Thyroid diseases- Hypothyroidism and hyperthyroidism

Question 1

Describe how the thyroid axis works

Answer 1

The hypothalamus releases Thyroid releasing hormone (TRH) that acts on the anterior pituitary to cause it to release thyroid stimulating hormone (TSH) which acts on the thyroid gland that releases T3 and T4 which act as a negative feedback loop on the anterior pituitary and hypothalamus

Question 2

What is the common signalment for hypothyroidism

Answer 2

Middle-age (6 years) dogs, commonly in Goldens, labs, and doberman pinschers

Question 3

What usually happens to cause hypothyroidism

Answer 3

80% loss of thyroid function due to primary immune destruction of the thyroid

Question 4

What are the common clinical signs for hypothyroidism

Answer 4

Alopecia (rat tail), weight gain, lethargy and exercise intolerance, heat-seeking behaviors, personality change and depression
constipation in cats!

Question 5

What is endocrine alopecia

Answer 5

When there is a loss of hair because the hair cells get stuck in telogen phase so when they get rubbed off they don’t grow back, will see non-puritic alopecia in areas of high friction like the tail and trunk. Loss of guard hairs with a dry and flakey coat. Also may have hyper-pigmented skin

Question 6

What are the common biochemical abnormalities for hypothyroidism

Answer 6

non-regenerative anemia, hypercholesterolemia

Question 7

What is the screening test for hypothyroidism and how does it work

Answer 7

total thyroxine concentration (TT4), if >1.5-2ug/dL hypothyroidism is unlikely but you can’t trust a low test to mean they are positive for hypothyroidism. You can only trust a negative test because TT4 is very sensitive to suppression

Question 8

What is Euthyroid Sick Syndrome

Answer 8

When TT4 is surpassed due to stress, illness, trauma, or after surgery. So many illnesses can cause a low TT4

Question 9

How do you obtain a definite diagnosis for hypothyroidism and what could a “classic positive” look like and what is the problem

Answer 9

You run a thyroid panel- TT4, fT4, and TSH. A positive is a low TT4 and fT4 and a high TSH
The problem is the TSH is normal in 20-30% of hypothyroid dogs

Question 10

How might a dog present with euthyroid sick syndrome

Answer 10

With a low TT4 and maybe a low fT4 and not have TSH impacted (hard to tell from a hypothyroid dog with a normal TSH though), usually ESS won’t have 2/3 abnormal

Question 11

What class of antibiotics can cause clinical hypothyroidism

Answer 11

sulfonamide antibiotics like TMS, will cause low TT4 and fT4 but a high TSH

Question 12

What can cause a false positives for hyperthyroidism or false negatives for hypothyroidism

Answer 12

anti-thyroid antibodies which cross react with the TT4 assay and will falsely increase it because they are read as T4

Question 13

How do you treat hypothyroidism and how long until you monitor the therapy

Answer 13

with sodium levothyroxine (synthetic TT4), wait 4-8 weeks to monitor the therapy

Question 14

How do you monitor treatment for hypothyroidism

Answer 14

after waiting 4-8 weeks run a TT4 concentration 4-6 hours after giving the pill with a goal of having the TT4 normal to slightly elevated (3-6ug/dL)

Question 15

How do you determine if an animal can go to once daily dosing for hypothyroidism

Answer 15

need to wait 2-3 months at first and then do a TT4 before and 4-6 hours after giving the pill. You want the pre-pill to be >1.5ug/dL and the post pill to be 3-6ug/dL and if you have yes to both you can go to once daily pills!

Question 16

What are the most common causes (etiology) of hyperthyroidism

Answer 16

bilateral adenomatous hyperplasia (70% of cases) or an adenoma (rarely a carcinoma) of the thyroid gland
in dogs it is usually iatrogenic or you can palpate a mass

Question 17

What is the common signalment for hyperthyroidism

Answer 17

an older (>8yrs) cat that will commonly have other comorbidities

Question 18

What are the common clinical signs for hyperthyroidism

Answer 18

weigh loss with a ravenous appetite, unkempt and dull coat, vomiting and diarrhea, behavior change (restless, agitated), and PU/PD, hypertension (can also cause end-organ injury like retinal detachment or seizures)
OR in some cases you may have apathetic hyperthyroidism with opposite signs to the standard disease

Question 19

What are the common physical exam abnormalities in hyperthyroidism

Answer 19

hair coat changes, low BCS, cardiac abnormalities, thickened “ropey” intestines, thyroid slip- the enlarged glands will sink down the neck so you must palpate from the larynx to the thoracic inlet

Question 20

What are common biochemical abnormalities expected with hyperthyroidism

Answer 20

Erythrocytosis, elevated ALT, ALP, azotemia, increased blood pressure

Question 21

Why do you not use only a fT4 to diagnose hyperthyroidism in cats, and how should you test

Answer 21

Becuase it can be elevated even in healthy cats. You should run a TT4 as a screening test and if it is elevated it is probably hyperthyroidism. If it is high-normal then you should also submit a fT4 and TSH because if they have a high fT4 and low TSH they are also likely hyperthyroid

Question 22

What are the three ways to treat hyperthyroidism in cats

Answer 22

Medical management through methimazole or Hill’s Y/d diet or definitive treatment through radioactive iodine (I-131)

Question 23

How does Y/d work and what are the drawbacks and benefits

Answer 23

It is an iodine restricted diet to help prevent the formation of T4, so it is easy because it is just food not a pill and not hugely expensive. But it is the ONLY thing the cat can eat, not even a bug and the palatability can be an issue

Question 24

How does methimazole work and what are the side effects and issues with it

Answer 24

It blocks TPO which inhibits T4/T3 synthesis but there is still abnormal tissue forming so you may need to increase the dose over time. It is also a 2x a day pill so it may difficult to give.
It can cause GI upset, Iatrogenic hypothyroidism, facial excoriations, hepatotoxicity, thrombocytopenia, neutropenia, and myasthenia gravis

Question 25

What are options for the possible side effects with Methimazole

Answer 25

If there is GI upset you can transition to the transdermal form
If there is hypothyroidism you should decrease the dose, or take a drug holiday, or you may need supplementation temporarily
Any other side effect- you need to try another therapy

Question 26

How do you monitor and what are the treatment goals for methimazole

Answer 26

You should recheck TT4 and TSH in 2 weeks (or transdermal every 3-4 weeks) and monitor the CBC and Chemistry for the first 1-2 months. THe goal is to have a TT4 in the mid to low-normal range and once controlled you can maybe go to once a day dosing

Question 27

What should you worry about if a cat is on >15mg of methimazole and still having clinical signs of hyperthyroidism (no response to treatment)

Answer 27

Consider the possibility of a carcinoma or ectopic tissue

Question 28

How does radioactive iodine work and what are the benefits and drawbacks

Answer 28

It uses radioactive iodine that is taken up by the hyper functioning thyroid tissue cells and then destroys them. There is a 90-95% success rate. The downside is it is expensive, may make the animal hypothyroid, the animal may need a second dose, and there is a required isolation time of 5 days (so the animal needs to be stable)

Question 29

Describe the risks with renal disease and radioactive iodine treatment and how to mitigate it

Answer 29

Hyperthyroidism masks how bad kidney disease is, so in a cat that is moderately azotemic you may want to try a methimazole trial before radioactive iodine treatment because you want to see what the bodies reaction will be, especially before the 5 day isolation period

Question 30

What should you monitor after radioactive iodine treatment or starting methimazole and why

Answer 30

You need to monitor TSH because even though the TT4 may be normal if the TSH is high because the body is perceiving the thyroid to not be working it can cause hypothyroidism and azotemia is much more likely in hypothyroid cats, and it significantly decreases their survival

Question 31

What would it look like and what should you do if a cat develops Iatrogenic hypothyroidism because of treatment for hyperthyroidism

Answer 31

The cat would have elevated TSH, at first you monitor and it should improve in 1-3 months however if the cat also becomes azotemic you should supplement with levothyroxine