Canine hypothyroidism is not difficult to diagnose. In fact, it is one of the most overdiagnosed diseases of domestic dogs, mainly because the symptoms of hypothyroidism mimic those of so many other disorders.
When presented with a dog showing some of the classic signs of hypothyroidism, a veterinarian will typically perform a complete blood count, a serum biochemistry panel and a urinalysis, as part of an initial database. The results of these tests can be suggestive of hypothyroidism.
There are several additional tests that can help confirm the diagnosis, including measurement of circulating thyroxine hormone concentrations. If the first round of testing does not rule hypothyroidism in or out, the veterinarian can conduct more advanced testing, including a thyroid stimulating hormone (TSH) or a thyrotropin releasing hormone (TRH) stimulation test. These tests are rather expensive and are not commonly used with our domestic dogs. Ultrasonography and scintigraphy of the thyroid gland can also be diagnostically useful when performed by an experienced veterinary radiologist.
Currently, the best available tests to diagnose canine hypothyroidism are the modified equilibrium dialysis test (which measures levels of T4 hormone in the blood), combined with the measurement of circulating TSH. Almost all dogs with low free T4 and high TSH in circulation are suffering from hypothyroidism.
Another less accurate way to diagnose hypothyroidism is to assess a dog’s response to thyroid hormone replacement therapy. Most affected dogs will respond within about one week, showing markedly increased activity levels and alertness. Other signs of the disease take longer to resolve.