Canine thyroid disease is the most common endocrine disease and can appear in dogs of any age. Since the thyroid controls your dog's metabolism, including growth and development as well as breaking down proteins, fats and carbohydrates, thyroid disease can cause many health problems.
What the Thyroid Test Measures
The thyroid test measures the levels of hormones produced by the canine thyroid. The main hormones are referred to as T3 and T4. T3 refers to triiodothyronine, which affects almost every physiological process in your dog's body, and T4 refers to thyroxine, the prehormone secreted by the thyroid gland and later converted to the more potent triiodothyronine. Both hormones are required in proper amounts in your dog's body for it to function properly. Thyroid hormones are tested through a simple blood test and then analyzed in the lab.
Types of Thyroid Disease
There are two types of thyroid disease: hypothyroidism and hyperthyroidism. Hypothyroidism is when your dog doesn't produce enough hormones, and hyperthyroidism is when it produces too much. The blood test will show a normal range next to your dog's actual hormones levels. From this, you can compare if the value is too high or too low.
These diseases produce nearly opposite symptoms. Hypothyroidism causes weight gain, lethargy and loss of appetite while hyperthyroidism causes weight loss and increased energy and appetite. Both can cause skin and coat problems.
T4 Hormone Levels
The normal range for thyroid levels is quite large and varies depending on your veterinarian. A typical normal range falls around 1.0 to 4.0. However, many thyroid experts feel that this range is too large and can be more variable depending on the age, breed and size of your dog.
Low T4 hormones indicate hypothyroidism because the body is not producing enough of the prehormone necessary to be converted into the actual hormone. A TSH test can confirm this. TSH is thyroid-stimulating hormone, which converts T4 into T3. If hypothyroidism is the correct diagnosis, thyroid levels will remain low, even if TSH is administered.
If the problem is with the T3 hormone, the levels will elevate, signaling the problem is in the conversion of T4 to T3, not the production of T4.
High levels of T4 hormones indicate hyperthyroidism, which is rare in dogs but more common in cats.
T3 Hormone Levels
T3 hormone levels are rarely used to detect thyroid problems unless used in conjunction with T4 results, because T3 can be normal, even if T4 is dramatically low. This makes the tests less reliable.
Six-Panel Test
New research is showing that the majority of thyroid disease cases are genetic and can be detected at a much younger age than previously thought. This is detected by using a six-panel test that tests T3, T4, free T4, free T3, T3 antibodies and T4 antibodies. The antibodies reveal the potential for thyroid problems later in life.
The normal range of these tests vary with your dog's age, breed and size. Even if the T3 and T4 levels fall into the 1.0 to 4.0 normal range, a young dog should be in the top half of this or may need to begin treatment to prevent worse problems from developing later in life.