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Clinical Endocrine, Metabolic and Dermatology > Canine Thyroid: Hypothyroidism > Flashcards

Canine Thyroid: Hypothyroidism Flashcards

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1
Q

What are the 3 classifications of Hypothyroidism?

A
  • Primary (95%+): disease is within the thyroid, and the thyroid is unable to synthesize and/or release
  • Secondary (<5%): disease is within the pituitary gland, resulting in deficient TSH and lack of stimulation on the thyroid
  • Tertiary (only 1 case reported): disease is within the hypothalamus, resulting in deficient TRH
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2
Q

What are the most common causes of Primary Hypothyroidism?

A
  • Lymphocytic thyroiditis (50%) = seen in young dogs
  • Idiopathic Thyroidal Atrophy (50%) = seen in older dogs
  • Congenital: enzyme deficiencies leading to an inability to produce thyroid hormone
  • Neoplasia

note: it is believed that Lymphocytic thyroiditis eventually results in Idiopathic Thyroidal Atrophy, and therefore the divide in age in which they are seen
Though these diseases can potentially be distinct entities in some dogs with no transition between the two

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3
Q

What are the 2 classifications of Secondary Hypothyroidism?

A
  • Congenital

- Acquired

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4
Q

What are the 2 main body systems affected by hypothyroidism and the clinical signs associated with them?
What are the other body systems that can be affected (but rarely are)?

A
  • Metabolic: lethargy, weight gain, exercise intolerance, cold intolerance
  • Dermatologic: poor quality dry coat, failure to regrow hair, hair thinning and alopecia+ hyperpigmentation of the skin

Rarely:

  • Reproductive System
  • Neuromuscular
  • Ocular
  • Cardiovascular
  • GIT
  • Neurological
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5
Q

What are the patterns/ areas of Alopecia seen in dogs diagnosed with or suspected with Hypothyroidism?

A

Often areas of friction:

  • Neck
  • Tail (rat tail)
  • Lateral extremities

Tends to be patchy and asymmetric

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6
Q

What is Myxedema?

A

Myxedema: this is where deposition of mucopolysaccharides in the dermis result in swelling, thickening, cool-to-the-touch, puffy but not pitting and a “tragic facial expression”

note: it is an indicator of severe hypothyroidism

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7
Q

What are the top dermatological changes seen in dogs with hypothyroidism? (6)

A

1) Alopecia
2) Myxedema
3) Altered wound healing: delayed or excessive fibrous tissue
4) Hypertrichosis: retarded turnover of hair coat, giving the appearance of long bleached coat
5) Bacterial pyoderma
6) Seborrhea

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8
Q

What are the less commonly seen reproductive clinical signs associated with hypothyroidism?

A
  • Infertility in the bitch
  • prolonged parturition
  • Increased puppy mortality
  • Galactorrhea: milky nipple discharge not associated with normal milk production
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9
Q

What are the less commonly seen neuromuscular clinical signs associated with hypothyroidism?

A
  • Myopathies

- Neuropathies

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10
Q

What are the less commonly seen cardiovascular clinical signs associated with hypothyroidism?

A
  • Bradyarrhythmias
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11
Q

What are the less commonly seen ocular clinical signs associated with hypothyroidism?

A
  • Corneal lipidosis: giving a milky appearance to the cornea

- Reduced tear production

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12
Q

What are the less commonly seen gastrointestinal clinical signs associated with hypothyroidism?

A
  • Constipation
  • Vomiting
  • Diarrhea
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13
Q

What are the less commonly seen neurological clinical signs associated with hypothyroidism?

A
  • Myxedema coma

- Central vestibular disease

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14
Q

What is the list of unrelated signs to hypothyroidism? (3)

A
  • No effect on male reproductive performance
  • Does not result in PU/PD
  • Unrelated to dilated cardiomyopathy
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15
Q

How can Hypothyroidism be diagnosed?

A
  • Screening lab tests: mild anemia, mild increase in CK, Hypercholesterolemia, Hypertriglyceridemia
  • Hormone analyses: Total and free T4, Basal TSH and TSH/TRH response test
  • Thyroglobulin autoantibodies
  • Thyroid imaging
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16
Q

What are the stages of Hypothyroidism, specifically Lymphocytic thyroiditis?

A
  • Silent: majority of thyroid tissue is normal, positive TgAA + normal levels of TSH T4/T3
  • Subclinical: more marked infiltration, positive TgAA, increased TSH but normal T4/T3
  • Clinical: 75%+ thyroid tissue lost, negative TgAA, increased TSH but decreased T4/T3
17
Q

Why must breed of the dog be taken into consideration when diagnosing hypothyroidism?

A

Breed has been shown to play a major effect on the levels of T4/T3
- Sighthounds: greyhound, saluki, wolfhound and whippet are known to have lower T4/T3 levels, which is normal for these breeds

caution: do not accidentally diagnose a healthy sighthound as having hypothyroidism

18
Q

What are the drugs used in vet medicine that can lower TSH and T4/T3 levels, and give the illusion of hypothyroidism?

A
  • Glucocorticoids
  • Phenobarbital
  • Potassium bromide
  • Propranolol
  • Sulphonamides
  • Clomipramine
  • Aspirin
  • Ketoprofen
  • Carprofen
19
Q

A dog presents as potentially being hypothyroid, what tests will you do initially to confirm your suspicion?

A
  • Total T4

- Basal TSH

20
Q

If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: normal
TSH: normal

A

Euthyroid

21
Q

If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: low
TSH: high

A

Hypothyroid

22
Q

If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: low
TSH: normal

A
  • Non-thyroidal illness
  • Certain drug administration
  • Hypothyroid is still a possibility, so additional tests are warranted
23
Q

If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: normal
TSH: high

A
  • Recovery from Non-thyroidal illness
  • Certain drug administration
  • Subclinical Hypothroid, so additional tests are warranted
24
Q

What are the additional diagnostic tests that can be done to help confirm Hypothyroidism?

A
  • Free T4
  • Thyroglobulin autoantibodies
  • Thyroid imaging
  • Dynamic thyroid function tests: TSH/TRH response tests
25
Q

Why is Free T4 levels a helpful indicator of hypothyroidism?

A
  • Free T4 is less affected by non-thyroidal illness and drug administration, meanwhile, T3 is affected first

therefore, when Total T4 and TSH are inconclusive of a diagnosis, Free T4 is a good additional test

note: Free T4 can still be suppressed in severe illness and steroid or phenobarbital administration

26
Q

What is the purpose of measuring Thyroglobulin autoantibodies?

A

Their presence will help support the presence of Lymphocytic thyroiditis

note: a negative result does not eliminate diagnosis as animals can have other forms of hypothyroidism (e.g. atrophy)
note: TgAA is present early on in the disease and can be a predictor of onset

27
Q

What radioactive substance is used for Thyroid scintigraphy?

A

Pertechnetate

28
Q

What is the benefit of using Thyroid scintigraphy?

A

Can be used to distinguish between Non-thyroidal illness and Hypothyroidism

29
Q

What is the treatment of Hypothyroidism in dogs?

A
  • L-thyroxine supplementation (T4)
30
Q

What are some important points relating to L-thyroxine supplementation?

A
  • Give at the same time each day
  • Absorption decreased by food: either always with or without food
  • Avoid administration with other medications
31
Q

How long will it take for metabolic and dermatological signs to improve once a dog is started on thyroxine supplementation?

A
  • Metabolic: days to weeks

- Dermatological: weeks to months

32
Q

How long after adminstration of thyroxine supplementation should you wait to collect a blood sample and analyze it for Total T4?

A

2 weeks after starting treatment, have the patient take the meds in the morning, then come in 4 - 6 hours after to get a total T4 measurement

33
Q

A patient returns to the clinic with the O’s complaining of MORE alopecia after starting treatment for hypothyroidism, what do you suggest to the O?

A

Much more severe and widspread alopecia can be seen in some dogs following initial therapy with thyroxine supplementation, this is due to follicular turnover, as old hairs are shed prior to replacement by new growth
This can be concerning to owners, but should be considered a positive sign that treatment is being effective

34
Q

What are the potential reasons for failure of therapy following treatment of hypothyroidism?

A
  • Inappropriate diagnosis
  • Inadequate dosing: usually start by doubling the dose
  • Inactive preparation or batch of meds
  • Inhibition of absorption: concurrent intake of food or other drugs, or GIT disease
  • Poor owner compliance
35
Q

Obesity can be a sign of what endocrine diseases and why?

A
  • Hypothyroidism: decreased metabolic rate, lethargy and reduced exercise intolerance
  • Hyperadrenocorticism: increased endogenous corticosteroids induces polyphagia, exercise intolerance due to muscle mass loss and myopathy
  • Hyperinsulinism: increased insulin promotes glucose uptake into cells, this energy is stored as fat
  • Acromegaly: extreme polyphagia, GH induces anabolic effects, decreased exercise tolerance due to OA or dyspnea

Clinical Endocrine, Metabolic and Dermatology (17 decks)

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