Hypoadrenocorticism
Primary hypoadrenocorticism (Addison's disease) is a clinical syndrome resulting from a deficiency of cortisol and aldosterone.35 Hypoadrenocorticism is most common in young and middle-aged female dogs, with an increased prevalence and autosomal recessive
inheritance in standard poodles, Portuguese water dogs, and Nova Scotia duck tolling retrievers.36 Golden retrievers and Labrador retrievers may also be predisposed to the disease.
Clinical signs of hypoadrenocorticism are typically related to fluid and electrolyte imbalance, circulatory insufficiency,
and abnormal carbohydrate metabolism.35 Inappetence, lethargy, vomiting, and diarrhea are common. Muscular weakness or seizures that occur with exercise may be
the only presenting complaint, presumably caused by hypoglycemia.32,35
Hyponatremia, hyperkalemia, and increased blood urea nitrogen concentration are common findings in dogs with mineralocorticoid
deficiency, but about 25% of dogs (32% of Nova Scotia duck tolling retrievers) with hypoadrenocorticism are only deficient
in glucocorticoid secretion.35,36 Dogs with a pure glucocorticoid deficiency are said to have atypical Addison's. These dogs exhibit similar clinical signs
to dogs with primary hypoadrenocorticism, but they have normal electrolytes, making the diagnosis more challenging.35 Suggestive laboratory findings in cortisol-deficient dogs may include hypoglycemia, hypoalbuminemia, and the absence of
a stress leukon.35 Exercise-intolerant dogs with hypoadrenocorticism will have a basal serum cortisol concentration less than 2 μg/dl, but
definitive diagnosis requires confirmation with an ACTH stimulation test.37 The prognosis for recovery and function as a working dog is excellent with appropriately managed therapy.
Hypothyroidism
Hypothyroidism is common in the retriever breeds and can be associated with obesity, lethargy, and exercise intolerance caused
by a decrease in metabolic rate.38 Severe chronic hypothyroidism can also cause a reversible peripheral neuropathy characterized by weakness, exercise intolerance,
and hyporeflexia.39 Central nervous system atherosclerosis and thromboembolic events may be responsible for acute and chronic neurologic syndromes
in hypothyroid dogs, particularly in Labrador retrievers with concurrent severe hyperlipidemia.40 Labrador retrievers with coronary artery atherosclerosis may also be predisposed to myocardial infarction.41
Dermatologic abnormalities such as alopecia, seborrhea, and pyoderma are present in over 85% of dogs with hypothyroidism,
increasing the index of suspicion for this disorder.38 Laboratory testing of thyroid function, including measurement of serum total thyroxine (T4), free T4 by equilibrium dialysis (ED), and thyroid stimulating hormone (TSH), is recommended in all dogs with exercise intolerance.42 When evaluating retrievers involved in performance sports, it is important to consider that rigorous training and endurance
exercise have been shown to decrease serum total T4 and free T4 in some canine athletes.43,44
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