Introduction

Hypothyroidism is a condition caused by abnormally low circulating levels of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). It is a relatively common problem in mid-to-large-sized, middle-aged dogs of either gender and often is accompanied by vague, nonspecific symptoms that mirror those of other diseases. Once hypothyroidism is definitively diagnosed, it can be treated fairly easily, although treatment must continue for the dog’s lifetime. The goals of treating hypothyroidism are to restore normal levels of circulating thyroid hormones and to eliminate or at least manage the symptoms of the disorder so that affected dogs have a good, symptom-free quality of life.

Treatment Options for Hypothyroidism

The effects of hypothyroidism in dogs are caused by the thyroid gland’s failure to produce the proper amount of thyroid hormones. Hypothyroidism is a chronic disorder that does not normally require emergency care but does require life-long treatment. It is treatable with relatively affordable, daily oral medication, which reverses all of the clinical abnormalities associated with the disease. With consistent daily treatment, almost all dogs with hypothyroidism can lead long, healthy and normal lives.

Treatment is typically accomplished by giving daily doses of a synthetic T4 thyroid hormone called sodium levothyroxine, in the form of oral tablets. Successful treatment depends upon maintaining proper thyroxine levels in the dog’s blood. The frequency of dosing will vary between dogs, but most protocols call for either once or twice daily treatment. The attending veterinarian will periodically test the dog’s blood to make sure that circulating hormone levels are maintained at normal levels. These tests probably will be more frequent at first (maybe every 4 to 6 weeks), and then will decrease in frequency as thyroid hormone levels are stabilized.

Owners can expect to see dramatic improvement in their dogs within the first 1 to 2 weeks of treatment. Normally, energy and an interest in activities return first, and the dog just seems to be feeling better. Dermatologic and neurologic signs improve over several months, along with hair regrowth, improved overall skin condition and improved tolerance to exercise. With time, a healthy diet and moderate exercise, weight loss can be achieved as well. Reproductive abnormalities can take a bit longer to resolve.

If major clinical improvement is not seen in a dog within 3 months of hormone replacement therapy, despite normal blood levels of thyroxine as monitored during treatment, the diagnosis of hypothyroidism in probably incorrect, and another underlying cause for the dog’s symptoms should be explored.

Prognosis

The prognosis for dogs with primary hypothyroidism is very good to excellent, once they are put on an appropriate hormone replacement protocol. Secondary hypothyroidism caused by cancer of the pituitary gland has a much more guarded prognosis.