Angus waddled in to the exam room for his yearly checkup. This seven year old Lab had normally been springy enough he presented a challenge to examine, but between last year and this year he had gained ten pounds, and today all four feet stayed on the ground for the entire exam. His owners reported that it seemed like he was starting to act more like an old guy, just laying around and taking it easy more.

Angus’ physical exam revealed a fairly normal looking, overweight dog with some excessive dandruff along his back and a mild ear infection. He could easily be written off as a normally aging large breed dog, but something seemed not quite right. His condition was very typical of hypothyroidism, or low thyroid function in dogs, and some simple blood tests could tell us whether he was just getting old, or if he had an easily treatable medical condition.

Hypothyroidism is a very common hormonal disorder in dogs. The thyroid gland makes hormones that keep the metabolism revved up. When the immune system, for unknown reasons, erroneously attacks the thyroid gland it eventually causes it to fizzle away. The lack of thyroid hormone causes body systems to slow to a crawl. Large breed dogs are most commonly affected, but any type of dog can develop the problem. Because the disease is the result of a low grade, long term attack on the thyroid gland most affected dogs do not show symptoms until they are at least 4-5 years old

The classic hypothyroid dog is a middle aged large breed dog with tremendous recent weight gain, dandruff or bald patches of hair, and a sluggish attitude. "I swear I am feeding him nothing but air and he still gains weight!" is the common claim. Owners sometimes report that the dog is always lying at the foot of the refrigerator, presumably because he is hot, but in reality because the refrigerator is venting hot air from the bottom and a cold intolerant hypothyroid dog will camp out there to soak up as much heat as possible.

Hypothyroidism can affect the body in a wide variety of ways, so some dogs don’t fit the classic pattern. Some are thin, some have swallowing problems or facial nerve paralysis. It is a common enough disease that if there is any suspicion that low thyroid could be at play it is generally a good idea to check. There are a variety of blood tests that can get an answer. Because other severe systemic diseases can cause thyroid levels to read low on blood tests it is also important to look for other medical problems before diagnosing thyroid problems alone.

Treatment is usually simple. Safe, effective thyroid hormone replacement is readily available and is usually given as a twice daily pill. Because we are supplying the hormone that the body is no longer making the treatment will be life long. We monitor the thyroid levels in treated dogs periodically with blood tests to make sure we are giving the right dosage to the patient.

Getting an actual diagnosis with blood test is important before starting thyroid hormone supplementation. Not every fat dog is hypothyroid, and as much as we all would love to solve obesity issues with a twice daily pill, it is not only ineffective, but actually harmful to give thyroid hormone supplementation if there is not an actual thyroid hormone deficiency.

Unfortunately for owners of fat cats (myself included), who might hope that the weight issues might be a treatable hormonal issue rather than an overeating issue, cats do not develop low thyroid problems. They do have their own separate thyroid problems that drive their thyroid levels sky high, but almost never develop the need for thyroid supplementation. Good old portion control is usually what we are stuck with for cats.