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Most hypothyroidism in dogs is ___________, which is caused by... (2)
primary; idiopathic lymphoplasmacytic thyroiditis or idiopathic thyroid atrophy.
What are the RARE congenital defects of the thyroid that cause primary hypothyroidism? (2)
thyroid peroxidase mutation (toy fox terriers- dysmorphogenesis), failure of thyroid gland to develop
__________ is NOT a common cause of primary hypothyroidism in dogs?
How common is secondary hypothyroidism caused by pituitary disease and low TSH production?
so extremely rare (<5% of hypothyroid dogs)
How do glucocorticoids affect the thyroid?
suppress TSH--> "secondary" hypothyroidism
What dogs usually present with hypothyroidism?
middle-aged (2-6 years), any breed
What clinical signs are associate with hypothyroidism? (12)
lethargy, inactivity, mental dullness, weight gain (with no change or reduction in food intake), hyperkeratosis, seborrhea, thin haircoat ("rat tail"), hyperpigmentation, myxedema, otitis externa, dermatitis, rarely neuromuscular signs
What CBC changes can be associated with hypothyroidism? (1)
What biochem abnormalities can be associated with hypothyroidism? (4)
high TG, hypercholesterolemia, mild increase in ALP, rarely increase in CK
What UA abnormalities can be associated with hypothyroidism? (1)
NONE- UA IS NORMAL (hypothyroidism does NOT cause PU/PD....look for another cause!!!!)
What is the goal when testing for hypothyroidism?
differentiate b/w hypothyroidism and euthyroid sick syndrome
Why is measuring tT4 useless to run for hypothyroidism?
tT4 varies with time of day, season, ambient temperature, age
How is tT4 useful in the diagnosis of hypothyroidism?
[screening- usually a waste of money b/c T4 will probably be low in ANY sick dog] sensitive but not specific- can rule OUT hypothyroidism if T4 is normal, but SO MANY other things can cause low T4
What is the single most accurate test of thyroid function?
serum free T4 by equilibrium dialysis
Serum free T4 by equilibrium dialysis has _________ specificity and ___________ sensitivity than tT4 measurement for detecting hypothyroidism.
Why can't we determine if a dog has secondary hypothyroidism?
the reference range for TSH starts at 0
How can TSH be used in the diagnosis of hypothyroidism?
low T4 with high TSH supports primary hypothyroidism [only supportive- TSH is released in bursts, recovering from non-thyroidial illness can cause high TSH, breed differences]
What is the gold standard for diagnosing hypothyroidism? What is the caveat to this test?
TSH response test; dogs with ESS and certain medications can be false + for hypothyroidism and misdiagnosed
Why is it important to diagnose hypothyroidism/ ESS right the first time?
treatment with T4 will blunt the thyroid gland's responsiveness to TSH for up to 2 months after discontinuation of the drug--> TSH response test won't work to get a true diagnosis
How do you evaluate the TSH response test?
- No response to TSH- hypothyroidism
- T4 post-TSH > 1.5 basal T4- healthy or ESS
- Intermediate results- grey area
Why are tests for thyroid autoantibodies not really useful in diagnosing hypothyroidism?
thyroglobulin antibodies can be found in up to 47% of euthyroid dogs
If you have a dog with markedly high T4 and the dog does not have signs of hyperthyroidism, it is most likely caused by...
autoantibodies to T4 cross reacting with the assay.
The severity of decrease in thyroid hormones is related more to the ____________ than to the _____________.
severity of the illness; type of illness (hypothyroidism vs ESS)
Is thyroxine replacement advised with ESS?
What most common drugs have the potential to affect thyroid function? (4)
glucocorticoids, phenobarbitol, sulfonamides, NSAIDs
How do glucocorticoids affect the thyroid gland? (3)
decreased total and free T4, inhibit TSH, decreased binding of T4 to plasma proteins
How does phenobarbital affect the thyroid gland? (2)
decreases total and free T4 by increasing metabolism and biliary excretion of T4, increase TSH due to lower thyroid hormone concs
How do sulfonamides affect the thyroid gland? (2)
decreased total and free T4 by inhibiting thyroid peroxidase, TSH increases as a consequence of decreased T4 (laboratory findings indistinguishable from primary hypothyroidism)
How do NSAIDs affect the thyroid gland? (2)
may decrease total and free T4 and TSH
How is hypothyroidism treated?
Levothyroxine (T4) 0.02mg/kg PO BID (max is 0.8mg/kg BID)
What is the most common reason for treatment failure with hypothyroidism?
no response after 8 weeks- inappropriate diagnosis- the dog does not actually have hypothyroidism
Does weight loss and increased activity with thyroxine treatment confirm your diagnosis of hypothyroidism?
absolutely not- if a healthy individual is put on thyroxine supplements, they will lose weight and become more active, just like a hyperthyroid patient
How do you monitor treatment of a hypothyroid patient?
- 2 months after starting treatment, check tT4-
- are you giving too much? hyperthyroidism will kill your patient... tT4 should be at the high end of the RR
- are you giving enough? resolution of clinical signs
What are some complications of untreated hypothyroidism? [pretty rare] (2)
atherosclerosis/TE, myxedema coma (brain edema)
What is the prognosis for hypothyroidism?
excellent with lifelong treatment
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