Page 380 - WSAVA2018
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 25-28 September, 2018 | Singapore
and has been shown to be effective in dogs. However, its high cost makes routine use prohibitive.
Thyroid biopsy is another way to diagnose thyroid disease. It gives histologic evaluation of gland, but
no information on function. These glands are usually atrophied and a surgical biopsy is required. Usually it is not necessary in order to obtain a diagnosis.
Thyroid autoantibodies are circulating antibodies to
T3, T4, and thyroglobulin in the serum. Elevations of these antibodies may indicate early immune-mediated destruction of the gland. However, there are problems with specificity of antibody production. In a recent
large study, 6.3% of samples submitted from dogs with clinical signs of hypothyroidism were positive for thyroid hormone antibodies.
Problems with the diagnosis of hypothyroidism occur when concurrent illness results in euthyroid sick syndrome. This is a syndrome defined in humans. It is a physiological adaption to decrease cell metabolism during periods of illness. Serum levels of T4 and T3
are depressed, although the animal is euthyroid at
the cellular level. Levels of fT3 and fT4 appear to be affected less, but may also be decreased. In addition this can be due to changes in thyroid hormone binding proteins, changes in activation of the 5-deiodinase or 5’-deiodinase enzymes, changes in TSH secretion, or increased metabolism of thyroid hormones. Things that can cause this in dogs include hyperadrenocorticism, exogenous glucocorticoid treatment, diabetes
mellitus, starvation, systemic diseases, and pyoderma. These animals are euthyroid and do not need to be supplemented with thyroid hormone. Other factors that can affect thyroid hormone measurement include drugs (especially glucocorticoids, phenobarbitol, and clomipramine) and fasting.
So how does the clinician diagnose hypothyroidism.
It can be difficult and confusing. First the clinician should have a clinical index of suspicion. Basal T4, fT4, T4 autoantibodies, and TSH can be measured at the same time. Alternatively, measurement of TSH and T4 or fT4 is quicker and gives the same specificity, although the antibody status of the animal is not determined.
If clinical index of suspicion is high and initial thyroid testing comes back as normal, the animal should be retested in 1-2 weeks. Animals should be off thyroid supplementation for 6-8 weeks before thyroid testing is attempted.
In an emergency situation with myxedema stupor or coma, treat with intravenous L-thyroxine, and supportive care. Oral sodium levothyroxine (L-thyroxine) is the treatment of choice for longterm management of hypothyroidism. The dose is 0.01-0.02 mg/kg PO q 12 -24 hours. Start at a lower dose for dogs with cardiac illness, severly debilitated dogs, or geriatric patients and raise the dose slowly if necessary. Monitor and adjust dose
by measuring T 4-8 hours post-pill; dogs should be in
the high-normal to slightly-high range at that time if on
the correct dose. Monitor for signs of hyperthyroidism. Side effects are few. Levothyroxine (0.02 mg/kg) given once per day may be sufficient for most dogs after they are initially controlled. Obtain a pre-pill T4 to determine whether once/day dosing is appropriate for this patient.

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