- Adenomatous hyperplasia
- Adenoma
- Benign
- 95-98% of hyperthyroid cats
More common to have unilateral or bilateral disease in hyperthyroid?
- Bilateral in ~70%
How common is thyroid carcinoma in cats with hyperthyroid?
- <2-5%
Prevalence of hyperthyroid?
- Steadily increasing since around 1980
- Cause for changes unknown
- Reason for increased prevalence unknown
Proposed etiologies for hyperthyroid
- Genetic
- Nutritional
- Environmental
- Immune-mediated
- Infectious
Nutritional etiology possibilities for hyperthyroid
- Wide variability in iodine consumptions
- Selenium deficiency
- Commercial diets, especially canned food (liver, fish, giblet)
Environmental etiology possibilities for hyperthyroid
- Cat food cans (BPA)
- Water pollutants
Age of cats with hyperthyroid
- Older
- Mean 13 years
- less than 5% <10 years
Clinical signs of cats with hyperthyroid
- Weight loss
- Polyphagia
- PU/PD***
- Hyperactivity
- Vomiting
- “Apathetic” ~10%
- Anorexia, depression
- May not be noticeable early in the disease
Physical exam findings of cats with hyperthyroid
- Thin
- Palpable thyroid (“slip”)
- Hyperactive
- Poor hair coat
- Dehydration
- Poorly behaved
- Possible cervical ventroflexion due to muscle weakness
- Cardiac changes in ~50%
Cardiac changes in hyperthyroid cats
- Increase in Beta receptors
- Tachycardia (>220 bpm)
- Murmur
- Gallop rhythm
Differentials for PU/PD
- Diabetes mellitus
- Renal disease (often concurrent)
Differentials for cardiac disease
- Hypertrophic cardiomyopathy
DfDx for Weight loss/GI disease
- Exocrine pancreatic insufficiency
- Inflammatory bowel disease
- GI lymphosarcoma
Diagnosis of hyperthyroid
- History
- Clinical signs
- PE
- Minimum Database findings
- These are suggestive, but definitive diagnosis based on thyroid levels
CBC changes hyperthyroid
- Increased HCT (dehydration)
Chemistry changes hyperthyroid
- 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
Urinalysis changes hyperthyroid
- Isosthenuria common
- If you see it around 1.030
What is the screening test of choice for hyperthyroid?
- Total T4
- High sensitivity and specificity for hyperthyroidism if increased
What to do if you have a cat that appears hyperthyroid and has CBC/Chem/UA changes of hyperthyroid but normal TT4?
- 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
Why can total T4 be normal in a patient with hyperthyroid?
- Daily fluctuation
- Non-thyroidal illness or euthyroid sick
Repeating T4
- Look for daily fluctuation
- Either at a different time point or using a different lab method
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
T3 suppression test when to use?
- As a last resort after TT4 and FT4
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
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)
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)
Normal nuclear scinitgraphy
- Iodine in salivary glands and thyroid lobes
Blood pressure in cats with hyperthyroid
- > 180 mmHg
- Also associated with cardiac disease and renal disease
End organ damage with hypertension
- Brain
- Eyes
- Heart
- Kidneys
Hypertensive fundus
- Tortuous retinal arteries and venules
- Intraretinal hemorrhage
Echocardiogram on a cat with hyperthyroid
- “Thyrotoxic” cardiomyopathy
- Similar to HCM
- Usually resolves with therapy
Additional diagnostics to perform on a hyperthyroid cats
- Blood pressure, fundic exam, and echo
Treatment categories for hyperthyroid
- Medical
- Surgical
- Radioactive iodine
- Nutritional
Medical management for hyperthyroid primary treatment
- Methimazole primarily
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
Advantages of methimazole
- Inexpensive
- No anesthesia or surgery
- No hospitalization
- Reversible
Disadvantages of methimazole
- Side effects
- Frequent blood monitoring
- Daily medication
- NOT permanent
Reversibility of side effects of methimazole
- MOST are reversible with discontinuation IF caught in time
Methimazole side effects
- GI upset (10%, less with transdermal)
- Neutropenia/Thrombocytopenia
- Facial excoriation +/- erythema and pruritus
- Hepatotoxicity (necrosis, degeneration)
- Renal decompensation
“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
What would you use to treat hypertension in a patient with hyperthyroid?
- Amlodipine
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
Advantages of thyroidectomy for hyperthyroid treatment
- Relatively inexpensive
- Usually permanent
Disadvantages of thyroidectomy for hyperthyroid treatment
- Anesthesia
- Hypoparathyroidism
- Ectopic tissue
- Permanent
- Recurrent laryngeal nerve damage
- Risk of hypothyroid especially with bilateral removal
I-131
- I-131 concentrated in hyperfunctional thyroid cells
- Beta particles destroy local hyperactive tissue
- Normal tissue should be preserved
How long after I-131 will plasma T4 become normal?
- Within 3 months in 95%
Advantages of I-131
- Usually effective
- No pills
- No anesthesia or surgery
- Rapid correction
- Ectopic tissue
- Carcinomas (high dose)
- Permanent
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
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)
Advantages of Hill’s y/d
- Promising
- Good survival
- Can be used in azotemic cats
- Simple if one-cat household
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?
Prognosis for hyperthyroid
- Variable but generally good
- Affected by age at diagnosis, physical condition, and concurrent disease
