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Clinical Pathology > Endocrine System: Thyroid > Flashcards

Endocrine System: Thyroid Flashcards

1
Q

Where is the thyroid found? What 2 cells are found in it?

A

lateral to the first 5 tracheal rings

  1. parafollicular (C) cells - produce thyroglobulin, transport thyroid hormones, transport iodine
  2. follicular epithelial cells - produce colloid, T3, and T4
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2
Q

What regulates the thyroid? What 2 hormones does it produce?

A

synthesis and secretion regulated by TSH

  1. thyroxine (T4) = 100% synthesized by thyroid
  2. triiodothyronine (T3) = more potent than T4, 40/60 produced in thyroid and systemic cells
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3
Q

Which thyroid hormone is produced more? How is the other one produced?

A

T4

T3 is produced outside of the thyroid from deiodination of T4 (also producing a small amount of reverse T3)

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4
Q

How do T4 and T3 compare?

A

T4 = less reactive on target cells, 99.9% bound to albumin or thyroid binding globulin, 0.1% unbound (fT4)

T3 = more reactive on target cells, 99% bound, 1% unbound (fT3)

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5
Q

What are the biologically active forms of T3 and T4?

A

fT3 (1%), fT4 (0.1%) —> can diffuse into target cells

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6
Q

How does the HPT axis control thyroid hormone production and secretion?

A

high concentrations of fT3 and fT4 will have a negative feedback loop to decrease function and secretions from the hypothalamus and pituitary

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7
Q

What are 7 major functions of thyroid hormones?

A
  1. maintain basal metabolic rate (BMR)
  2. controls oxygen consumption (basal RBC concentration), GFR, and glucose metabolism
  3. required for proper development of musculoskeletal system and organs in juveniles
  4. essential for normal catecholamine actions
  5. enable lipoprotein lipase (LPL) activity for lipolysis
  6. maintain normal haircoat and sebaceous gland activity
  7. maintain normal neural signal transduction
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8
Q

What 4 baseline tests are used to assess thyroid gland function? What collection is required to test these on?

A
  1. T4 and fT4*
  2. endogenous TSH*
  3. T3 and fT3
  4. T3 suppression test

SERUM (not part of typical routine biochem)

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9
Q

What is included in thyroid panels for dogs and cats?

A

DOGS = total thyroxine (tT4), free thyroxine (fT4), TSH

CATS = total thyroxine (tT4) +/- free thyroxine (fT4)

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10
Q

What does thyroxine (T4) testing measure? What is the gold standard? What else is done?

A

total circulating T4 (protein-bound and free)

radioimmunoassay (RIA)

Snap ELISA kit —> not reliable for diagnosing hypothyroidism, reliable for normal [T4]

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11
Q

What 3 medications can cause a false decrease in tT4 value? What commonly causes a false increase?

A
  1. glucocorticoids
  2. sulfonamides
  3. phenobarbital

~10% of hypothyroid dogs produce anti-T4 antibodies, which confound results

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12
Q

What does free thyroxine (fT4) testing measure? What is the gold standard? What else can be done?

A

unbound, circulating T4

equilibrium dialysis (ED)

radioimmunoassay (RIA) —> underestimates fT4 in dogs

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13
Q

Why is fT4 testing preferred over tT4 testing?

A
  • better at assessing thyroid status, as fT4 is the biologically active form
  • results are less affected by medications

(more expensive and time-consuming, so it is less commonly done)

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14
Q

Why is T3 and fT3 testing not commonly done in dogs? In which breeds may it be beneficial?

A

more falsely increased or decreased results due to greater prevalence of T3 autoantibodies

sighthounds (Greyhounds, Whippets) —> tend to have lower T4 and fT4 levels than other dogs, but comparable T3

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15
Q

What are the 3 types of assays used for TSH testing? Which one is preferred?

A
  1. immunoradiometric*
  2. chemiluminescent
  3. ELISA
  • = differentiates normal from increased TSH concentrations, but cannot detect lower concentrations
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16
Q

What is considered the single gold standard test to measure thyroid status? What are the 4 steps?

A

TSH stimulation test (expensive and time-consuming)

  1. measure baseline T4
  2. administer TSH (bovine or recombinant human)
  3. re-measure T4 6 hours after
  4. should see minimal increase in T4 following TSH administration in hypothyroid dogs
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17
Q

What are the 2 major disease syndromes of canine hypothyroidism? What must they be differentiated from?

A
  1. primary - canine adult-onset hypothyroidism
  2. secondary - RARE, TSH is not synthesized or secreted due to pituitary neoplasia, congenital malformation, or trauma/surgery

euthyroid sick syndrome, spurious changes

18
Q

What most commonly causes canine hypothyroidism?

A

AUTOIMMUNE DISEASE - lymphocytic thyroiditis causes idiopathic thyroid atrophy due to the production of thyroglobulin autoantibodies, lymphocyte and macrophage invasion, and replacement of endocrine tissue by fibrous tissue

19
Q

What is the most common signalment for canine hypothyroidism? What are the 6 most common clinical signs?

A

middle-ages, usually pure-bred

  1. lethargy, inactivity, mental dullness
  2. weight gain
  3. alopecia, often symmetrical (rat tail)
  4. dry, brittle hair coat
  5. seborrhea
  6. myxedema (drooping eyelids)

(apparent when there’s <25% of thyroid tissue, gradual onset, affects many systems)

20
Q

What are 3 indications on CBC of canine hypothyroidism?

A
  1. non-regenerative anemia (50%)
  2. normocytic, normochromic (decreased metabolic rate = decreased oxygen demand = decreased EPO)
  3. codocytes on blood smear
21
Q

What are 2 indications on chemistry of canine hypothyroidism?

A
  1. fasting hypercholesterolemia (75%) - decreased cholesterol catabolism by liver
  2. fasting hypertriglyceridemia (88%)
22
Q

What are 3 indications on thyroid function tests of canine hypothyroidism?

A
  1. decreased tT4 - not specific, but if normal there is a strong indication that the dog is not hypothyroid
  2. decreased fT4 - usually not affected by non-thyroid illness
  3. increased TSH - confirmatory when elevated with low tT4 and fT4
23
Q

What is the most common signalment for feline hyperthyroidism? What are the 2 possible pathogeneses?

A

old to geriatric cats (>10 y/o) “acting like a kitten”

  1. functioning adenoma (or hyperplasia)* - bilateral > unilateral > ectopic
  2. thyroid adenocarcinoma (1-2%)
24
Q

What are the 4 most common clinical signs associated with feline hyperthyroidism?

A
  1. weight loss and polyphagia* due to increased basal metabolic rate and energy consumption
  2. vomiting, diarrhea
  3. aggression, hyperactivity
  4. PU/PD
25
Q

What are the 3 most common physical exam findings associated with feline hyperthyroidism?

A
  1. bilateral thyroid slip (70%)
  2. tachycardia +/- heart murmur
  3. poor hair coat
26
Q

What 3 findings on CBC are indicative of feline hyperthyroidism?

A
  1. erythrocytosis due to increased metabolic rate, increased oxygen demand, and increased EPO; T4 directly stimulates BM erythropoiesis
  2. Heinz body formation NOT associated with hemolytic anemia
  3. stress leukogram (neutrophilia, lymphopenia)
27
Q

What are 3 indications on chemistry panes of feline hyperthyroidism?

A
  1. increased liver enzyme activity: mild to moderate increases in ALT correlating with tT4 levels
  2. altered bone metabolism contributes to increased ALP
  3. azotemia (elevated BUN/CREA): pre-renal = dehydration; renal = concurrent primary renal insufficiency
28
Q

What 2 conditions commonly mask azotemia in feline hyperthyroidism? How can it be unmasked?

A
  1. increased GFR due to increased cardiac output
  2. less muscle mass = lower basal CREA

treatment

29
Q

What is the single most reliable indicator of feline hyperthyroidism on thyroid function tests? What other 3 options are used to confirm?

A

increased tT4

  1. recheck tT4 in 1-2 weeks
  2. treat underlying disease and recheck tT4
  3. submit fT4 ($$)
30
Q

What is euthyroid sick syndrome? What 3 effects does it have on thyroid function testing?

A

common adaptive response where sick animals have decreased tT4, but lack clinical signs of hypothyroidism (no treatment necessary)

  1. decreased tT4
  2. normal fT4
  3. normal TSH
31
Q

How is the T3 suppression test used in cats? What are the 3 steps? What is the interpretation?

A

differentiates hyperthyroid cats from euthyroid sick cats when T4 is normal

  1. measure baseline T4 and T3
  2. client administers T3 to cat orally several times over 2 days
  3. measure T4 and T3 2-4 hours after the last dose of T3

if final T4 and T3 is above cutoff value = lack of T3 and T4 suppression = confirms HYPERTHYROIDISM

32
Q

What are the 4 diagnostic criteria used to diagnose feline hyperthyroidism?

A
  1. consistent clinical signs
  2. palpable thyroid tumor
  3. increased tT4
  4. response to therapy
33
Q

What are the 3 general causes of spurious cases of altered thyroid test results?

A
  1. daily variation - tT4 is sporadic and unpredictable, commonly low around 8 AM and high at 2PM
  2. age - progressive decline in tT4, fT4, and T3
  3. breed - medium to large breeds, greyhounds, and sighthounds trend to lower tT4
34
Q

What 2 drugs can cause spurious cases of altered thyroid test results? How?

A
  1. glucocorticoids - changers peripheral metabolism of T4 and inhibits TSH formation
  2. sulfonamindes - blocks iodination of thyroglobulin
35
Q

What is the most common chemistry abnormality associated with feline hyperthyroidism?

a. hypercholesterolemia
b. hypocholesterolemia
c. increased ALT activity
d. decreased ALT activity

A

C

36
Q

What is the most common chemistry abnormality associated with canine hypothyroidism?

a. hypercholesterolemia
b. hypocholesterolemia
c. increased ALT activity
d. decreased ALT activity

A

A

(+ hypertriglyceridemia)

37
Q

Interpret the canine thyroid panel:

a. euthyroid sick syndrome
b. primary hypothyroidism
c. secondary hypothyroidism

A

B

  • decreased tT4
  • decreased fT4
  • increased TSH
38
Q

Interpret the canine thyroid panel:

a. euthyroid sick syndrome
b. primary hypothyroidism
c. secondary hypothyroidism

A

A

  • decreased tT4
  • normal fT4
  • normal TSH
39
Q

Interpret the canine thyroid panel:

a. euthyroid sick syndrome
b. primary hypothyroidism
c. secondary hypothyroidism

A

C

  • decreased tT4
  • decreased fT4
  • decreased TSH (TSH not produced/secreted)
40
Q

Diagnosing primary hypothyroidism, secondary hypothyroidism, and euthyroid sick syndrome:

A

Clinical Pathology (33 decks)

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