522: Jan 18 Feline hyperthyroidism

What is the three potential causes of feline hyperthyroidism? Highlight the most common

*1. Adenomatous hyperplasia* 2. Adenocarcinoma (2-5%) 3. Fucntional ectopic tissue

True or False, other cats in the home can also eat the thyroid diet without consequence.

*false* although one study found that as long as they get a tablespoon of maintenance food per day they will be okay

True or False, while hyperthyroidism "masks" renal disease, it can also be contributing to renal damage.

*true* Hypertension and increased GFR can cause damage to the kidneys themselves.

How do you differentiate thyrotoxic cardiac disease from primary hypertrophic cardiomyopathy?

*you cannot* However, with thyrotoxic cardiac disease it is often reversible with thyroid therapies.

What is thyrotoxic cardiac disease?

- Hyperthyroid cats often present with tachycardia, murmur and other evidence of hypertrophic heart disease

Describe 2 reasons why hyperthyroidism can "mask" renal disease:

1) hyperthyroidism causes hypertension which leads to increased GFR --> increased GFR helps excrete BUN/creatinine keeping them in reference ranges. 2) Cachexia secondary to hyperthyroidism can cause decrease creatinine because of the drop in muscle mass

If you have a cat whose total T4 is in the mid to high portion of the reference range and you highly suspect hyperthyroidism how should you proceed diagnosticalyl?

1) retest 2) free T4 test

What three major changes do you expect on serum chemistry in a patient with hyperthyroidism?

1. Increase in ALP (likely the bone isoform) 2. Dramatic T4 elevations 3. Increased P (bone turn over)

What are the four potential treatment options for feline hyperthyroidism?

1. Methimazole 2. Radioactive iodine 3. Thyroidectomy 4. Diet

Who are considered good candidates for dietary control of hyperthyroidism?

1. Newly diagnosed cases 2. Cases wishing to switch from daily meds (methimazole) 3. cases not eligible for radioactive iodine

What four components are used to diagnose feline hyperthyroidism?

1. Palpable thyroid nodule 2. History 3. Elevated T4 4. Appropriate clinical signs

What are the five most common clinical signs of feline hyperthyroidism?

1. Palpable thyroid nodule 2. Weight loss 3. Polyphagia 4. Hyperactivity 5. Tachycardia

What is the proper way to palpate a cats thyroid gland?

1. Raise the cat's chin to 45 degree angle 2. Run your index finger and thumb down either side of trachea 3. move from the larynx to the thoracic inlet 4. feel for nodule passing under your finger

What five factors should you consider when creating a treatment plan for a hyperthyroid cat?

1. Reversible 2. Cost 3. What tissue will it be effective against 4. Separation from pet 5. Ease

What are three severe adverse reactions to methimazole that require drug cessation?

1. facial excoriations 2. cytopenias 3. hepatic toxicity *all of which are rare*

How does concurrent illness affect total T4?

Concurrent illness can falsely elevate total t4 (6-12% of cases)

Is concurrent illness in cats a concern for confounding your total T4 results like it is in dogs?

Concurrent illness will lower Total T4 in cats but not to the same extent as dogs and usually not to the low point of the refeerence range.

How do dogs with thyroid tumors typically present?

Dogs will typically have a non functional malignant adenocarcinoma that will show up as a large mass on their neck.

What is the most common side effect of methimazole?

GI upset - it is self limiting and can be avoided by starting at a lower dosage

What is radioactive iodine therapy?

Injection of radioactive iodine subQ attacks abnormal functioning thyroid tissue while sparing the normal tissue.

What is the main benefit to treating hyperthyroidism with methimazole?

It is reversible - you can balance the hyperthyroidism with clinical signs of kidney disease using dose.

If you have mild hyperthyroidism with marked hepatic enzyme changes what should you suspect?

Liver enzymes should be related to the increase in thyroid hormone - if liver enzymes are markedly higher you should consider possible concurrent hepatic disease.

What is a potential adverse effect of radioactive iodine therapy?

May develop low T4 after therapy - usually not clinical and will resolve in weeks to months

How must you fee the diet in order for it to be effective?

Must be fed *exclusively* to be effective

What specific abnormalities do you expect on a urinalysis of a cat with hyperthyroidism?

None *But 20% will have a UTI, just like other endocrinopathies you should always culture urine*

What specific abnormalities do you expect on a CBC of a cat with hyperthyroidism?

None may see mild erythrocytosis

What is the typical signalment of cats presenting with hyperthyroidism?

Older cats, usually > 10 y.o.

What is a "thyroid storm"?

Pathophysiology unclear but it is a severe manifestation of hyperthyroidism: - severe hypertension - tachypnea - hyperthermia - seizures or other neuro signs

What is methimazole?

Pill that inhibits thyroid hormone synthesis given orally BID

Why is thyroidectomy not a common treatment choice for feline hyperthyroidism?

Requires anesthesia and these patients are not always great anesthetic candidates. Its a busy area, lots of vessels and nerves: potential for hypothyroidism, hypoparathyroidism and laryngeal paralysis.

What is the big downside to undergoing radioactive iodine therapy?

Requires special facilities and isolation of the animal for 5-7 days. Can be problematic for some owners or animals not healthy enough for isolation.

How does diet work as a treatment for hyperthyroidism?

Specially formulated diet (*Hills Y/D*) that is low in iodine limiting body's ability to produce thyroid hormones.

If you have a feline hyperthyroid patient on methimazole who comes back to your clinic with crusting around the ears what do you suspect?

This is a severe adverse reaction to methimazole and the drug should be stopped immediately.

What diagnostics should you run on a dog with a large palpable mass on his neck?

Thoracic radiographs, ultrasound and basic database. These tumors are highly metastatic.

What is a good first line test for feline hyperthyroidism?

Total T4

What is the pathogenesis of thyrotoxic cardiac disease?

Up regulation of beta receptors in the heart and hypotrophic changes to heart muscle.

You have a cat recently diagnosed with feline hyperthyroidism who is also showing signs of severe hypertension - are you worried?

Yes! Increased thyroid hormone leads to only *mild-moderate* hypertension in 10-20% of patients but it is not usually associated with clinical signs. if you have severe hypertension, look for an underlying cause

How long does the diet take to return thyroid hormone levels back within the reference range?

~8 weeks