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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
Reproductive signs associated with HypoTh
- Reproductive dysfunction
26
Other clinical signs associated with Hypothyroidism
- 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
27
Other disease that can be confused with or dfdx's for hypothyroidism?
- Cushing's | - Atopic dermatitis
28
What is the most important question you should ask yourself before trying to diagnose hypothyroidism?
- ****IS THE DOG SICK?**** | - Base upon history, clinical signs, and physical exam
29
Other tests involved in diagnosing Hypothyroidism
- Minimum data base (CBC/Chem/UA) | - Thyroid-specific testing
30
Common signs on CBC with Hypothyroidism
- Normocytic, normochromic, nonregenerative anemia (30%) | - Based on chronic inflammation if disease and chronic disease in general
31
Hypothyroidism Common signs on Chemistry Panel
- Fasting hypertriglyceridemia - Fasting hypercholesterolemia (>75%) - Increased hepatic enzymes
32
Total T4 overview
- Good screening test - Highly sensitive - Moderately specific (~75%)
33
How will non-thyroidal illness impact tT4/fT4/TSH?
- Decreased tT4 - Normal to decreased fT4 - Normal TSH - caused by concurrent illness and that effect on thyroid hormone levels - Normal physiologic response
34
Should you supplement an animal with low total T4 if they have a concurrent illness?
- DO NOT NEED SUPPLEMENTATION - Normal physiologic response - Treat by correcting underlying illness
35
Total T4 in a healthy animal with no clinical signs of Hypothyroidism
- Don't run it
36
Total T4 in a dog with clinical signs attributable to another disease process and not hypothyroidism?
- Be wary of running or interpreting tT4 test in dogs with clinical signs like this
37
Interpretation of tT4 within middle to upper half of reference range
- Normal dog
38
Interpretation of tT4 with low-normal or low range
- Normal fluctuation in euthyroid dog? - Non-thyroidal illness (sick euthyroid syndrome)? - Hypothyroid?
39
What should you do after you see a low tT4?
- fT4 and/or TSH
40
Free T4
- should be by equilibrium dialysis - ACTIVE form of T4 - Less affected by non-thyroidal illness
41
What is the advantage of free T4 over total T4?
- Separate out the T4 from the forms that are bound to the antibodies or are structurally larger - More SPECIFIC than tT4
42
Cost of TT4 vs FT4
- FT4 is more expensive
43
Primary purpose of TT4 vs FT4
- TT4 is a good screening test | - FT4 is used to help confirmation
44
TT4 vs FT4 in terms of being affected by other illness
- TT4 is more affected by euthyroid sick syndrome | - FT4 is less affected by illness but STILL CAN BE
45
TT4 vs FT4 in terms of sensitivity
- TT4 is more sensitive than FT4 (95% compared to 90%)
46
TT4 vs FT4 in terms of specificity
- FT4 is more specific than TT4 (93% compared to 75%)
47
TSH overview
- 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
TSH elevation in what % of dogs?
- Only high in ~70% of hypothyroid dogs
49
TSH and tT4 or tT4 results for a dog with hypothyroidism
- High TSH combined with low TT4 or free TT4 | - Diagnostic for hypothyroidism (99% specific)
50
If you have a normal TSH can you rule out hypothyroidism?
- NO | - Because 30% of hypothyroid dogs have normal TSH
51
If a patient is on ____ or _____, avoid testing thyroid function (if possible).
- Steroids - Phenobarbital - e.g. derm patients
52
Effects of glucocorticoids on: Total T4 Free T4 TSH Treat or not?
- TT4: D to N - FT4: D to N - TSH: N to D - No tx
53
Effects of phenobarbital on: TT4 FT4 TSH Treat or not?
- TT4: D - FT4: D - TSH: Slight elevation - No tx
54
Effect of TMS on: TT4 FT4 TSH Treat or not?
- TT4: D - FT4: D - TSH: I - May tx if clinical
55
Thyroglobulin antibody testing
- 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
Are thyroglobulin antibodies diagnostic of hypothyroidism if elevated?
- No, just supportive
57
T3 and fT3
- Significant fluctuation during the day (even more than fT4) - Not useful - DO NOT LOOK AT THEM - IGNORE THEM
58
TSH stimulation testing
- 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
TRH stimulation test
- Equivocal results
60
Ultrasound
- Insensitive | - Operator-dependent
61
Therapeutic trial for hypothyroidism
- 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
Levothyroxine overview
- Synthetic thyroxine (T4)
63
Route and Frequency of Levothyroxine
- q12hr PO initially - It's a big dose - Even in big dogs they do not like to exceed this
64
Monitoring Levothyroxine
- 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
Side effects of levothyroxine
- Hyperthyroidism (PU/PD, Polyphagia, Tachycardia, nervousness) - Lack of response/treatment failure (inaccurate diagnosis, inadequate dose/compliance, poor absorption)
66
When should you start to see improvement after starting levothyroxine?
- 1-2 weeks
67
When should you start to see improvement in terms of activity levels after starting levothyroxine?
- First 1-2 weeks
68
When should you start to see improvement in terms of weight loss after starting levothyroxine?
- seen in 8 weeks
69
When should you start to see improvement in terms of hair coat after starting levothyroxine?
- Normal hair coat may take several months and may appear worse before it gets better
70
When should you start to see improvement in terms of Neuro signs after starting levothyroxine?
- Improve rapidly, may take 8-12 weeks for complete resolution

SA Medicine Endocrine (8 decks)

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