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Clinical Signs
Hypothyroidism most often occurs in middle-aged dogs (4-10 years). Large breed dogs appear to be predisposed including Golden Retrievers, Doberman Pinschers, and Labrador Retrievers (not documented). Spayed females and castrated male dogs may be at increased risk.
Clinical signs are often subtle and have a gradual onset. Metabolic signs reflect a decreased cellular metabolism including lethargy, exercise intolerance, heat seeking, weight gain, mental dullness, decreased appetite, and constipation. Dermatologic signs are often presenting complaints and include bilateral truncal alopecia that is nonpuritic, alopecia of the caudal thighs and a “rat” tail, loss of guard hairs (puppy coat), seborrhea, chronic otitis, hyperpigmentation, failure of hair growth, secondary pyoderma, or myxedema. Neuromuscular signs including profound lethargy and muscle weakness, peripheral nerve paralysis, slow nerve conduction velocities, type
II myofiber atrophy, and loss of peripheral vestibular disease are also reported. Reproductive signs include prolonged interestrus intervals, failure to cycle, and inappropriate galactorrhea in the female. Congenital hypothyroidism results in growth retardation, mental retardation, and disproportionate dwarfism (retarded epiphyseal growth) in puppies. Dilated cardiomyopathy that was responsive to thyroid supplementation has been reported in two Great Danes.
These animals are also predisposed to development
of polyglandular endocrine gland destruction and may also develop hypoadrenococticism, diabetes mellitus, and/or hypoparathyroidism. Some hypothyroid animals can also present comatose with cerebral myxedema. Physical examination findings include a mildly overweight to severely obese animal, although some patients may be of normal body condition. Profound lethargy may
be present, as well as, hypothermia, bradycardia, skin disease, myxedema of skin, mostly on head, or stunted growth.
A biochemical database should be performed on
every animal suspected of having hypothyroidism.
This is necessary to identify concurrent disease and determine the metabolic status of the animal before potential therapy is initiated. A CBC may reveal mild/ moderate normocytic, normochromic, nonregenerative anemia, potentially from decreased erythropoiesis. Findings on a routine serum chemistry include fasting hypercholesterolemia that may be severely elevated. Hypercholesterolemia is seen in approximately 80% of hypothyroid dogs and can help you identify the disease. Other findings may include fasting hypertriglyceridemia and mild elevations in liver enzymes. Exercise-induced hyperkalemia has been reported in some dogs. A urinalysis is usually normal. ECG abnormalities include a sinus bradycardia and decreased amplitude of the P and R waves.
TSH-stimulation test is the gold standard of testing for hypothyroidism. It tests for reserve in a hypofunctioning gland. In normal dogs, cats, and birds serum T rises
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Hypothyroidism is an overdiagnosed disease! Baseline hormone measurements can be very helpful in identifying this disease, but are often not interpreted correctly leading to misdiagnosis. T4 is the major thyroid hormone secreted into the bloodstream and, therefore, would be thought to be the hormone to measure. However, measurement of serum T4 has consistently confused the diagnosis of the disease because > 99% of T4 is protein bound in plasma. The amounts and affinity of binding proteins can change by many physiologic and pharmacologic factors and artificially lower the T4 measurement while thyroid status is actually normal. Measurement of T4 is useful as a screening test. If the result is in the middle-high normal range, hypothyroidism would be placed lower on the list of differential diagnoses. T4 in serum is relatively stable and can be sent to outside laboratories for measurement by RIA. Interference by anti-T4 antibodies can cause spuriously high readings. The practitioner must remember that certain breeds of dogs, notably sighthounds, have
lower T4 than other dogs. The correct breed-associated reference range should be used to interpret serum T4 measurements.
FreeT4 (fT4) measurement is that in which the free hormone is separated by equilibrium dialysis
and measured by RIA. This factors out influence
of nonthyroidal illness and drugs that lower the concentration of T4 serum binding proteins and subsequently decrease the total T4. It is more
expensive and takes longer than T4 measurement; however, it more accurately diagnoses the disease.
In one study, 98% of hypothyroid dogs had low fT4, although a small percentage of euthyroid dogs with concurrent nonthyroidal illness have low serum fT4.TSH measurement (cTSH) is used to diagnose hypothyroidism in people and takes advantage of measuring the negative feedback (or lack thereof) of fT4 and fT3 on pituitary secretion of TSH. As fT4 and fT3 decrease in primary hypothyroidism, TSH levels will increase due
to lack of negative feedback. In dogs and cats,this
test has been hampered by availability of good assay reagents. As is stands now it is not valid as a test on its own. Studies show that 25-38% of hypothyroid dogs had normal TSH levels. In euthyroid dogs with concurrent illness, only 12% had elevated TSH levels. This test has low sensitivity, but can be very specific when combined with fT4 or T4 measurement.
in response to TSH administration. Hypothyroid dogs
have a blunted response. Injectible bovine source TSH is available as a research grade compound which may cause anaphylaxis. Human recombinant TSH is available

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