Hypothyroidism in Dogs: Signs, Diagnosis and Treatment

What it is

Hypothyroidism means the thyroid gland, located in the neck, makes too little thyroid hormone. Because thyroid hormone sets the pace of metabolism, a shortage slows many body systems. In most dogs the cause is in the gland itself: either the immune system gradually attacks and destroys thyroid tissue (thyroiditis), which often has a hereditary component, or the gland shrinks and is replaced by fat. Tumors, pituitary disease, and birth defects are much rarer causes.

It is mainly a disease of middle-aged dogs, most often diagnosed between about 4 and 10 years of age. Merck notes it usually affects mid- to large-size breeds and is rare in toy and miniature breeds, and spayed or neutered dogs may be affected a bit more often.

The signs creep in slowly: weight gain without eating more, tiredness, less interest in exercise, seeking warm spots, and a dry, dull coat with excess shedding, thinning, or symmetrical hair loss. The skin may darken, and repeat skin and ear infections are common. Rare signs include facial puffiness and nerve problems such as head tilt, weakness, or foot dragging. Many of these overlap with other conditions, including obesity and hair loss from other causes, so testing is essential.

Causes and risk factors

What you can do at home

Hypothyroidism cannot be diagnosed or treated at home, so the best use of home time is observation and consistency. Keep a short log of your dog's energy, appetite, weight, coat, and any ear or skin trouble, and bring it to appointments. It makes it easier to judge whether treatment is working.

If your dog is diagnosed, give the thyroid medication exactly as your vet prescribes, on a consistent schedule, and never stop or change it on your own. Keep up with the recheck blood tests, which show whether the dose is right. Too little medication lets signs linger, while too much can cause restlessness, hyperactivity, weight loss, and increased thirst, so tell your vet if you notice those.

Be patient. Energy often improves within a few weeks, but coat, skin, and weight changes can take several months. Keep to a measured diet and regular exercise, because weight gain is common before treatment and a healthy weight helps. Dogs with hypothyroidism often feel the cold, so a warm spot to sleep is a kindness. Skin and ear infections may need their own treatment alongside the thyroid medication.

What happens at the vet

Your vet will examine your dog, ask about the signs, and run routine blood work, which can show high cholesterol and sometimes anemia. The usual first thyroid test is total T4. A low result is not proof on its own, because other illnesses, some medications (such as steroids), and breed differences (sighthounds like Greyhounds normally run lower) can lower T4. Confirmation usually involves more tests, such as free T4, TSH, and sometimes thyroid antibodies.

Treatment is a synthetic thyroid hormone replacement (levothyroxine) given by mouth, usually for life. Your vet chooses the amount for your dog and adjusts it based on follow-up blood tests: typically a recheck within weeks of starting, then every few months during the first year, and once or twice a year after that. Skin and ear infections are treated separately.

If your dog does not improve on medication, your vet will look again for another disease. With an accurate diagnosis, treatment, and monitoring, Cornell describes the outlook as good. The condition is treatable but not curable, so the medication and rechecks continue for life.

Prevention

Veterinary references describe most cases as immune-mediated with a hereditary component, and none lists a reliable way to prevent them. What you can do is catch the disease early and protect your dog's health around it.

Take your dog to regular checkups, especially as your dog reaches middle age, and mention slow changes such as weight gain, low energy, or a thinning coat instead of assuming it is just aging. Do not start thyroid medication without testing first. Other illnesses and some medications can mimic hypothyroidism on blood tests, and too much thyroid hormone causes problems of its own.

Keep your dog at a healthy weight; see obesity in dogs. If your dog has hypothyroidism, stay on schedule with medication and rechecks to keep signs from returning and to lower the chance of repeat skin and ear infections.