Hyperthyroidism & Hypothyroidism Flashcards
T/F: Total T4 is very sensitive for diagnosing hypothyroidism, but not specific.
True
90% of hypothyroid animals will have a LOW/Low-Norm TT4. So this test is sensitive for detecting those with the disease.
However, 25% of individuals that show LOW TT4, will not have hypothyroidism (false +) due to non-thyroidal illness
T/F: FreeT4 is highly sensitive but not specific for diagnosing hypothyroidism
false – highly sensitive AND specific.
It is considered the best test available.
T/F: TSH is insensitive for hypothyroidism
True
TSH is only elevated in 60-75% of true hypothyroid dogs, missing the other ~25% of individuals.
However, TSH is very specific; so if there is a low T4 and high TSH, then the animal is likely hypothyroid.
What do you expect the TSH to do in an animal that is experiencing thyroid gland disease?
If the thyroid gland is destroyed/damaged, then TSH should increase because there is LESS negative feedback d/t the thyroid gland secreting LESS T4/T3.
what is the most common cause of hyperthyroidism?
Bilateral adenomatous hyperplasia or adenoma (causing autonomous production of thyroid hormones)
and rarely carcinoma
What are clinical signs associated with hyperthyroidism in cats?
- weight loss + good appetite
- PU/PD
- vomiting/diarrhea
- unkempt haircoat
- thyroid nodule or slip
- murmur or gallop
- muscle loss
What are 5 clinical pathology changes you may see in cats with hyperthyroidism?
- erythrocytosis
- elevated ALT and ALP (mild)
- BUN and creatinine will be norm/decreased usually UNLESS there was pre-existing kidney disease (hyperT4 can mask kidney disease by increasing GFR and RBF)
- unconcentreated urine (<1.035 d/t increased RBF and PU/PD)
- proteinuria
What are 2 MOST likely causes of a cat that has clinical signs of hyperthyroidism, but has a NORMAL T4?
early hyperthyroidism** or nonthyroidal illness
T/F: Free T4 is very sensitive for diagnosing hyperthyroidism, but has poor specificity and should only be used when TT4 is elevated.
TRUE!
20% of euthyroid cats with nonthyroidal illness have an elevated fT4.
Only rule fT4 with TT4 and if there are signs of hyperthyroidism
TT4 is the screening test of choice for hyperthyroidism
T/F: TT4 is the screening test of choice for hyperthyroidism
true
if TT4 is within the normal range, but in the upper 1/3 of the reference –> Look for NTI; if not present then perform fT4 +/- TSH.
How does hyperthyroidism MASK preexisting CKD?
increases renal blood flow and GFR
this means that in hyperthyroid cats, BUN, creatinine, USG, and SDMA are not reliable.
what is the MEDICAL treatment for hyperthyroid cats?
oral or transdermal methimazole (blocks thyroid peroxidase –> no more thyroid hormone production)
Takes 2-4 weeks to become effective.
Side effects: GI, hepatotoxicity, blood dyscrasias, immune disease, and facial pruritus
When starting a hyperthyroid cat on methimazole, what should follow-up look like?
Recheck patient in 4 weeks – do CBC, chem, UA, and T4
Then recheck again in 2 months if the T4 is normal and there are no side effects.
Recheck periodically if remaining stable.
Why might methimazole dose need to be increased for a cat with hyperthyroidism?
- Cats that are treated long term may develop resistance to methimazole at their current dose.
- The tumor may progress and require more drug to control the thyroid hormone production
- they may have large goiter
- the tumor may progress into carcinoma and be resistant to antithyroidal drugs
What is the SURGICAL treatment option for hyperthyroidism in cats?
Thyroidectomy
definitely there are risks for developing hypothyroidism, hypoparathyroidism (hypocalcemia), and horners syndrome
How does radioiodine treatment work for hyperthyroidism in cats and what does follow up look like?
It is a SQ injection of I131. It emits beta particles which destorys abnormal thyroid tissue but spares normal thyroid tissue.
You must follow up at 1, 3, and 6 months to check chemistry, UA, and T4/TSH.
What is the nutritional treatment for hyperthyroidism in cats?
Low iodine diet
this works because iodine is required for thyroid hormone production
Takes time to work – T4 normalizes by 4-8 weeks, but clinical signs should improve within a few weeks.
when is nutritional (low iodine therapy) appropriate for cats with hyperthyroidism?
- cats who cannot tolerate methimazole
- concurrent diseases that preclude I131
- concurrent early renal insufficiency
- mild-mod HT4 cats
requires you to be strict with the diet, so if you cannot, its not worth it.
T/F: >95% cases of hypothyroidism in dogs is primary hypothyroidism (disease of the thyroid gland itself)
true
The 2 types are 1. lymphoctic thyroiditis (an autoimmune disease) and 2. idiopathic atrophy of the thyroid gland
What is the common signalment for hypothyroidism in dogs?
middle-aged
goldens, dobermans, boxers, shelties, etc.
what are the clinical signs of hypothyroidism in dogs?
Consistent with decreased metabolic rate
1. lethargy
2. weakness
3. myopathy
4. exercise intolerance
5. Bilaterally symmetric alopecia + hyperpigmentation
6. seborrhea (dry or oily)
7. possible secondary cutaneous infections
8. Myxedema (non-pitting edema –> tragic facial expression)
9. bradycardia, weak pulses, muffled heart sounds, reduced cardiac contractlity (usually all clinically insignificant)
10. peripheral neuropathies (facial paralysis, vestibular signs, megaesophagus)
11. multifocal CNS signs (hemiparesis, hypermetria, ataxia, CN deficits)
12. repro abnormalities (infertility, low birth weight)
What are common bloodwork findings in hypothyroid dogs?
- HYPERCHOLESTEROLEMIA (**)
- mild, NR anemia
- Hyponatremia (mild)
T/F: Low T4 is NOT specific for hypothyroidism
true
meaning, you can have false +. Dogs can have low T4 and not be hypothyroid.
If a dog has clin signs + low T4, that is adequate for diagnosis.
which assay is the MOST reliable to diagnose a dog with hypothyroidism?
Equilibrium dialysis assay - fT4
more sensitive and specific
AND less affected by non-thyroidal factors.
T/F: primary hypothyroidism causes elevated TSH
true
T/F: a combination of elevated TSH and decreased T4 or fT4 is diagnostic for hypothyroidism
true
if elevated TSH and low-normal T4 or fT4, can be early hypothyroidism
elevated TSH adds specificity to T4 and fT4
List some factors that affect thyroid function tests and falsely lower thyroid hormone levels based on the test result.
- breed – sighthounds
- corticosteroids
- sulfonamides
- phenobarbital
- clomipramine
- illness (NTI)
Which value is commonly decreased on thyroid function tests with non-thyroidal illness - TT4, fT4, or TSH?
TT4
you should delay testing until the NTI is resolved.
How can you test for autoimmune thyroiditis which interferes with assays for T3 and T4 (falsely elevates them)?
testing for anti-thyroglobulin antibodies
which test is NOT affected by T4 autoAb?
fT4
you may see increased TT4, but if you do fT4, it will be low.
How do you treat hypothyroidism and what would follow-up need to look like for these patients?
Levothyroxine
Response is rapid (1-2 weeks), derm signs can take months to resolve though.
Follow up in 6-8 weeks to do a 4-6 hr post-pill TT4 (it should be 35-80 nmol/L which is high-normal or above RI)
T/F: if you are treating a hypothyroid dog with levothyroxine and you cause iatrogenic hyperthyroidism, this is irreversible
false – reversible; stop treatment for 2-3 days and restart at 50-75% of the original dose.
In what instances would you see elevated TT4 in dogs?
- hyperthyroidism (rare – diet-related, iatrogenic, or thyroid carcinoma)
- Autoimmune thyroiditis (the antibodies are interfering with T3/T4)
T/f: if fT4 and TSH are normal, the dog is euthyroid
true
What can cause LOW total T4?
- hypothyroidism
- NTI
- Breed (sighthound)
- Drugs – corticosteroids, phenobarb, sulfonamides, clomipramine
why would CK be increased on a dogs bloodwork when you are suspecting hypothyroidism?
muscle breakdown
T/F: fT4 in cats is very sensitive by not specific
true – can be falsely increased by NTI.
It should never be used as a stand alone test to diagnose hyperthyroidism.
Elevated fT4 when TT4 is in upper 25% of the RR is diagnostic of hyperthyroidism
T/F: Elevated fT4 when TT4 is in upper 25% of the RR is diagnostic of hyperthyroidism
true
