Endocrinology - Hyperthyroidism

Question 1

What is feline hyperthyroidism?

Answer 1

A multi-system disorder resulting from the overproduction and secretion of thyroid hormones

Question 2

True or false: Feline hyperthyroidism is the most common endocrinopathy of cats.

Answer 2

TRUE

Question 3

Whatis the pathophysiology of hyperthyroidism?

Answer 3

Autonomous production of T4 from funcitonal thyroid tumors

Question 4

What can cause hyperthyroidism in cats?

Answer 4

Benign multinodular adenomatous hyperplasia and/or thyroid adenomas (>95%) or thyroid carcinoma

Question 5

What can cause hyperthyroidism in dogs?

Answer 5

Thyroid adenocarcinomas

Question 6

What are the risk factors for hyperthyroidism (unless I mention otherwise, all questions are regarding cats)?

Answer 6

Canned food, variable iodine content and other dietary factors, age, use of cat litter, indoor housing, regular exposure to lawn and flea control products

Question 7

True or False: Hyperthyroidism is likely multifactorial.

Answer 7

TRUE

Question 8

What is the signalment for hyperthyroidism?

Answer 8

Senior cats - 8 years or older (mean age is 13 years)

Question 9

What historical clinical features are associated with hyperthyroidism?

Answer 9

Weight loss (>90%), polyphagia (80%), hyperactivity, vomiting, polydipsia/polyuria, hair coat changes, diarrhea/increased fecal volume, panting, and anorexia/hyporexia

Question 10

What clinical features on PE are associated with hyperthyroidism?

Answer 10

From most to least common: palpable thyroid, thin, tachycardia, hyperactive/difficult to examine, heart murmur, skin changes, small kidneys, increased temperature, and gallop rhythm

Question 11

How does hyperthyroidism affect the cardiovascular system?

Answer 11

Increase sympathetic tone and increased metabolic rate +/- systemic hypertension. It can cause hypertrophied cardiac muscle

Question 12

On PE what physical exam findings will you find with hyperthyroidism?

Answer 12

Tachycardia, murmurs, and gallop rhythm

Question 13

Since hypertension is an associated complication with hyperthyroidism, what must you do when you suspect hyperthyroidism?

Answer 13

evaluate the BP

Question 14

How is hyperthyroidism diagnosed?

Answer 14

Consider signalment, history, and PE findings (palpate thyroid)
Consider clin path findings
Specific endocrine diagnostics

Question 15

What specific endocrine diagnostics can be done for hyperthyroidism?

Answer 15

thyroid hormone concentrations, T3 suppression test, and thyroid scintigraphy

Question 16

What will you find on CBC in hyperthyroid patients?

Answer 16

Mild erythrocytosis with a variably present stress leukogram

Question 17

What will you find on serum biochemistry in patients with hyperthyroidism?

Answer 17

Elevated ALT and/or ALP and variably present azotemia

Question 18

Why can hyperthyroidism mask underlying disease?

Answer 18

Because it increases renal blood flow

Question 19

What will you find on UA in a patient with hyperthyroidism?

Answer 19

variable USG and proteinuria

Question 20

True or False: A majority of cats with hyperthyroidism will have a high total T4.

Answer 20

TRUE

Question 21

Is a high TT4 diagnostic?

Answer 21

Yes - typically it is safe to assume that if a cat has an elevated TT4 then it is hyperthyroid

Question 22

Does a normal TT4 rule out hyperthyroidism?

Answer 22

No, cats with early or mild hyperthyroidism will have a normal TT4 therefore one TT4 within the referencerange does not rule it out

Question 23

If hyperthyroidism is suspected but TT4 is normal, what can be done?

Answer 23

Recheck in 3-6 weeks or use additional diagnostics such as fT4ed, T3 suppression test, or thyroid scintigraphy

Question 24

What does a fT4ed measure?

Answer 24

free, unbound fraction of thyroid hormone

Question 25

What is a fT4ed sensitive diagnostic for?

Answer 25

hyperthyroidism

Question 26

Since fT4ed is sensitive for diagnosing hyperthyroidism, is it diagnostic as a stand alone test in cats?

Answer 26

no - it should not be used as a stand alone

Question 27

When is a T3 suppression test indicated?

Answer 27

To confirm diagnosis when clinical suspicion of hyperthyroidism and high-normal TT4

Question 28

In nnormal cats, what will the T4 level be during the T3 suppression test?

Answer 28

T4 will fall tp <50% of pre-levels

Question 29

In hyperthyroid cats, what will happen during a T3 suppression test?

Answer 29

There will be minimal suppression

Question 30

When is thyroid scintigraphy indicated?

Answer 30

To determine the extend of disease or to confirm hyperthyroidism in equivcal cases

Question 31

How is hyperthyroidism treated?

Answer 31

Anti-thyroid medication, dietary iodine restriction, radioactive iodine administration, and surgery

Question 32

What are the indications for anti-thyroid drugs?

Answer 32

Long-term management of hyperthyroidism, short-term stabilization of patients with hyperthyroidism prior to definitive treatment, and challenge therapy to determine effect of return to euthyroid state on clinical signs and renal function

Question 33

What anti-thyroid drug is commonly used?

Answer 33

methimazole

Question 34

What is the mechanism of action of methimazole?

Answer 34

Inhibitor of thyroid hormone synthesis via inhibition of thyroid peroxidase

Question 35

What are the adverse effects of methimazole?

Answer 35

GI effects, facial excoriatiions, blood dyscrasias, and hepatotoxicity

Question 36

Why would you want to restrict iodine in hyperthyroid patients?

Answer 36

because iodine is required for hormone synthesis

Question 37

What is the curative therapy (and gold standard) for hyperthyroidism?

Answer 37

radioactive iodine therapy

Question 38

How does radioactive iodine therapy work?

Answer 38

It isolates in the funcitoning cells of the thyroid, which are the ones that are overproducing thyroid hormone, and spares the atrophied cells

Question 39

What precautions need to be taken if radioactive iodine therapy is done?

Answer 39

The patient should be isolated for a variable period of time | Their feces needs to be collected and isolated until the radiation wears off

Question 40

How long does it take for the patients to become euthyroid after radioactive iodine therapy?

Answer 40

within 3 months

Question 41

What surgical option is there for hyperthyroid patients?

Answer 41

thyroidectomy - surgical resection of affected thyroid tissue

Question 42

What are the potential complications associated with thyroidectomies?

Answer 42

Laryngeal paralysis, hypoparathyroidism, and hypothyroidism

Question 43

What should be considered when chosing the appropriate treatment for a hyperthyroid patient?

Answer 43

effectiveness of therapy, durability of therapy, potential side effects or complications, ease for the client, quality of life for the cat, if a follow up is required, cost, and feasibility

Question 44

Canine hyperthyrodidsim is almost always associated with what?

Answer 44

thyroid adenocarcinomas

Question 45

What is treatment for canine hyperthyroidism focused on?

Answer 45

treating the tumor

Question 46

What are the treatments for canine hyperthyroidism?

Answer 46

surgical resection, radiation therapy, and methimazole for palliative care

Question 47

What can cause dietary hyperthyroidism?

Answer 47

raw meat diets and fresh/dried gullets

Question 48

What should the fT4 level be in patients with canine hyperthyroidism?

Answer 48

It can be normal - there amy be a false elevation from T4-autoantibodies

Question 49

How should you approach a canine with high TT4?

Answer 49

Get a thorough history, do a PE, or so an fT4 to rule out antibody interference

Question 50

If dietary contributors are ruled out and fT4 is high in a canine patient, what should be done?

Answer 50

Evaluate for thyroid neoplasia with thoracic radiographs, cervical and abdominal ultrasound, cross-sectional imaging, and thyroid scintigraphy