Canine Thyroid: Hypothyroidism Flashcards
What are the 3 classifications of Hypothyroidism?
- 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
What are the most common causes of Primary Hypothyroidism?
- 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
What are the 2 classifications of Secondary Hypothyroidism?
- Congenital
- Acquired
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)?
- 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
What are the patterns/ areas of Alopecia seen in dogs diagnosed with or suspected with Hypothyroidism?
Often areas of friction:
- Neck
- Tail (rat tail)
- Lateral extremities
Tends to be patchy and asymmetric
What is Myxedema?
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
What are the top dermatological changes seen in dogs with hypothyroidism? (6)
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
What are the less commonly seen reproductive clinical signs associated with hypothyroidism?
- Infertility in the bitch
- prolonged parturition
- Increased puppy mortality
- Galactorrhea: milky nipple discharge not associated with normal milk production
What are the less commonly seen neuromuscular clinical signs associated with hypothyroidism?
- Myopathies
- Neuropathies
What are the less commonly seen cardiovascular clinical signs associated with hypothyroidism?
- Bradyarrhythmias
What are the less commonly seen ocular clinical signs associated with hypothyroidism?
- Corneal lipidosis: giving a milky appearance to the cornea
- Reduced tear production
What are the less commonly seen gastrointestinal clinical signs associated with hypothyroidism?
- Constipation
- Vomiting
- Diarrhea
What are the less commonly seen neurological clinical signs associated with hypothyroidism?
- Myxedema coma
- Central vestibular disease
What is the list of unrelated signs to hypothyroidism? (3)
- No effect on male reproductive performance
- Does not result in PU/PD
- Unrelated to dilated cardiomyopathy
How can Hypothyroidism be diagnosed?
- 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
What are the stages of Hypothyroidism, specifically Lymphocytic thyroiditis?
- 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
Why must breed of the dog be taken into consideration when diagnosing hypothyroidism?
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
What are the drugs used in vet medicine that can lower TSH and T4/T3 levels, and give the illusion of hypothyroidism?
- Glucocorticoids
- Phenobarbital
- Potassium bromide
- Propranolol
- Sulphonamides
- Clomipramine
- Aspirin
- Ketoprofen
- Carprofen
A dog presents as potentially being hypothyroid, what tests will you do initially to confirm your suspicion?
- Total T4
- Basal TSH
If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: normal
TSH: normal
Euthyroid
If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: low
TSH: high
Hypothyroid
If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: low
TSH: normal
- Non-thyroidal illness
- Certain drug administration
- Hypothyroid is still a possibility, so additional tests are warranted
If a dogs hormone results come back as follows, what is the likely diagnosis?
Total T4: normal
TSH: high
- Recovery from Non-thyroidal illness
- Certain drug administration
- Subclinical Hypothroid, so additional tests are warranted
What are the additional diagnostic tests that can be done to help confirm Hypothyroidism?
- Free T4
- Thyroglobulin autoantibodies
- Thyroid imaging
- Dynamic thyroid function tests: TSH/TRH response tests
