What we know
Hypothyroidism is a common endocrine disorder of middle-aged, purebred dogs resulting in decreased production of the thyroid
hormones, thyroxine (TT4) and tri-iodothyronine (T3) from the thyroid gland. The majority is believed to be due to acquired
primary hypothyroidism. Destruction of the thyroid gland can result from autoimmune lymphocytic thyroiditis, idiopathic thyroid
atrophy, or more rarely, neoplastic invasion. Secondary hypothyroidism is rare.
Autoimmune lymphocytic thyroiditis is common in some breeds (English Setters, old English sheepdogs, boxers, giant schnauzers
and American pit bull terriers) as evidenced by thyroglobulin autoantibodies (TgAA). Hypothyroidism in some breeds is commonly
associated with TgAA while others have a lower prevalence. For example only 26% of Doberman pinschers are positive for TgAA
compared to 69% of golden retrievers. A TgAA-positive does not provide any assessment of thyroid function, only that thyroiditis
is present. Dogs with positive TgAA tests should have thyroid function monitored every 6 – 12 months. There is no scientific
evidence that vaccination induces autoimmune thyroid disease.
In hypothyroid dogs, basal metabolism is slowed as evidenced by lethargy, weight gain, and exercise intolerance. These signs
gradually appear so owners attribute changes to aging. Dermatologic changes are common including dry, scaly skin, changes
in haircoat quality or color, alopecia, seborrhea sicca or oleosa, and superficial pyoderma. Neurological manifestations are
occasionally present. Generalized polyneuropathy is displayed as generalized weakness, ataxia, and hyporeflexia. Central nervous
signs occur, with central vestibular signs most common.
Clinical suspicion of canine hypothyroidism should be obtained by evaluation of the history, physical and blood test results.
A complete blood count (CBC), serum chemistry profile (SMA) and urinalysis, as well as measurement of TT4 and TSH concentration
should be obtained. The CBC may reveal normocytic, normochromic, and non-regenerative anemia. The SMA might have fasting hypertriglyceridemia
and hypercholesterolemia because thyroid hormones stimulate virtually all aspects of lipid metabolism, with decreased degradation
affected mostly with the net effect of plasma lipid accumulations.
For routine cases of suspected hypothyroidism, serum concentrations of TT4 and TSH are recommended at a minimum. Decreased
TT4 AND elevated TSH confirm the diagnosis, but do not occur in all cases. In the presence of non-thyroidal illness or uncommon
clinical signs a serum free T4 (fT4) should be measured by equilibrium dialysis.
Total T4 is a sensitive but not specific test for diagnosing hypothyroidism. Although the majority of hypothyroid dogs have
low TT4 levels, some normal dogs with other illnesses have low TT4 levels as well. And it is important to use a veterinary
laboratory as dogs have a much lower range of TT4 than humans so non-veterinary laboratories do not accurately measure TT4.
Serum T3 concentration is an unreliable test for evaluation of thyroid function as some studies tout almost 90% of hypothyroid
dogs have a normal serum T3 concentration.