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60 Cards in this Set
- Front
- Back
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What are the 5 functions of thyroid hormone?
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1) Maintain metabolic rate
2) Normal thermogenesis 3) Stimulate growth & maturation 4) Stimulate heart rate, cardiac output, blood flow 5) Increase gene transcription |
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What are the 2 main thyroid hormones?
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T4 (thyroxine)
T3 (triidothyronine) |
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What is the precursor for T3 & T4?
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Thyroglobulin
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What do T3 and T4 circulate bound to?
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Thyroid-binding globulin
Albumin |
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What is free T4? How much of total T4 does it make up?
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The fraction that circulates unbound to thyroid-binding globulin
-Small amount (0.1%) of T4 |
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What is the biologically active form of T4?
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The free T4
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What 2 hormones regulate thyroid hormone secretions?
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TRH=thyrotropin-releasing hormone from Hypothalamus-->
TSH=thyroid stimulating hormone from Pituitary gland |
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T3 and T4 provide negative feedback to what part of the thyroid-pituitary feedback regulation?
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Negative feedback on TSH from pituitary
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What T4 is included in the serum total T4?
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Includes protein-bound and free T4
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What is the serum total T4 assay ran on?
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Serum
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True or false. When mailing in serum for a serum total T4, you need to put the sample on ice.
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False, relatively stable so not necessary
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Serum total T4 may be used in diagnosis of what?
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Hypo or hyperthyroidism
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When is there always a decrease in serum total T4?
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In patients with hypothyroidism
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What are 5 non-thyroid situations in which there is sometimes a decrease in serum total T4?
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1) Non-thyroidal illness
2) Glucocorticoids 3) Sulfonamides 4) Phenobarbital 5) NSAIDs |
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What does the serum free T4 measure?
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Non-protein bound fraction of T4
-Which is about 0.1% of total -Fraction that's biologically active |
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What test is more specific for hypothyroidism, serum total T4 or serum free T4, why?
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Serum free T4 because less affected by non-thyroid factors so more specific test for hypothyroidism
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What are the 2 methods used to measure free T4? Compare each.
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1) Equilibrium dialysis
-Better -Harder -More expensive 2) Non-dialysis immunoassay -Not as good -Easier (automated) -Less expensive |
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Why aren't serum total and free T3 usually performed?
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Don't really give you any additional information for diagnosis of thyroid disease, but may be part of comprehensive thyroid panel
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Serum endogenous TSH is useful in assessing _____.
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Hypothyroidism
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What is the body's response to decrease in free T4?
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Pituitary gland senses it and increases production of TSH
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What combination of lab results is most often used for diagnosis of hypothyroidism?
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Decreased free T4 and increased TSH
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What combination of lab results is most often used for diagnosis of hypothyroidism?
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Decreased free T4 and increased TSH
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True or false. TSH levels and free T4 levels can be used as a sole diagnostic test for hypothyroidism.
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False, there are a few non-thyroidal factors that affect TSH so don't use it as a sole diagnostic test for hypothyroidism.
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True or false. TSH levels and free T4 levels can be used as a sole diagnostic test for hypothyroidism.
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False, there are a few non-thyroidal factors that affect TSH so don't use it as a sole diagnostic test for hypothyroidism.
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True or false. TSH assays are species specific so the lab has to use a canine TSH assay.
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True
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True or false. TSH assays are species specific so the lab has to use a canine TSH assay.
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True
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What would be a reason for measuring T3 and T4 autoantibodies?
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They can interfere w/ assays for T3 and T4, leading to very high or low results, so sometimes tested to clarify puzzling test results
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What would be a reason for measuring T3 and T4 autoantibodies?
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They can interfere w/ assays for T3 and T4, leading to very high or low results, so sometimes tested to clarify puzzling test results
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Are T4 and T3 autoantibodies only present in dogs that are hypothyroid?
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No, can also be present in normothyroid dogs
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Are T4 and T3 autoantibodies only present in dogs that are hypothyroid?
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No, can also be present in normothyroid dogs
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What does the presence of anti-thyroglobulin antibodies indicate?
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autoimmune thyroiditis, and present in 36-60% of hypothyroid dogs, but not specific for hypothyroidism
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Anti-T4 antibodies don't affect ______ T4.
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Free T4- so serum free T4 is better test for hypothyroidism
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What are 3 thyroid function tests?
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1) TSH stimulation test
2) TRH stimulation test 3) T3 suppression |
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What USED to be the gold standard for testing for hypothyroidism? Why isn't it used anymore?
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TSH stimulation test-used bovine TSH, which is no longer available and recombinant human TSH is too expensive
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How does the TSH stimulation test work?
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Take baseline (T4) blood sample, give 0.1 units TSH/kg body weight, then draw second sample 6 hours later and normal dog will have 2X increase in T4
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What is TRH stimulation used for?
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-Not used in dogs(no better than measuring endogenous TSH)
-Sometimes used to diagnose hyperthyroidism in cats or hypothyroidism in horses |
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When is T3 suppression used?
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In cats if total T4 is inconclusive for hyperthyroidism
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Explain how T3 suppression works.
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Draw baseline T4, then give oral T3 TID for 2 days, then take 2nd sample on third day. Normal cats will suppress to <1.5 ug/dL
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Hypothyroidism is most common in what domestic specie?
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Dogs
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What are 2 causes of primary hypothyroidism?
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1) Lymphocytic thyroiditis
2) Idiopathic atrophy |
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What is a cause of secondary hypothyroidism?
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Pituitary tumors
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Diagnosis of hypothyroidism in dogs is based on ______, ______, _____ and _______.
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History, clinical signs, lab data and thyroid testing
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What are 2 common abnormalities in the CBC and biochem of dogs with hypothyroidism?
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1) Mild normocytic, normochromic anemia
-Decreased tissue consumption of O2 decreases need for erythropoietin 2) Hypercholesterolemia |
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What are the most common history (clinical signs) associated with hypothyroidism?
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Lethargy, weight gain, mild exercise intolerance, cold intolerance
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What are the 3 most commonly-used laboratory tests used for diagnosis of hypothyroidism?
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1) Total T4
2) Free T4 by equilibrium dialysis 3) Endogenous TSH |
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True or false. If the total T4 is within the reference range, it normally rules out hypothyroidism.
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True
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What are 3 causes of a low total T4?
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1) Hypothyroidism
2) Euthyroid sick syndrome 3) Drugs (steroids, phenobarb, furosemide) |
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If you run a total T4 and it comes back low, what's your next diagnostic step?
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Run free T4 by equilibrium dialysis and measure endogenous TSH
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What is diagnostic for hypothyroidism in dogs (for the most part)?
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Combination of low free T4 and high TSH
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If a dog is on thyroid medication, how long must you wait before testing the free T4 and endogenous TSH?
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4-8 weeks for accurate assessment
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What are 3 ways to monitor the therapeutic success of treating hypothyroidism?
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1) Look for decrease/elimination of clinical signs
2) Can measure serum T4 concentration 3) Measure peak (4-6 hours after meds), should be high normal and trough (just before next dose) should be low normal |
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Hyperthyroidism is most commonly seen in what domestic animal?
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Cats
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What usually causes hyperthyroidism in cats?
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Bilateral hyperplasia
benign adenomas -Carcinomas rare |
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Diagnosis of hyperthyroidism in cats is made on ____, _____ and ______.
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History, clinical signs, and laboratory tests
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What is a common history of a hyperthyroid cat?
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Weight loss, older cat (>8 yrs), PU/PD, polyphagia, vomiting, tachycardia, often depressed, enlarged glands may be palpable
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What are 3 common abnormalities in the CBC and biochem of a hyperthyroid cat?
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1) Mild increase in PCV
2) Mild increase in ALT 3) +/- mild hyperphosphatemia |
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*______ is usually increase in hyperthyroid cats.
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Total T4
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What should you do if the total T4 on a cat you suspect is hyperthyroid is high normal?
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-Retest total T4 in a few weeks OR perform T3 suppression test
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Why isn't free T4 routinely measured in cats suspected to be hyperthyroid?
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Because there are some non-thyroidal illnesses that can increase fT4 concentrations so get false positives
-Can be an option when cat has typical clinical signs and history of hyperthyroidism but a total T4 within reference range |
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True or false. They're not sure if adult hypothyroidism in horses exists.
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True, but experimental hypothyroidism has shown free and total T4 decrease and TSH is increased
-No commercially available equine TSH assay |
