The Fat Dog: Disease or Diet?
By Robert Young, DVM // June 26, 2013
(VIDEO BY: Pet Health Network)
BREVARD COUNTY • MELBOURNE, FLORIDA – Is your dog lazy and out of energy all the time? Does it have a rather large waist line and not seem to eat much at all? Does it have skin problems and a general dullness about its day to day activities? If so, then it may be more than a case of “couch potatoitis.” There is a real chance that Fido could have a medical problem.
Hypothyroidism is a hormone deficiency in which the body has inadequate active thyroid hormone. This condition that affects both humans and dogs, and is most common hormone imbalance in dogs. The disease is a result of the body not producing enough thyroid hormone, and once identified can be treated with a pill.
What is the thyroid gland, what does it do?
The thyroid gland is one of the most important glands in the body. It is located in the neck near the trachea (windpipe) and is composed of two lobes, one on each side of the trachea. The thyroid gland is controlled by the body’s master gland, the pituitary gland, located at the base of the brain.
The thyroid gland regulates the rate of metabolism. If it is overactive, metabolism speeds up. If it is less functional than normal, metabolism slows down. The latter is the cause of the clinical signs of hypothyroidism.
The thyroid gland makes two forms of thyroid hormone. One form, called T3 is the active form. T4 is the inactive form created to circulate in the blood stream. T4 is converted into T3 inside tissue cells. Most T4 is bound to proteins in the blood and cannot be absorbed. The portion that is not bound (free T4) is able to enter the cells. The thyroid gland can also convert T4 to T3. In dogs half of the T3 comes from the thyroid gland and half comes from the tissue. In humans 80% comes from tissue activation.
The pituitary gland is the main controller of T4 production and when levels are low it produces a hormone called thyroid stimulating hormone (TSH).
Active thyroid hormone controls metabolism throughout the body. The entire body is influenced by thyroid function and so decreased levels lead to signs in more than one area of the body.
Causes of hypothyroidism
Hypothyroidism is almost always caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy. The former disease is the most common cause of hypothyroidism and is thought to be an immune-mediated disease. This means that the immune system decides that the thyroid is abnormal or foreign and attacks it. It is not known why the immune system does this. Idiopathic thyroid gland atrophy is also poorly understood. The general belief is that normal thyroid tissue is replaced by fat tissue in what is considered a degenerative disease.
These two causes of hypothyroidism account for more than 95% of the cases. The other five percent are due to uncommon diseases, including iodine deficiency, congenital problems or cancer of the thyroid gland.
Hypothyroidism generally occurs in middle age to older dogs. Breeds more likely to be affected include: the Doberman pinscher, the Golden retriever, the Dachshund, and the boxer.
When the rate of metabolism slows down, virtually every organ in the body is affected in some manner. Most affected dogs demonstrate one or more symptoms:
- Weight gain without an increase in appetite (49%)
- Lethargy and lack of desire to exercise
- Cold intolerance (gets cold easily)
- Skin problems (88%)
- Dry hair coat with excessive shedding
- Very thin hair coat to near baldness
- Increased pigmentation in the skin
- Increased susceptibility to skin and ear infections
- Failure to regrow hair after clipping or shaving
- High blood cholesterol, anemia
Some dogs also have other abnormalities that are not as typical. These include but are not limited to:
- Thickening of the facial skin creating a tragic facial expression
- Neurologic weakness and reproductive abnormalities
- The eyes can be affected with fat deposits in the corneas or a lack of normal tear production (Keratoconjunctivitis sicca)
The most common test is evaluating the T4 level. This is a measurement of the main thyroid hormone in a blood sample. If it is below normal and the correct clinical signs are present, the test is meaningful. However, testing for the T4 level can be misleading because some dogs that are not hypothyroid may have subnormal levels. This happens when another disease is present or when certain drugs are given. Among the drugs causing this drop are seizure medications, steroids, NSAIDS, sulfa antibiotics, and some heart and behavioral medications .
If hypothyroidism is suspected but the T4 is normal, other tests can be performed. These are more expensive so they are not used as first line tests. More extensive lab evaluations generally include a free T4, TSH, and antithyroglobulin autoantibodies.
Hypothyroidism is treatable but not curable. The treatment is relatively straight forward. It is done with oral administration of the thyroid replacement hormone thyroxine (T4). This drug must be given for the rest of the dog’s life.
When using long term medications, periodic testing is needed. It is important to make sure the dose is not too high or low. Thyroxine is viewed as a safe medication, but if the dose is too low then the problem is not being adequately controlled. If the dose is too high then signs like increased water consumption, weight loss and restlessness can be seen.
There is a standard dose that is used initially based on the dog’s weight. After about one month of treatment, further testing is done to verify that the thyroid hormone levels are normal. You may notice an increase in your pet’s activity after therapy has begun.
If your dog has been diagnosed with hypothyroidism, the good news is that with proper treatment your pet will be able to live a normal, healthy life.
ABOUT THE AUTHOR
Dr. Young graduated with honors from the University of Florida College of Veterinary Medicine in 2004. He also did his undergraduate work at the University of Florida, graduating with a B.S. in Animal Science in 2000. After graduating he completed an internship at Hollywood Animal Hospital in Hollywood, Florida. He enjoys all aspects of small animal medicine and surgery. He has been with Animal Medical Clinic in Melbourne since 2006.