What is the Thyroid Gland?
The thyroid gland, located on either side of the trachea, is one of the largest endocrine glands in a dog’s body. It secretes active thyroid hormone, which serves as a sort of volume dial for metabolism and affects almost every cell in the body. In most dogs, the thyroid releases appropriate levels of this hormone, but sometimes dogs can suffer from a disease that results from too little circulating thyroid hormone called hypothyroidism. This is the most common hormonal imbalance in dogs, leading to symptoms in multiple body systems.
What is Hypothyroidism?
As previously stated, hypothyroidism is the natural deficiency of thyroid hormone. This can be the result of immune-mediated destruction of the thyroid gland, natural atrophy of the gland, dietary iodine deficiency or a congenital problem. In dogs, the first two causes listed account for almost all cases, although it is generally accepted that atrophy of the gland actually represents the end result of earlier immune-mediated destruction.
Hypothyroidism generally develops in middle-aged or elderly dogs. Breeds with a predisposition to develop hypothyroidism include: the Doberman Pinscher, the Golden Retriever, the Irish Setter, the Great Dane, the Dachshund, and the Boxer.
Manifestations of Hypothyroidism
Hypothyroidism is a disease with a classic collection of clinical signs. The most common of these are skin abnormalities (including hair loss, scaly/oily skin, skin infection, dry/brittle hair coat), obesity, lethargy or listlessness at home, high cholesterol and anemia.
Testing for Hypothyroidism
The most common way to diagnose hypothyroidism is to check the T4 level (also called the total T4 level), a blood test that is often included in routine screenings. Usually, a normal T4 indicates normal thyroid function and a low T4 indicates hypothyroidism, but in some cases it isn’t that simple. Dogs taking certain drugs (most notably the seizure medications phenobarbital and potassium bromide, prednisone and other corticosteroids, non-steroidal anti-inflammatory drugs, the heart
medication propranolol, the behavior drug clomipramine, or sulfa class antibiotics) or with illnesses other than thyroid disease often have depressed T4 secretion. These dogs will have low T4 levels but are not truly hypothyroid. There is also a grey zone where T4 results are considered not definitively normal and not definitively abnormal. In short, sometimes T4 alone
is not sufficient to make a diagnosis of hypothyroidism. Occasionally, additional tests, such as free T4 by equilibrium dialysis (ED), thyroid stimulating hormone (TSH) and endogenous TSH levels, antithyroglobulin antibodies (TGAA test) or a trial of the medication, will be recommended by your veterinarian for a more definitive diagnosis.
Treatment of Hypothyroidism
At least treatment of hypothyroidism is relatively straight-forward; active thyroid hormone is restored to a normal level through an oral medication containing that very hormone (T4). Pills are given twice daily for the remainder of the pet’s life. There are many brands of thyroid supplementation available and prices are somewhat variable depending on the manufacturer.
Occasionally we are asked if it is reasonable to use dried or powdered thyroid glands of hogs or cattle as a more “natural” form of treatment. The answer is simply no. These products are not produced with adequate quality control to ensure that they contain a reliable amount of thyroid hormone. Each dose may be completely different when such a product is used.
Periodic blood testing is required in the treatment of hypothyroidism, because it is important to know if the current dose is too low or too high. Thyroxine (T4) is a safe medication but if it is not given in adequate doses, obviously the patient will not be adequately treated. And if the dose is too high, excessive water consumption, weight loss, and restlessness can result.
The highest level of the day is found by taking the blood sample 4 to 6 hours after it is given. This blood test is recommended every 6 months (after the correct dose has been established) for the rest of the dog’s life.
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