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71 Cards in this Set
- Front
- Back
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What proportion of T4 is bound to albumin in circulation?
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99%
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High levels of T4 and T3 provide negative inhibition on the secretion of these 2 hormones.
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TRH from hypothalamus
TSH from pituitary |
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Form of T4 that enters cells
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Free T4
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Biologically active form of thyroid hormone
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T3
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Two causes of primary hypothyroidism
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Lymphocytic thyroiditis
Idiopathic atrophy |
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Is primary or secondary hypothyroidism more common?
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Primary (95%)
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Typical signalment of a hypothyroid dog
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Middle aged
Purebred (Goldens, Dobies, Boxer, Sheltie) |
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CS of hypothyroidism
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Decreased BMR
Weight gain Lethargy Weakness and myopathy Exercise intolerance Symmetrical alopecia and seborrhea Myxedema |
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Nonpitting edema, often causing a "tragic" facial expression in hypothyroid dogs
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Myxedema
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What causes myxedema?
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Increased deposition of CT components in SQ space
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How can hypothyroidism affect the CV system?
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Bradycardia
Weak pulses Muffled heart sounds Reduced myocardial contractility |
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Nerves most commonly affected by hypothyroidism
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Cranial nerves like facial and vestibular
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Possible cause of CNS signs in a hypothyroid dog
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Atherosclerosis and compromised blood flow
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Gender more commonly affected in a reproductive sense by hypothyroidism
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Females
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Reproductive effects of hypothyroidism
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Infertility
Increased puppy mortality Low birth weight |
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CS of congenital hypothyroidism
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Dwarfism
Mental retardation Broad skull, macroglossia, delayed dental eruption Ataxia, hypermetria |
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How frequently is hypercholesterolemia seen with hypothyroidism?
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75% of cases
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Ddx for elevated cholesterol.
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Hypothyroidism
Post-prandial Diabetes mellitus Cushing's Cholestatic liver disease Pancreatitis PLN Familial hyperlipidemia |
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How is a dx of hypothyroidism made?
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CS
Blood chem: Hypercholesterolemia and low Na Thyroid function tests Response to tx |
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How useful is total T4 for measuring thyroid function?
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Sensitive, 90% hypothyroid dogs have low T4
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What can cause a false elevation of T4?
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T4 autoantibodies
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What does it mean if an animal is "euthyroid sick?"
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Artificial lowering of T4 b/c another illness affects T4 protein binding
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How specific is low T4 as a diagnostic criteria for hypothyroidism?
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Not very specific b/c levels fluctuate during the day
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How does human T4 compare to canine T4 levels?
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Human T4 4-5X greater
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Value of T3 test in diagnosing hypothyroidism
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Poor
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Drug class that can affect protein binding of T4
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Glucocorticoids
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Only accurate method for measuring free T4
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Equilibrium dialysis assay
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WHhch test is more sensitive and specific: Total T4 or free T4?
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Free T4
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When T4 is low, how should TSH levels respond?
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Should increase
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What is the value of TSH test in diagnosing hyperthyroidism?
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Adds specificity to T4 and fT4...elevated TSH levels help with dx
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True or false: A dog with early primary hypothyroidism may have high TSH and low-normal T4.
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True
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Non thyroidal factors that influence thyroid function tests
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Breed
Drugs Illness |
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4 examples of drugs that can affect thyroid function tests
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Corticosteroids
Sulfonamides Phenobarbital Clomipramineq |
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How does the baseline thyroid hormone level in sighthounds compare to that of other dogs?
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Lower
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Major protein in colloid of the thyroid
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Thyroglobulin
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Used as markers of autoimmune thyroiditis
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T4 antibodies
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Which thyroid function test is unaffected by T4 auto-antibodies
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free T4
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Treatment for hypothyroidism
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Levothyroxine 0.02mg/kg QD
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How soon after hypothyroidism treatment begins should improvement of CS be seen?
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1-2 weeks
(months for skin to improve) |
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How should hypothyroid dog be monitored once treatment starts?
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Recheck in 6-8 weeks:
Resolution of CS T4 should be measured 4-6 h post pill |
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A dog has been treated for hypothyroidism, and presents with PU/PD, panting, hyperactivity, and polyphagia...What is likely happening?
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Hyperthyroidism from oversupplementation or concurrent illness
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How should oversupplementation of levothyroxine be addressed?
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Stop treatment for 2-3 days
Reinstitute at 50-75% of original dose |
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What % of thyroid hyperplasias are benign in cats?
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98%
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What disease is caused by feline adenomatous hyperplasia of the thyroid?
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Hyperthyroidism
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Hyperthyroidism is more common in what species: Dog, or cat?
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Cat
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CS of hyperthyroidism in cats
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Polyphagia
Weight loss Hyperactivity |
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Ddx for polyphagia with weight loss include:
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Hyperthyroidism
Diabetes mellitus Malabsorption Neoplasia |
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How does hyperthyroidism cause PU/PD?
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Increased renal blood flow
Increased tubular fluid |
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How does hyperthyroidism affect the digestive system?
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Diarrhea
Vomiting Sometimes anorexia |
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How does hyperthyroidism affect the CV system?
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Heart murmur, gallop rhythm, arrhythmias
LV hypertrophy |
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What may be seen in a hyperthyroid cat on PE?
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Thin
Palpable thyroid nodules Heart murmur or gallop Tachycardia Hypertension and retinopathy Weakness CNS signs |
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Is thyroid screening done more commonly in dogs or in cats?
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Cats
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How does hyperthyroidism lead to an increase in liver enzymes?
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Possibly hypoxic injury to the hepatocytes
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How frequently is T4 falsely elevated with a hyperthyroid cat?
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Rarely
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Good screening test for feline hyperthyroidism
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T4 (total)
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Gold standard of diagnosis for feline hypothyroidism
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Nuclear scintigraphy
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Test of choice for diagnosing feling hyperthyroidism
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Total T4
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You suspect a cat is hyperthyroid but his total T4 is normal...What do you do?
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Repeat T4 in 1-4 weeks
Measure fT4 Search for other disease |
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Why is fT4 not used for initial testing for hyperthyroidism in cats?
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Can be falsely elevated in some cats with nonthyroidal illness
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Treatment options for feline hyperthyroidism
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Methimazole
Radioactive iodine Surgical |
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Why can treating feline hyperthyroidism unmask underlying kidney disease?
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Restoration of normal blood flow decreases GFR which can cause increase in BUN
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How does methimazole treat hyperthyroidism?
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Inhibits synthesis of thyroid hormone by thyroid peroxidase
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Appropriate dosage regimen for methimazole in the cat
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5 mg/day in 1 or 2 treatments
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Time to full effect when treating a cat with methimazole
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4 weeks
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Side effects of methimazole
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Hemologic
GI (most commonly) |
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How often should hyperthyroid cats be re-evaluated?
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Every 6-12 months after initial reckecks at 2 and 4 weeks.
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Pros of using transdarmal methimazole to treat feline hyperthyroidism
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Well tolerated
Easier than pilling a cat |
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Cons of using transdarmal methimazole to treat feline hyperthyroidism
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Requires higher dose td than po
Owner has to wear gloves to apply |
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If SE of methimazole are going to happen, how long does it take them to appear?
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Within 8 weeks of beginning Tx
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Alternatives to methimazole for treating feline hyperthyroidism
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Iopanoic acid (Radioactive iodine!)
Propanolol/ Atenolol for B blocking effect to spare heart |
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Risk associated with surgical thyroidectomy
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Hypoparathyroidism
Horner's syndrome Laryngeal paralysis Hypothyroidism |
