Hypothyroidism is the most common endocrine disease of dogs. This disease is far less common in cats, however cases do occasionally occur. This article will therefore focus on dogs, but the clinical consequences similar in cats. Hypothyroidism is most commonly seen in pets four to six years of age. Male and female dogs are equally affected, however, some vets believe that neutered dogs are more susceptible than intact dogs. 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 consequences may occur.
It is common for dogs with hypothyroidism to gain weight while only eating moderately. 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, However, when an animal’s backs become flattened instead of curved and they pant heavily with every exertion, some owners bring them in for a check up. In most cases such as these, a thyroid panel is warranted.
Most cases of hypothyroidism stem from the dog’s own immune system attacking thyroid gland tissue. This condition is called autoimmune thyroiditis, or, Hashimoto’s Syndrome. Another form of hypothyroidism in dogs is idiopathic thyroid atrophy, or shrinking of thyroid gland tissue for no apparent reason. In both cases, the gland fails to produce enough of the hormone, thyroxine, and signs and treatment are the same.
Clinical Signs Of Hypothyroidism
Adequate levels of thyroid hormone are necessary for proper hair growth. When hormone levels are low, hair growth tends to be thin over the lumbar area equally on both sides. We call this bilaterally symmetrical alopecia, which is one of the hallmark signs of hypothyroidism. The back of the rear legs are also commonly affected. The pet’s hair coat is often scurfy, flaky and lack luster. The coat is commonly deficient in finer body hairs and undercoat. The tail may be bald, like the tail of a rat. An important differentiating feature of thyroid deficiency is that this hair loss is not itchy as it would be from fleas, allergic or infectious skin disease. Hypothyroid dogs commonly have excess black pigment in the skin of their groin, a condition termed acanthosis nigricans. Sometimes this pigment is also present over a large part of the body and the skin becomes flaky, oily, and thickened. Also, frayed
or broken toenails and are common.
Hypothyroidism often causes infertility is intact female dogs, as hypothyroidism commonly leads to erratic reproductive cycling. Pseudopregnancy or false pregnancy with milk flow and abdominal distension is common in these dogs.. Male dogs may also have low sperm levels, decreased libido, and subsequent infertility issues.
Some other signs 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 exercise intolerance may also indicate thyroid disease. It has also been reported that hypothyroid dogs have a higher incidence to KCS (dry eye).
Breeds Most Commonly Affected
Hypothyroidism seems to most commonly present in Labrador and Golden Retrievers, Dachshunds, Cocker Spaniels, Boxers, and Rottweilers.. Hypothyroidism tends to be rare in small or giant breed dogs..
Diagnosis of Hypothyroidism
Diagnosis is obtained through blood test. The serum separated from the blood sample is often creamy whitish in color due to the presence of large amounts of fats (triglycerides and cholesterol) in the blood of hypothyroid patients (a condition called lipemia). The name of the thyroid panel performed on the blood sample is TSH/T3/T4/freeT4. . 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/deciliter and normal levels of Free T-4 are 11-43 picomols/liter. T-4 hovering about one unit and T-3 and Free T-4 levels are low-normal still create suspicion of hypothyroidism if clinical signs are significant. 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.
Fortunately, thyroid hormone is easily synthesized and available in inexpensive tablet form. The T4 form of the hormone is generally prescribed , l-thyroxine (levothyroxine sodium). The initial dose is 10 micrograms per pound of body weight (0.1 mg/10 lbs) twice a day. Borderline dogs are best put on thyroid hormone for a sixty-day trial. This beginning dose is in reality 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 the dose is lowered and the thyroid level resckecked. T4 is assessed 40 weeks follwing the beginning of treatment, or any time the dose has to tbe changed. Once steady state levels have been acheived, the T4 should be checked once every 6 months.
By: Roger L. Welton, DVM
President, Maybeck Animal Hospital
Author Canine and Feline 101