canine hypothyroid

Question 1

normal thyroid - what stimulates it, what it produces?

Answer 1

Normal thyroid
◦ Stimulated by TSH from anterior pituitary (AP is stimulated by TRH from hypothalamus)
◦ Produces T3, T4
> increases metabolism

Question 2

primary canine hypothyroidism is generally due to what?

Answer 2

◦ Thyroid atrophy or lymphocytic thyroiditis/destruction
◦ Genetic component, likely environmental factors

Question 3

Clinical Presentation: Canine Hypothyroid
- age, breed

Answer 3

• Median age onset 7 years
  > Uncommon in dogs <2 years
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  Breed predispositions:
• Boxer, Golden Ret, OES, Cocker Spaniel, Ridgeback, Eng Setter

Question 4

canine hypothyroid clinical signs
- metabolic signs
- skin and haircoat abnormalities

Answer 4

** ~80% of dogs have these metabolic and skin & haircoat changes**
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Metabolic signs:
- Lethargy
- Weight gain / obesity
- Decreased exercise tolerance
- Cold intolerance (“heat seeker”)
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Skin & haircoat abnormalities:
- Alopecia / thin hair coat / “rat tail”
- Dry hair coat / excess dander
- Otitis externa
- Pyoderma
- Seborrhea
- Hyperpigmentation (skin)
- Bilaterally symmetrical changes
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Other (less common):
- Neuropathy
- Myopathy
- Bradycardia
- Reproductive abnormalities

Question 5

Hypothyroidism & Obesity - how common is canine obesity

Answer 5

40% of hypothyroid dogs are overweight
(Most obese dogs suffer from over-nutrition)

Question 6

Routine Labwork Findings for canine hypothyroid
- CBC, biochem, urinalysis

Answer 6

CBC
- Mild anemia: normocytic, normochromic
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Biochemical profile
- Increased cholesterol (~75% dogs)
- Increased triglycerides
- Increased liver enzymes (~30% dogs)
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Urinalysis
- no specific changes

Question 7

Testing for canine Hypothyroidism - is it simple? why?

Answer 7

• Can be challenging
• Multiple tests available
• No single test has 100% diagnostic specificity or sensitivity
• Non-thyroidal illness can impact results
• Maximize diagnostic utility through appropriate patient selection

Question 8

first steps for canine hypothyroid dx

Answer 8

First steps – consistent abnormalities on:
* History & PE
* Routine labwork (CBC, biochemical profile)
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If high index of suspicion for hypothyroidism
* Specific thyroid testing

Question 9

canine Thyroid Testing: Laboratory Options

Answer 9

Total T4 (TT4)
Free T4 (fT4)
Canine thyroid stimulating hormone (cTSH)
Autoantibodies against:
* Thyroglobulin (TgAA)
* T3 and T4 (T3AA; T4AA)

Question 10

T4 Level (Total and Free) - difference between them? what do levels mean?

Answer 10

◦ Free T4 is the more biologically active form versus total T4
◦ Good screening tool
> If normal T4, hypothyroidism unlikely
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Low T4 not specific finding for hypothyroidism
◦ Could be hypothyroidism or falsely lowered by other illness/medications
◦ FT4 less affected than TT4 (but can still be affected)

Question 11

TSH Level - what does it tell us?

Answer 11

◦ Hypothyroid dogs
◦ Low circulating thyroid hormone
◦ Lack of negative feedback
◦ TSH should be elevated (~75% of hypothyroid dogs)
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◦ Up to 1/3 of hypothyroid dogs have normal TSH

Question 12

Thyroid Autoantibodies - why do we look for them, what do levels tell us? when do we do this?

Answer 12

◦ Hypothyroidism often originates with lymphocytic thyroiditis
> Antibodies against thyroglobulin, T3, and/or T4 can be elevated in this phase
◦ Over time, antibodies become negative as thyroid atrophies/inflammation
resolves
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Antibody measurement nor routinely recommended
◦ Primarily performed in breeding dog screening
◦ Significance of positive antibodies in normal dogs questionable: does not 100% predict future hypothyroidism

Question 13

Euthyroid Sick Syndrome - what is this? what do we see?

Answer 13

A dog has a non-thyroidal illness that suppressed the circulating levels of thyroid hormone
◦ Can result in a low or low-normal T4
◦ Variable effects on TSH
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Mechanism is not fully understood
◦ Might be a protective response to illness
◦ Patient is not actually hypothyroid

Question 14

Euthyroid Sick Syndrome - what drugs can cause this? other causes?

Answer 14

Drugs:
◦ Steroids
◦ Phenobarbital
◦ Sulfonamides
◦ Possibly other drugs
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- Non-thyroidal illness
- Any especially those that are severe systemic disorders, inducing a catabolic state

Question 15

Patient comes in for check up:
In dogs with consistent clinical signs and low T4, what should we do?

Answer 15

Perform FT4, TSH

Question 16

Finding a Low T4 as part of routine bloodwork - what do we do?

Answer 16

If you find a low T4 as part of routine bloodwork, and no clinical evidence of hypothyroidism / other illness
◦ Recommend to monitor dog in 6 months
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Low T4 and signs of other illness
◦ Address non-thyroidal illness
◦ Re-investigate thyroid in future if clinical signs warrant

Question 17

Diagnosing Hypothyroidism
- Strongest evidence of hypothyroidism:

Answer 17

◦ Supportive clinical signs; laboratory abnormalities (CBC/chem)
◦ No other illness / medications / etc
◦ Thyroid testing results:
> Low TT4 and/or fT4
> Plus high TSH (could be normal in some cases)

Question 18

Treating Hypothyroidism

Answer 18

Synthetic thyroid hormone (levothyroxine)
◦ Veterinary and human brands available
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Initial dose 0.01-0.02 mg/kg every 12 hours
◦ Some concurrent diseases (e.g., cardiac disease) – lower dose

Question 19

Monitoring Hypothyroidism Treatment

Answer 19

Evaluate response at 4-8 weeks after starting treatment
◦ History, PE
◦ Recheck TT4 (4-6 hours after dose given)
◦ Could also check TSH (should normalize)
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◦ TT4: aim to be in middle / upper end of reference interval

Question 20

Therapeutic Adjustments for hypothyroid - when to make them

Answer 20

T4 recheck values:
* If too low, increase dose, recheck in 4 weeks
* If too high, decrease dose or go to every 24 hours, recheck in 4 weeks
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Once stable, less frequent rechecks
* Eventually recheck 1-2 times per year

Question 21

Hypothyroidism: Prognosis
- when do we see improvements?

Answer 21

Adults with primary hypothyroidism have an excellent prognosis
- Improvement in activity: 1-2 weeks
- Weight loss (if obese): Evident ≥ 8 weeks
- Hair coat improvement: Several months