Care of Hypothyroid Dogs

Ron Hines DVM PhD 10/1/03

A sluggish thyroid gland or hypothyroidism is the commonest endocrine gland disease of dogs. I often diagnose this condition when a pet reaches four to six years of age. Male and female dogs are equally affected but I have noticed that neutered animals are more susceptible than intact pets.

The thyroid gland consists of two lobes located at the base of the neck. This gland produces thyroxine, a hormone that regulates the body’s metabolic rate, that is the rate at which it burns calories. When thyroxine is not produced in sufficient quantity a number of things happen.

It is common for dogs with hypothyroidism to gain weight while only eating moderately. These dogs have been described as “easy keepers” because they gain weight so easily. Not withstanding, the majority of plump and fat dogs do not have thyroid disease – they just eat too much and get too little exercise. Many owners are oblivious to weight gain in their pets. But when an animal’s backs become flattened instead of curved and they huff and puff with every exertion some owners bring them in for a check up. I run thyroid tests on all these dogs. Most cases of hypothyroidism stem from the dog’s own immune system attacking thyroid gland tissue. This condition is called autoimmune thyroiditis. Common hypothyroidism is further broken down into two types, lymphocytic thyroiditis and idiopathic thyroid atrophy. In both cases, the gland fails to produce enough of the hormone, thyroxine. The signs and treatment are the same.

Hair and Skin Changes

Adequate levels of thyroid hormone are necessary for hair to grow. When hormone levels are low, hair growth sparsely over the lumbar area equally on both sides. The back of the rear legs is also commonly affected. The pet’s hair coat is often scurfy, flaky and dull. The coat commonly lacks finer body hairs and undercoat. The tail may be bald as a rat’s tail. An important clue pointing to thyroid deficiency is that this hair loss is not itchy as it would be from fleas , allergic skin or infectious skin disease. Hypothyroid dogs commonly have excess black pigment in the skin of their groin. This pigment results in a condition called acanthosis nigricans. Sometimes this pigment is present over much of the body and the skin becomes oily and thickened. Broken toenails and toenail infections are common. Hair coat color may change.

Infertility

Female dogs with hypothyroidism often cycle erratically. When they do pass through estrus or heat, they are often infertile. Pseudopregnancy or false pregnancy with milk flow and abdominal distension is common in these dogs (especially dachshunds). Male dogs may have low sperm levels and decreased libido.

Breeds Commonly Affected

I see Hypothyroidism most commonly in Labrador and Golden Retrievers, Dachshunds, Cocker Spaniels, Boxers Doberman Pinchers and Greyhounds in that order of frequency. I rarely encounter this disease in terriers or other small breeds and I have never encountered it in giant breeds.

Other Recognized Signs of Hypothyroidism

Some other symptoms of sluggish thyroid function are seen occasionally and are seen with a number of diseases that are not related to the thyroid gland. These symptoms include mental dullness or depression, cold intolerance, slow heart rate, constipation, anemia, muscle weakness and atrophy, nerve disturbances, edema, stunted growth, and slowed clotting of the blood. Hypothyroid dogs have more than their fair share of joint pain and swelling and ear and skin infections. Lethargic behavior – such as increased sleeping, less play activity and easy fatigue may also indicate thyroid disease. It has also been reported that hypothyroid dogs have more “dry eye” disease (keratoconjunctivitis sica) but I have never seen a case.

Laboratory Diagnosis of Hypothyroidism

When I recognize one or more of the signs I have mentioned, I draw blood for thyroid function tests. The blood I remove is often creamy whitish in color due to the presence of large amounts of fats (triglycerides and cholesterol) in the blood of hypothyroid animals. I have the blood analyzed for thyroid hormones (T-4, free T-4 and T-3). I will occasionally also run at thyroglobin autoantibody test to determine if autoimmune thyroid disease is present. Low hormone levels in the absence of signs of other diseases are diagnostic of hypothyroidism. Blood levels of T-4 are normally 1.0-4.0 micrograms/deciliter. Normal levels of T-3 are 45-150 nanograms/decileter and normal levels of Free T-4 are 11-43 picomols/leter. I become suspicious of hypothyroidism when the numbers for T-4 hover about one unit and T-3 and Free T-4 levels are low-normal - even if the lab reports the case as normal. Falsely low thyroid hormone levels can be due to administration of steroids (cortisone) or concurrent systemic disease. A TSH stimulation test can be run if the diagnosis is in doubt.

Treatment

Fortunately, thyroid hormone is easily synthesized and available in inexpensive tablet form. I generally prescribe the T-4 form of the hormone, l-thyroxine (levothyroxine sodium). We begin by administering 10 micrograms per pound of body weight (0.lmg/10 lbs) twice a day. Borderline dogs are best put on thyroid hormone for a sixty-day trial. This beginning dose is only an estimate. All dogs need their dose individually tailored to their needs. Signs that the initial dose may be too high are agitation, excessive thirst, and diarrhea. When these occur I lower the dose. Thirty days after beginning treatment I assay a second blood sample for Free T-4. If levels are still not adequate the dose is increased. I then retest the dog every six months. Once a dog is placed on medication, it should be given for the rest of the pet’s life.
Hypothyroidism runs in families so it is unwise to breed hypothyroid dogs or their normal littermates. When the disease is diagnosed early and treated one can expect the pet to live a long and happy life.