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SA Medicine Endocrine > Hypothyroidism > Flashcards

Hypothyroidism Flashcards

1
Q

Review the HPA axis do it

A

do it

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2
Q

Which thyroid hormone is the most biologically active?

A

T3

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3
Q

T3

A
  • Most biologically active
  • Enters cell more rapidly
  • 3-5x more potent than T4
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4
Q

How is T3 typically made?

A
  • only 20% made in the thyroid

- Majority produced by peripheral deiodination of T4

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5
Q

How are thyroid hormones produced in the thyroid gland?

A
  • Thyroid peroxidase binds together tyrosine on the thyroglobulins to oxidized iodide = iodinated tyrosine residues
  • Iodinated tyrosine resudes are transformed into T3 and T4 and secreted bound to thyroglobulin
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6
Q

How common is primary hypothyroidism relative to secondary hypothyroidism?

A
  • Most common
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7
Q

Definition of hypothyroidism

A
  • Decrease in T3 and T4
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8
Q

Possible hypothyroidism etiologies

A
  • Thyroiditis
  • Idiopathic Atrophy
  • Neoplasia
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9
Q

Thyroiditis

A
  • Lymphocytic inflammatory infiltration
  • Replacement with fibrous connective tissue
  • Suspected to be immune mediated
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10
Q

Idiopathic atrophy

A
  • Replacement by adipose and connective tissue

- End-stage thyroiditis?

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11
Q

Thyroid neoplasia

A
  • Most thyroid tumors are non-functional
  • Potentially destructive
  • Uncommon overall
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12
Q

Breeds associated with hypothyroidism

A
  • CAN BE ANY BREED
  • Golden
  • Dobie
  • Beagle
  • Great Dane
  • English and Irish setters
  • Shetland Sheepdog
  • Borzoi
  • Toy Fox Terrier
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13
Q

Typical age of Hypothyroid

A
  • Middle-aged
  • Mean of 7 years with a big range
  • Breeds predisposed to thyroiditis tend to be younger at diagnosis
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14
Q

Sex of dogs with hypothyroid

A
  • Spayed/neutered dogs of either gender
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15
Q

Common categories of clinical signs with hypothyroid

A
  • Metabolic
  • Dermatologic
  • Neurologic
  • Cardiovascular
  • Other
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16
Q

Metabolic clinical signs underlying cause

A
  • Decreased metabolic rate

- Insidious onset

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17
Q

Metabolic clinical signs

A
  • Lethargy
  • Weight gain
  • Heat-seeking/cold intolerance
  • Mental dullness
  • Unwillingness to exercise
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18
Q

Dermatologic clinical signs

A
  • Symmetric alopecia (failure of hair to grow back)
  • Hyperpigmentation
  • Dry, scaly skin
  • Superficial pyoderma
  • Otitis
  • Rat-tail (hair loss in areas with more trauma, and the hair has a harder time re-growing)
  • Seborrhea
  • Change in hair quality (dry, brittle)
  • Myxedema (thickening of skin, forehead, eyelids due to mucin deposits secondary to loss of thyroid hormone)
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19
Q

Tragic expression in hypothyroidism

A
  • Myxedema (thickening of skin, forehead, eyelids due to mucin deposits secondary to loss of thyroid hormone)
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20
Q

Neurologic clinical signs in HypoTh affecting the Peripheral Nervous System

A
  • Ranges from weakness and exercise intolerance to ataxia and quadriparesis
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21
Q

Neurologic clinical signs in HypoTh affecting the Central Nervous System

A
  • Seizures, central vestibular disease, mentation changes
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22
Q

Neurologic clinical signs in HypoTh affecting the Cranial Nerves

A
  • Facial nerve paralysis

- Vestibular disease

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23
Q

Cardiovascular Clinical signs with HypoTh

A
  • Bradycardia
  • Low QRS voltage
  • Inverted T wave
  • Reduced left ventricular pump function
  • Rarely a primary cause of myocardial failure
  • Leads to a weak heart
  • NOT MAJOR
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24
Q

Concurrent DCM with Hypothyroidism

A
  • They can occur, e.g. with a Great Dane
  • Weak heart gets weaker
  • Treatment for hypothyroidism might help DCM clinical signs
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25
Q

Reproductive signs associated with HypoTh

A
  • Reproductive dysfunction
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26
Q

Other clinical signs associated with Hypothyroidism

A
  • Megaesophagus (treatment can resolve)
  • Laryngeal paralysis (treatment does not usually resolve)
  • Ocular changes
  • Not proven to be causal
  • Treatment does not consistently reverse clinical signs
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27
Q

Other disease that can be confused with or dfdx’s for hypothyroidism?

A
  • Cushing’s

- Atopic dermatitis

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28
Q

What is the most important question you should ask yourself before trying to diagnose hypothyroidism?

A
  • IS THE DOG SICK?

- Base upon history, clinical signs, and physical exam

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29
Q

Other tests involved in diagnosing Hypothyroidism

A
  • Minimum data base (CBC/Chem/UA)

- Thyroid-specific testing

30
Q

Common signs on CBC with Hypothyroidism

A
  • Normocytic, normochromic, nonregenerative anemia (30%)

- Based on chronic inflammation if disease and chronic disease in general

31
Q

Hypothyroidism Common signs on Chemistry Panel

A
  • Fasting hypertriglyceridemia
  • Fasting hypercholesterolemia (>75%)
  • Increased hepatic enzymes
32
Q

Total T4 overview

A
  • Good screening test
  • Highly sensitive
  • Moderately specific (~75%)
33
Q

How will non-thyroidal illness impact tT4/fT4/TSH?

A
  • Decreased tT4
  • Normal to decreased fT4
  • Normal TSH
  • caused by concurrent illness and that effect on thyroid hormone levels
  • Normal physiologic response
34
Q

Should you supplement an animal with low total T4 if they have a concurrent illness?

A
  • DO NOT NEED SUPPLEMENTATION
  • Normal physiologic response
  • Treat by correcting underlying illness
35
Q

Total T4 in a healthy animal with no clinical signs of Hypothyroidism

A
  • Don’t run it
36
Q

Total T4 in a dog with clinical signs attributable to another disease process and not hypothyroidism?

A
  • Be wary of running or interpreting tT4 test in dogs with clinical signs like this
37
Q

Interpretation of tT4 within middle to upper half of reference range

A
  • Normal dog
38
Q

Interpretation of tT4 with low-normal or low range

A
  • Normal fluctuation in euthyroid dog?
  • Non-thyroidal illness (sick euthyroid syndrome)?
  • Hypothyroid?
39
Q

What should you do after you see a low tT4?

A
  • fT4 and/or TSH
40
Q

Free T4

A
  • should be by equilibrium dialysis
  • ACTIVE form of T4
  • Less affected by non-thyroidal illness
41
Q

What is the advantage of free T4 over total T4?

A
  • Separate out the T4 from the forms that are bound to the antibodies or are structurally larger
  • More SPECIFIC than tT4
42
Q

Cost of TT4 vs FT4

A
  • FT4 is more expensive
43
Q

Primary purpose of TT4 vs FT4

A
  • TT4 is a good screening test

- FT4 is used to help confirmation

44
Q

TT4 vs FT4 in terms of being affected by other illness

A
  • TT4 is more affected by euthyroid sick syndrome

- FT4 is less affected by illness but STILL CAN BE

45
Q

TT4 vs FT4 in terms of sensitivity

A
  • TT4 is more sensitive than FT4 (95% compared to 90%)
46
Q

TT4 vs FT4 in terms of specificity

A
  • FT4 is more specific than TT4 (93% compared to 75%)
47
Q

TSH overview

A
  • Screening test in people

- Due to a lack of negative feedback TSH should be high, but dogs don’t like to follow the rules

48
Q

TSH elevation in what % of dogs?

A
  • Only high in ~70% of hypothyroid dogs
49
Q

TSH and tT4 or tT4 results for a dog with hypothyroidism

A
  • High TSH combined with low TT4 or free TT4

- Diagnostic for hypothyroidism (99% specific)

50
Q

If you have a normal TSH can you rule out hypothyroidism?

A
  • NO

- Because 30% of hypothyroid dogs have normal TSH

51
Q

If a patient is on ____ or _____, avoid testing thyroid function (if possible).

A
  • Steroids
  • Phenobarbital
  • e.g. derm patients
52
Q

Effects of glucocorticoids on:

Total T4
Free T4
TSH

Treat or not?

A
  • TT4: D to N
  • FT4: D to N
  • TSH: N to D
  • No tx
53
Q

Effects of phenobarbital on:

TT4
FT4
TSH

Treat or not?

A
  • TT4: D
  • FT4: D
  • TSH: Slight elevation
  • No tx
54
Q

Effect of TMS on:

TT4
FT4
TSH

Treat or not?

A
  • TT4: D
  • FT4: D
  • TSH: I
  • May tx if clinical
55
Q

Thyroglobulin antibody testing

A
  • Found in up to 50% of hypothyroid dogs
  • Can increase tT4 measurement
  • Increase suspicion of hypothyroidism
  • Presence can indicate progression into hypothyroidism in the future
56
Q

Are thyroglobulin antibodies diagnostic of hypothyroidism if elevated?

A
  • No, just supportive
57
Q

T3 and fT3

A
  • Significant fluctuation during the day (even more than fT4)
  • Not useful
  • DO NOT LOOK AT THEM
  • IGNORE THEM
58
Q

TSH stimulation testing

A
  • GOLD STANDARD
  • Very expensive
  • If you have an animal with clinical signs (lab changes, total T4 low, TSH normal) and want to make a slam dunk
59
Q

TRH stimulation test

A
  • Equivocal results
60
Q

Ultrasound

A
  • Insensitive

- Operator-dependent

61
Q

Therapeutic trial for hypothyroidism

A
  • Treat with levothyroxine
  • Wait 4-8 weeks
  • Assess for any (even subtle0 improvements in clinical signs
  • If response seen, stop supplementation to prove diagnosis (wait 2-3 weeks)
  • If see signs return, then hypothyroidism likely
  • Don’t do this in a healthy animal!
62
Q

Levothyroxine overview

A
  • Synthetic thyroxine (T4)
63
Q

Route and Frequency of Levothyroxine

A
  • q12hr PO initially
  • It’s a big dose
  • Even in big dogs they do not like to exceed this
64
Q

Monitoring Levothyroxine

A
  • Recheck TT4 after 4-8 weeks of therapy
  • Most dogs show good control at this dose, but treatment needs to be individualized to each patient
  • Recheck total T4 4 weeks after a dose change then every 6 months if doing well
65
Q

Side effects of levothyroxine

A
  • Hyperthyroidism (PU/PD, Polyphagia, Tachycardia, nervousness)
  • Lack of response/treatment failure (inaccurate diagnosis, inadequate dose/compliance, poor absorption)
66
Q

When should you start to see improvement after starting levothyroxine?

67
Q

When should you start to see improvement in terms of activity levels after starting levothyroxine?

A
  • First 1-2 weeks
68
Q

When should you start to see improvement in terms of weight loss after starting levothyroxine?

A
  • seen in 8 weeks
69
Q

When should you start to see improvement in terms of hair coat after starting levothyroxine?

A
  • Normal hair coat may take several months and may appear worse before it gets better
70
Q

When should you start to see improvement in terms of Neuro signs after starting levothyroxine?

A
  • Improve rapidly, may take 8-12 weeks for complete resolution

SA Medicine Endocrine (8 decks)

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