Hyperthyroidism

Question 1

What is the most common cause of hyperthyroidism? What lobes are most commonly affected?

Answer 1

functional adenomatous hyperplasia or adenoma that produce thyroid hormones (rarely carcinoma)

commonly both lobes are affect, but it can be unilateral

Question 2

How does hyperthyroidism affect the thyroid hormone production pathway?

Answer 2

increase in T4 or T3 production causes a decrease in TSH from the pituitary

Question 3

What are the most common signalment and clinical signs associated with hyperthyroidism?

Answer 3

cats over 8 y/o

• weight loss with good appetite
• PU/PD
• vomiting, diarrhea
• unkempt hair coat
• thyroid nodule or slip
• murmur, gallop
• muscle loss

Question 4

How is the erythrogram most commonly affected by hyperthyroidism? What 2 changes on biochemistry may be caused by comorbidities?

Answer 4

erythrocytosis

1. elevated ALT and ALP (mild, 2-3x URI)
2. changes in BUN and creatinine - azotemia in cats with concurrent kidney disease, most HT4 cats have normal or decreased creatinine due to increased GFR
   - treat hyperthyroidism, then check

Question 5

How is USG affected by hyperthyroidism? What else is seen in urine?

Answer 5

unconcentrated - <1.035 due to increased renal blood flow and PU/PD

proteinuria - resolves after treatment

Question 6

How is hyperthyroidism diagnosed? What are 4 challenges?

Answer 6

elevated thyroid hormones with clinical signs - straightforward in most cats

1. early or mild hyperthyroidism
2. concurrent nonthyroidal illness skews blood work (IBD, DM, CKD)
3. enlarged thyroid gland without clinical hyperthyroidism
4. elevated T4 without clinical signs or thyroid nodules

Question 7

What is expected on serum total T4 in hyperthryoid cats?

Answer 7

mid to high normal and elevated T4

(nonthyroidal disease can push T4 into reference range)

Question 8

What is expected to be seen in total T4 in hyperthyroid cats? What can explain a hyperthyroid cat with normal T4 levels?

Answer 8

elevated —> 10% with mid to high normal T4

early or mild hyperthyroidism (80%) or nonthyroidal illness (20%)

Question 9

What kind of test is free T4 for diagnosing hyperthyroid cats? How is it used for diagnosis?

Answer 9

very sensitive test for HT4, but has poor specificity —> accurately diagnoses hyperthyroid cats, but there are a lot of false positives (euthyroid cats with elevated fT4 and normal tT4)

never used as a sole test to confirm hyperthyroidism —> fT4 + tT4

Question 10

How do tT4 and fT4 levels compare in early hyperthyroid and euthyroid cats?

Answer 10

early hyperthyroid or nonthyroidal illness = mid to high normal tT4 and high fT4

euthyroid or nonthyroidal illness = low to normal tT4 and high fT4

Question 11

Other than free/total T4 levels, what is also used for diagnosing hyperthyroidism? What challenge is associated with this test?

Answer 11

TSH - canine TSH or feline optimized TSH (should be decreased)

suboptimal as sole test of hyperthyroidism - cannot distinguish between low normal and undetectable TSH ( <0.03 ng/mL)

Question 12

Canine TSH assay vs. feline optimized TSH:

Answer 12

• CANINE: poor LLOD can diagnose cats with lower normal values of TSH as hyperthyroid
• FELINE: LLOD is more accurate to truly low levels of TSH

Question 13

What is the test of choice for diagnosing hyperthyroidism? What are the 3 general observations made according to results?

Answer 13

total T4

1. increased T4 in a patient with compatible clincial signs supports hyperthyroidism
2. total T4 within normal range, especially upper 1/3rd of RI, look for NTI and perform fT4 and TSH
3. if total T4 is at the lower end of RR, look for another diagnosis

Question 14

What is thyroid scintigraphy? What is it used for?

Answer 14

an injected radioactive tracer – usually iodine – is used to evaluate function and structure of the thyroid gland

• anatomy and function = increased intensity correlates to increased size and function
• dose calculation for radioactive iodine treatment for lowest amount possible
• ectopic = reactions in unexpected areas of the body
• unilateral vs bilateral
• adenoma vs carcinoma (multinodular, metastasis)

Question 15

How is CKD associated with hyperthyroidism? What is used to provide a preview of renal function in hyperthyroid cats?

Answer 15

14-40% of hyperthyroid feline patients have preexisting CKD, where hyperthyroidism is masked due to increased RBF and GFR

methimazole trial - start treatment and measure T4; if normal, perform a UA to assess renal function (BUN, creatinine, USG, SDMA, GFR studies, and biomarkers are not reliable or impractical)

Question 16

What are the 4 most common management strategies used for hyperthyroidism?

Answer 16

1. Methimazole
2. surgical thyroidectomy
3. radioiodine therapy (curative!)
4. iodine-restricted diet

Question 17

What are the 2 goals of hyperthyroid therapy?

Answer 17

1. achieve euthyroidism - 1.5-3.0
2. avoid hyperthyroidism and hypothyroidism - worsens kidney function (azotemia, proteinuria, glomerular sclerosis)

Question 18

What is the mechanism of action of methimazole? How long does it take for effects to occur?

Answer 18

blocks thyroid peroxidase, resulting in no more production of thyroid hormones (PO or transdermal)

2-4 weeks - the already made thyroid hormones are still in the system

Question 19

What are some benefits to treating hyperthyroidism with methimazole?

Answer 19

• readily available
• cheap
• effective
• reversible - can be used as a trial to see how decreased levels of thyroid hormones affect kidney function

Question 20

What is the recommended dose of methimazole to start? What happens at higher doses?

Answer 20

5 mg/cat/day —> 2.5 mg BID

> 15 mg = intolerance

Question 21

What 6 side effects are associated with methimazole usage?

Answer 21

1. vomiting
2. inappetence
3. hepatotoxicity
4. blood dyscrasias - granulocytes, RBCs
5. immune disease- IMHA, myasthenia gravis
6. facial pruritis/excoriation

changes in administration rarely helps, except for changed to transdermal following GI signs after PO

Question 22

How often should patients have follow-ups on methimazole treatment?

Answer 22

• check CBC, chem, UA, and T4 4 weeks after starting
• if normal and no side effects, check again in 2 months
• if stable again, can go for longer periods since side effects are most common in the first few months

Question 23

What are 4 complications of methimazole treatment? How should treatment change when this occurs?

Answer 23

1. cats treated long-term begin to need higher and higher doses to get same results
2. tumor continues to progress and secrete more thyroid hormones, overwhelming current dose
3. large goiter
4. carcinoma transformation resistant to antithyroidal drugs

need to resort to more definitive therapies, like higher doses of radioiodine or surgery

Question 24

What is the main use of surgical thyroidectomies? Why are the not as commonly done to treat hyperthyroidism anymore?

Answer 24

removal of carcinoma for histopath

• increasing availability of radioiodine
• high risk of hypothyroidism and hypoparathyroidism (hypocalcemia!)
• surgery requires the proper candidtates
• recurrence still an issue!

Question 25

How is radioiodine used to treat hyperthyroidism?

Answer 25

SQ injection of I131 is taken up by abnormal thyroid tissue and emits beta particles to destroy it while sparing normal tissue

Question 26

What are 4 disadvantages to radioiodine treatment?

Answer 26

1. should use a methimazole trial in cats that are azotemic and hyperthyroid to ensure that decreased thyroid hormone levels will not cause kidney damage 2. the cat will be radioactive and must stay in specialty hospital facilities in isolation for a few days 3. iatrogenic hypothyroidism is possible 4. expensive up front ($1600 at the low end)

Question 27

What follow-up blood work should be done following radioiodine treatment?

Answer 27

chemistry, UA, T4/TSH 1 mo, 3 mo, and 6 mo after

Question 28

What type of diet is recommended for hyperthryoid cats? When is improvement commonly noted? In what way is it unique in compared to other hyperthyroid treatments?

Answer 28

low iodine (y/d) ---> only treatment, not used with radioiodine, methimazole, or surgery within 4 weeks creatinine will be decreased or have no change ---> GFR is not affected

Question 29

In what 4 situations is a low iodine diet beneficial?

Answer 29

1. cats who cannot tolerate methimazole 2. concurrent disease that preclude I131 3. cats with concurrent early renal insufficiency 4. mild to moderate hyperthryoid cats

Question 30

In what 3 situations is a low iodine diet difficult? Why?

Answer 30

1. indoor/outdoor cats 2. multi-cat household 3. cats requiring other diets this nutritional therapy is only helpful as the only source of food, other food sources will offer high iodine levels

Question 31

Medical, dietary, radioiodine, surigcal treatment of hyperthyroidism:

Answer 31