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63 Cards in this Set
- Front
- Back
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Where is TRH produced
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Hypothalamus
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What should normally be the relationship b/t TSH and T4
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Serums TSH should be inversely proportional to serum T4
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What is the major product of the thyroid
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T4
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What percentage of T4 is metabolically active in circulation
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0.1% (unbound "free" T4)
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Which is more representative of true thyroid fxn in most cases: fT4 or T4
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fT4
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What is more common: primary or secondary hypothyroidism
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Primary- 95% of cases
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What is the distribution of lymphocytic thyroiditis and idiopathic follicular atrophy as types of primary hypothyroidism
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50/50
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How common are thyroid neoplasia and congenital primary hypothyroidism as causes of decreased thyroid activity
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Rare
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What is the most common clinical sign of hypothyroidism in the dog
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Dermatologic signs:
Alopecia Dry, oily, or dermatitic seborrhea Hyperpigmentation Otitis externa (recurrent) Recurrent pyoderma Myxedema |
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How common are signs due to cardiovascular effects seen in cases of hypothyroidism
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Rare
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What is myxedema coma
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Rare, L/T form of hypothyroidism w/ depression, coma, bradycardia, hypotension, hypoventilation, and myxedema
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What are some findings of congenital hypothyroidism
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Mental dullness
Dwarfism Macroglossia Broad head |
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What is the single most accurate test for dx of hypothyroidism
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fT4
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Why is serum T4 not the most reliable test for dx of hypothyroidism
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Lots of non-thyroid causes can decrease the value:
Drugs Non-thyroid illness |
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What is the only reliable method of measurement of fT4
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Equilibrium dialysis
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What should an endogenous TSH test reflect in a case of hypothyroidism
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Increased TSH
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What is the best way to diagnose hypothyroidism
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Combo of clinical signs, elevated TSH,
and decreased T4 or fT4 This is nearly 100% diagnostic |
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What is the significance of autoAbs to T4 and T3
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Limited
Do not = hypo or hyper thyroid Interfere w/ assays for those hormones, though, increasing the values falsely BUT, AutoAb to T4 does not affect fT4 |
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Which is more resistant to the effects of non-thyroidal illness: T4 or fT4
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fT4; It still may be reduced, though
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What are two ways you can avoid non-thyroidal illness' effect on measurements of fT4
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Don't measure when there is another illness present
Include a measurement of TSH when you do the fT4 |
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Which drugs are known to cause readings of thyroid hormones suggestive of hypothyroidism
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Glucocorticoids
Pb Clomipramine |
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What is the drug of choice in tx of hypothyroidism
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Levothyroxine (synthetic T4)
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Why do some patients lose more hair initially when tx w/ Levothyroxine is started
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Old hair is replaced by new follicles
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When should you expect to see a response to Levothyroxine in management of hypothyroidism
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Increase in activity and improved attitude w/in 1-2 weeks
Other signs resolve w/in 2 mths (derm changes may take several more months) |
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When and how should the dog be rechecked after starting a regimen of Levothyroxine
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Recheck in 8 wks for response to tx and post-pill testing (4-6hr after administration)
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What is the ideal test result for a measurement of T4 after 8 wks of tx w/ Levothyroxine
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Want the T4 to be in the high normal to slightly above normal range at its peak concentration
(4-6 hrs after administration) |
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What is the only SE of Levothyroxine
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Thyrotoxicosis, and is uncommon
(wt loss, increased appetite, hyperactivity, PU/PD, tachycardia) Requires a large overdose, but can be confirmed by looking for substantial elevation in serum T4 (post-pill) |
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In the event that thyrotoxicosis occurs (as is evidenced by wt loss, increased appetite, hyperactivity, PU/PD, and tachycardia), what should you do
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Consider the possibility that there is an underlying dz
Stop Levothyroxine for 2-3 days and then restart @ 50-75% original dose |
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How common is it to find antithyroglobulin Ab
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In 40-50% of hypothyroid dogs
(Ab formed against thyroid colloid protein) |
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What combination of fT4 and TSH results equals hypothroidism (almost always)
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Low fT4 and High TSH
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What is the most common etiology of feline hyperthyroidism
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Adenomatous hyperplasia (fxnl thyroid adenoma)
Is bilateral in 70% of cases |
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How commonly is carcinoma the cause for feline hyperthyroidism
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Rare
Feline hyperthyroidism is almost always due to adenomatous hyperplasia |
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What is the typical signalment for dogs and cats w/ thyroid disorders
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Dogs- hypothyroid: young-adult to mid-age medium-lg
Cats- hyperthyroid: 12-13yrs avg (very rare <6yrs) |
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Why may patients w/ feline hyperthyroidism appear of normal weight
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They most likely were obese before
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Why is diarrhea commonly seen in cases of hyperthyroidism in cats
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Increased food intake
Increased GI motility |
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What are two reasons for why v+ occurs in feline hyperthyroidism
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Rapid intake of lg amts of food
Direct effects of T4 on CRTZ |
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How does feline hyperthryoidism affect the liver and kidneys
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Liver- hypoxia, CHF, and direct hepatotoxic effect of thyroid hormones causes increased ALT or ALP
Kidney- increased CO causes increased renal blood flow, which may decrease BUN and Creatinine and cause diuresis (and PU/PD) |
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Besides liver dysfxn, what is another reason you expect to see an elevated ALP w/ feline hyperthyroidism
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Increased bone turnover
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What are 4 reasons for why you often see L ventricular cardiac hypertrophy in feline hyperthyroidism
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Increased CO, HR, and SV
Decreased PVR Increased adrenergic response Direct effects of thyroid hormones on heart (increased peripheral demand for O2 and increased blood volume) |
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How commonly does heart failure and/or cardiac arrhythmias result from hyperthyroidism
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Uncommon
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What is the most common cause of hypertension in cats (after renal failure)
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Feline hyperthyroidism
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In how many cases of hyperthyroidism is the thyroid palpable
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>90% (w/ 70-80% having bilateral dz)
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Does a normal T4 measurement R/O hyperthyroidism
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No
The value can fluctuate into the normal range But, it is still abnormal in 90% of cases w/ few false positives (and a single elevated T4 in a cat w/ compatible signs is diagnostic) |
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Why is fT4 not used as commonly to diagnose hyperthyroidism as it is to dx hypothyroidism
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Measuring fT4 in cats yields more false +s than TT4
Use fT4 only if T4 has not provided and accurate assessment |
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Why do non-thyroidal illnesses cause elevations in fT4
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Illness may disrupt protein binding and make more free T4 available for the test
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If basal T4 is normal and a dx of hyperthyroidism is suspected, then what
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Repeat the T4 and/or do a fT4 before trying other, more complex methods of dx
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How commonly is a T3 suppression test done to dx hyperthyroidism
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Not commonly
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What is the benefit to radionuclide imaging (scintigraphy) in the dx of hyperthyroidism
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It can locate ectopic thyroid tissue
But, there is the radioactivity and limited availability to consider |
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Why may renal failure result from all txs for hyperthyroidism
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Because they all decrease renal blood flow b/c of decreased CO when euthyroid state is reached (w/ resulting decrease in GFR)
The renal failure was likely present before, but the hyperthyroidism was masking it |
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Why should you always treat w/ methimazole prior to more permanent txs for hyperthyroidism
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To evaluate the effects of "fixing" the hyperthyroidism on renal fxn
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What is the survival time for a hyperthyroid patient, being treated w/ Methimazole, that is now in renal failure
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Stable chronic renal dz has a pretty long survival time
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How does Methimazole work
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Prevents synthesis of thyroid hormones
(iodide cannot be incorporated into tyrosine residues on thyroglobulin molecule) |
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How should treatment w/ Methimazole for hyperthyroidism be monitored
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Check T4 every two weeks until mid-low normal value of T4 is obtained (adjust dose @ 4 weeks as needed)
After dosage is fine-tuned, subsequent evaluation every 6-12 mths |
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What are, and how common are, the SEs assoc'd w/ Methimazole
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Usually w/in first 2-3mths
15%- anorex, v+, lethargy Transient hemo abnormalities Serious hematologic effects possible (agranulocytosis and thrombocytopenia); usually reversible, but can lead to sepsis or severe hemorrhage |
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How can you prevent serious hematologic effects from occurring when using Methimazole
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CBC check every 2wks for the first 3 mths, and then q 6mths
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What is agranulocytosis
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Complete neutropenia
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How many cats will have SEs that prohibit the use of Methimazole
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5-6%
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What is the treatment of choice for hyperthyroidism, if available
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Radioactive Iodine Treatment
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What are 3 advantages to tx via Radioactive Iodine Tx
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Effective for thyroid tissue in any location b/c taken up by the gland, w/ destruction of follicular cells
No significant SEs Effective in 98% cases |
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If surgery is indicated, what steps should be taken pre-op, and which sides of the thyroid should be operated on
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Antithyroid tx for 4-6wks pre-op
Bilateral thyroidectomy (since so many cases are bilateral) |
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What are some SEs of surgery done to tx hyperthyroidism
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Risk of hyperparathyroidism
(check Ca2+ post-op) Horner's Lar-Par Hypothyroidism |
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Which thyroid hormone is the one w/ the ability to affect other cells
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T3
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What organ should be tested whenever there is peripheral neuropathy of unknown cause
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Thyroid gland
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