45 Disorders of the Thyroid Gland I

Feline hyperthyroidism

What is THE MOST COMMON endocrine disorders of cats (especially older cats)?

Functional thyroid adenomatous hyperplasia is the most common form = either from hyperplasia or benign adenoma

What are the more common forms of feline hyperthyroidism?

Excessive circulating T4 and T3

What hormones are the issue with feline hyperthyroidism?

OLDER CATS (only 5% of cats are less than 8-10 years of age)

What is the typical signalment for feline hyperthyroidism?

Feline hyperthyroidism

A 12 year old cat presents with weight loss but is polyphagic. You can tell the cat has an unkempt and ragged haircoat. He is stress intolerant and open mouth breathes in stressful situations. Thoughts?

Malabsorption, parasite burden, diabetes mellitus, hyperthyroidism, chronic kidney disease

What are some differentials for a cat which is ravenous polyphagic but excessive weight loss?

Thyroid slip is palpable, enlarged thyroid glands. This is often present in hyperthyroidism, but absence of thyroid slip doesn't rule it out

note: palpate from the mandible to the ribs for thyroid

What is "thyroid slip" and how is it related to feline hyperthyroidism?

One or both of the thyroid glands are palpably enlarged

note: enlargement is NOT ALWAYS associated with clinical signs, but you should warn the owner the cat MAY develop hyperthyroidism with thyroid slip

What physical exam finding in cats is present in over 80% of hyperthyroidism cases?

Feeling for thyroid slip

What is this?

Increased appetite/polyphagia with mild to severe weight loss

Describe the more common GI signs with feline hyperthyroidism

Increases blood pressure

What does feline hyperthyroidism do to blood pressure?

Because cats affected by hyperthyroidism are often older, they typically have comorbidities. The kidneys is often one of these problems, so a lot of hyperthyroid cats have mild azotemia. This could be related to hyperthyroid, which increases blood pressure, which increases pounding pressure on the glomerulus, which causes scar tissue and damages the kidneys long term

How is feline hyperthyroidism related to the kidneys?

Hyperthyroid increases blood pressure. In doing this, at the beginning, the increased blood pressure can actually be "good for" the kidneys because it increases GFR. This is because a chronic kidney failure kidney would have decreased GFR, and the thyroid issue would mask the kidney issue because it is "compensating" for low GFR with high blood pressure. Long term tho, remember this can cause issues with scar tissue buildup in the glomerulus and actually make the problem much worse.

We say feline hyperthyroidism can be related to azotemia because it increases blood pressure and puts pounding pressure on the glomerulus. However, feline hyperthyroidism can also MASK chronic renal failure. How? And what does this mean?

Thyrotoxic HYPERTROPHIC cardiomyopathy

=this means left ventricular hypertrophy, arrhythmias, effusion or edema consistent with heart failure

What cardiovascular problem might cats with hyperthyroid present with?

They could be affected by an "apathetic" or atypical form (very uncommon), and they usually have comorbidities

What does it mean if you suspect a cat has hyperthyroidism but they aren't eating?

Groups 1 (classical clinical disease), 2, 3, 4, 5, and 6 (clinically normal)

How many groups are there of feline hyperthyroidism?

Elevated liver enzymes (ALT, SAP, AST), SDMA (kidney function biomarker), elevated PCV, lymphopenia, USG over 1.035

note: liver affected because of inc. metabolism. if these values have over a 2-3x increase, consider comorbidities

Name some CBC, chem, and urinalysis changes that may come with feline hyperthyroidism

Increased

Overall, hyperthyroidism in cats is increased or decreased metabolism

Total T4 and Free T4

TOTAL: the typical screening wellness hormone = consistently elevated in hyperthyroidism

FREE: do this one when TotalT4 is normal, but you are still highly suspicious of hyperthyroidism. More SENSITIVE, less specific

Name the hormones you can use to test for feline hyperthyroidism. What is the difference?

HYPERTHYROID CAT: this is radionuclide thyroid scanning (thyroid scintigraphy) using Technetium m99 (pertechnetate) or m99 contrast agent

A cat presents with thyroid slip and you see this on your imaging modality. What is this imaging modality and what is the contrast agent of choice?

b. Technetium m99 (pertechnetate)

Select the contrast agent used in thyroid nuclear scintigraphy scanning:

a. Gandolinium-based agent

b. Technetium m99 (pertechnetate)

c. barium sulfate

d. iodine-based iohexol

1:1

What is uptake for the Technetium m99 (pertechnetate) contrast agent NORMALLY LIKE in nuclear scintigraphy?

1. medication/diet

2. surgery

3. radioactive iodine treatment

What 3 things can you do to manage/treat hyperthyroidism in cats?

If the cat does NOT HAVE renal insufficient, the goal is to get the T4 between 1 and 2.5 (normal is between 1-4)

What is the GOAL of treating feline hyperthyroidism?

Because if a cat has renal insufficiency, remember that hyperthyroid is "helping" the GFR in some cases. So, if you drop the thyroid levels low-normal and the blood pressure decreases dramatically, the kidneys may suffer and GFR will plummet. So, you may need to keep these cats higher than the normal limit (say somewhere around 5) so you can keep some of the compensation for chronic kidney disease without pounding them with insanely high blood pressure

this process is called "titrating treatment" for hyperthyroidism

The goal of treating a cat with feline hyperthyroidism that DOES NOT HAVE renal insufficiency is to get their T4 between 1-2.5. Why is the "no renal insufficiency" distinction so important?

Treat the downstream cardiac events like hypertrophic cardiomyopathy separately (with beta blockers) and there is a chance the effects will reverse with the thyroid corrected too

note: the effects with NOT reverse with DCM, but the problem in general is much less common

What can you do treatment wise if a cat has downstream cardiac effects from hyperthyroidism?

Ultra low iodine diet

note: this is because thyroid hormones are made from iodine and amino acids

What kind of diet change can you make to help supplement treatment of feline hyperthyroidism

a. methimazole

c. tapazole

d. felimazole

note: tapazole and felimazole are two types of methimazole

What medications can be used to treat cats with hyperthyroidism:

a. methimazole

b. itraconzaole

c. tapazole

d. felimazole

e. fluconazole

f. imidazole

Felimazole

What is the name of the FDA approved methimazole antithyroid medication in cats?

c. methimazole

All of the following medications can be used to treat hyperthyroidism in cats. Which is the DRUG OF CHOICE because of its lower incidence of side effects?

a. carbimazole

b. thiamazole

c. methimazole

Methimazole (oral and transdermal)

What medication to treat hyperthyroidism in cats has the MOA of inhibiting synthesis of thyroid hormones?

Dose per CAT per day, NOT per kg.

Start low then titrate up

What is important to remember about the dosing of antithyroid medications like methimazole in cats?

CBC, liver, kidneys

What organs should you monitor once you start to administer antithyroid medications to hyperthyroidism cats?

1. Cytopenia, bone marrow suppression

2. hepatotoxicity

3. facial pruritis

Name the CRUCIAL side effects you need to look out for when treating cats with anithyroid medications like methimazole (DRUG SPECIFIC EFFECTS)

I 131 = concentrates in thyroid gland and radiation destroys functioning follicular cells

What is the radioactive iodine treatment of choice for cats with hyperthyroidism and how does it work?

Adenocarcinomas or adenomas

You can treat hyperthyroidism with radioactive iodine when WHAT is causing it?

1. 2% require second treatment

2. 2% works but recurs

3. 2% develop hypothyroidism

What 3 outcomes do you need to warn the owner of when treating cats with radioactive iodine for hyperthyroidism?

Radioactive iodine

With which treatment modality of feline hyperthyroidism must you quarantine the cat for 8-14 days post treatment?

Hypocalcemia due to hypoparathyroidism (remember the parathyroid sits right on the thyroids)

What is the MOST IMPORTANT complication to look out for with surgery to remove thyroid glands?

Good with treatment and successful management (co-morbidities are a large determinant of long term survival)

What is the prognosis like for feline hyperthyroidism?

CATS: typically BENIGN neoplasia

DOGS: 1/2 benign and 1/2 malignant neoplasia but RARELY functional. because the likelihood for malignancy is higher, prognosis is variable compared to cats

Hyperthyroidism is more a cat than a dog disease. Talk about the difference in causes between cats and dogs

Radiation therapy and chemotherapy (remember the causes are often 1/2 benign or 1/2 malignant adenomas or carcinomas)

What are the big treatments of canine hyperthyroidism?