Canine Hypothyroidism Flashcards
The majority of T4 is bound to which protein while in circulation?
thyroxine binding protein
Most of the steps involved in thyroid hormone synthesis is catalyzed by which enzyme?
thyroid peroxidase
How is thyroid hormone controlled?
hypothalamus –> TRH –> anterior pituitary –> TSH –> thyroid gland –> iodine trapping, T4/T3 release
excess circulating free T4 and T3 = negative feedback loop –> anterior pituitary and hypothalamus
What are some major functions for thyroid homrones?
- metabolism
- growth
- CNS development
- tissue turnover
- (+) inotrope/ chronotrop
- cholesterol synthesis & metabolism
- stimulates erythropoiesis
What are the 3 types of thyroid disease?
- primary = thyroid gland
- secondary = pituitary
- tertiary = hypothalamus
What’s the most common form of hypothyroidism?
primary, acquired –> lymphocytic thyroiditis and thyroid atrophy
Which breeds with high prevalence of thyroglobulin autoantibodies (TgAAs)?
- Golden Retriever
- English Setter
- Rhodesian Ridgeback
- Cocker Spaniel
- Boxer
How is lymphocytic thyroiditis diagnosed?
- biopsy = gold standard
- can also be inferred with thyroglobulin autoantibodies (TgAAs)
What’s the typical signalment for canine hypothyroidism?
- 7yo
- predisposed purebreds may have an earlier onset
- intact males and spayed females
What are the most common c/s of canine hypothyroidism?
- dermatological and metabolic concerns
What are the metabolic concerns with hypothyroid dogs?
- obese/ weight gain
- decreased metabolism
- shivering, heat seeking
- lethargy, mental dullness
What are some dermatological abnormalities associated with canine hypothyroidism?
- poor hair coat
- alopecia (truncal, rat tail, bilateral symmetrical)
- non-pruritic
- dry, scaly skin; seborrhea, otitis, hyperpigmentation, secondary bacterial infection
What’s the cause of the “tragic facial expression”?
Myxedema - accumulation of hyaluronic acid in the dermis
What are some of the ECG changes noted in canine hypothyroidism?
- low voltage R wave
- inverted T wave
- bradycardia
What are some echocardiogram changes noted in canine hypothyroidism?
- decreased fractional shortening
- mild increased L ventricle end systole diameter
How does thyroid hormone affect the heart>
it increases the heart’s sensitivity to adrenergic stimulation –> (+) inotrope
Are hypothyroid dogs often clinical for the cardiovascular changes?
no
What are some neuromuscular changes noted in canine hypothyroidism?
- no direct causal effect, BUT
- central vestibular disease –> suspect related to atherosclerosis –> brain infarction or metabolic derrangement
- fatal myxedema coma has been reported
- may be difficult to ddx between neuropathy vs myopathy
What eye abnormality is noted in canine hypothyroidism?
arcus lipoides (esp in German Shepherd Dogs)
What reproductive changes are noted in canine hypothyroidism?
- short term: fertility is normal, but prolonged parturition and lowered pup survival
- long term: decreased fertility, and increased peri-parturient mortality , lower birth rate
- male repro indices are not affected
How is congenital canine hypothyroidism different than pituitary dwarfism?
hypothyroid = disproportional dwarfism
What’s autoimmune polyendocrine syndrome type 1 (APS-1) in people?
Triad of autoimmune hypothyroidism, Addison’s, and mucocutaneous candidiasis –> rare, autosomal recessive
What’s autoimmune polyendocrine syndrome type 2 (APS-2) in people?
2 or more of the following:
- Addison’s
- Grave’s
- autoimmune thyroiditis
- Type 1A diabetes mellitus
- primary hypogonadism
- myasthenia gravis
- Celiac
Similar to APS-2 is seen in dogs. Particularly with which ones?
Addison’s - if poor response to mineralocorticoids, persistent hyponatremia, etc
- diabetes mellitus also common to have in addition to hypothyroidism
What’s potential MOA of low thyroid induced diabetes mellitus?
transdifferentiating of somatotropic pituitary cells to thyrosomatotropes –> excessive growth hormone = highly diabetogenic
What are the most common blood work changes noted in canine hypothyroidism?
- anemia
- hypercholesterolemia
- hypertriglyceridemia
- increased CK
- decrease fructosamine?
How is the hypothyroid anemia characterized as?
mild, none regenerative, normocytic, normochromic
- physiological anemia, due to lack of erythropoietin stimulation and lack of stimulatory effect of thyroid hormone on the bone marrow
How is the platelet count in hypothyroid dogs?
increased; thrombopoiesis and erythropoiesis have an inverse relationship
What’s MOA for hypercholesterolemia with hypothyroidism?
Thyroid hormone is involved in synthesis, mobilization and degradation of cholesterol. Hypothyroidism has profound effect of the degradation.
Why does hypothyroidism increase CK?
- decreased metabolism or excretion
- myopathy
Why does hypothyroidism cause increased fructosamine?
- decrease protein turnover
Why does hypothyroidism lead to mild liver enzyme elevation?
- specifically in ALP and GGT –> lipid deposition
How is free T4 tested?
Free T4 by equilibrium dialysis
What are some storage/ interference with thyroid testing?
- free T4 significantly increased with stored at room temp for a long time
- severe hyperlipemia may falsely increase free T4 by equilibrium dialysis
How is TSH tested?
Using species specific hormone assay
Which breeds have low thyroid hormones?
sighthounds
How does sulfonamides induce hypothyroidism?
it reversibly inhibits thyroxin peroxidase
What are some drugs that can lead to hypothyroidism?
- glucocorticoids
- phenobarbital
- aspirin
- ketoprofen
- carprofen
- clomipramine
- sulfonamides
What’s the diagnostic utility of total T3?
typically not used due to low sensitivity and specificity.
BUT, can be useful in Greyhounds when they have low total T4 but normal total T3
What’s the diagnostic utility of total T4?
- highly sensitive but low specificity
- T4 autoantibodies can falsely increase the result
- should never be used alone for diagnosis of hypothyroidism
Which is the single most accurate test for hypothyroidism?
free T4
What’s the diagnostic utility of total cTSH?
- moderate sensitivity
- but highly specific, not really affected by non-thyroid illness
What’s the diagnostic utility of TgAA?
- increased = lymphocytic thyroiditis
- has no info in thyroid function
- an increase indirectly supports hypothyroidism
What are the utilities of provocative tests for canine hypothyroidism?
- human recombinant TSH, pretty good test
- TRH, not as reliable as the others
Which imaging modality is useful for diagnosing hypothyroidism in general?
ultrasound
hypothyroid thyroid gland = smaller, heterogenous echogenicity, irregular outline, more rounded
When is technetiusm-99M scans useful?
for Greyhounds
- or investigating central hypothyroidism
- ddx between dysgenesis from dyshormonogensis in congenital hypothyroidism
How is hypothyroidism treated?
with endogenous T4 supplementation
How long does it take for c/s to resolve once T4 supplements have been given?
- within days: metabolic changes, ex. lethargy, dullness
- first few months: 10% weight loss
- 2-3 months: improvement in dermatological signs
- first 3 months: serum cholesterol and triglycerides decrease rapidly, RBC increases gradually
- up to 6months: neurological signs
How to treat myxedema coma?
IV T4, improvement should be noted in 30hours
