The thyroid gland is a general regulator of the metabolism. Hypothyroidism is usually a spontaneous disorder that results in a progressive loss of thyroid function. Hypothyroidism results in a decreased metabolism, which results in weight gain, general loss of energy, dry or thin hair coat, decrease in muscle strength, decreased ability of tissues to heal, and decreased immune system function, often resulting in secondary infections. These infections can be in the skin, ears, urinary tract, or other areas. Hypothyroidism can also result in behavioral changes, as well.
We can screen for hypothyroidism by checking one of the thyroid hormones, T4. If this hormone is well within the normal range, then the thyroid function is likely normal. If the value is extremely low, then hypothyroidism is likely. However, the T4 level by itself can be affected by numerous other conditions and diseases. There is a broad range from the lower end of normal to the upper end of abnormal that is considered a “gray zone,” and may or may not indicate hypothyroidism. If the T4 comes back within this gray zone, there are 3 options:
1) We can run a complete thyroid panel, which is done through the diagnostic laboratory atMichiganStateUniversity. This panel tests for several defferent thyroid hormones, as well as hormones that affect thyroid function and antibodies that are being made against the thyroid or its hormones. This test is also reviewed by an endocrinology specialist. The cost of this test is generally about $150-175.
2) We can check an additional thyroid hormone, called a free T4 by equilibrium dialysis. The test costs about $100, but it gives more definitive results. This is a hormone that is not affected by other conditions. If this level is also low, it indicates hypothyroidism. This test takes about 2-4 days to come back after it has been submitted to the laboratory. If the original blood test was submitted to the laboratory less than 5 days prior to running this test, an additional blood draw is not required, because we can call the laboratory and simply request that they add this test on to the current blood work.
3) Re-check the T4 level again in 1-3 months. Hypothyroidism is not a condition that develops overnight, but is more of a long, slow decline in thyroid function. This test is less than $50. If the T4 is becoming progressively lower with time, then it is very suggestive of hypothyroidism.
In addition, if the screening T4 comes back near or above the high end of the normal range, this may also indicate problems. Dogs do not become hyperthyroid (too high of a thyroid level), but there are certain forms of hypothyroidism (primarily autoimmune thyroiditis or thyroid hormone autoantibodies) that can cause temporary or persistent elevations in the T4 value.
In the event of an elevated T4, the above testing options still apply.
Hypothyroidism is a lifelong condition. Once it develops, it does not ever resolve by itself. The treatment for hypothyroidism is with an oral form of the thyroid hormone. This medication must be given for the rest of the pet’s life. While on the medication, we have to intermittently check the pet’s T4 level. We will generally check the level 3-4 weeks after starting the medication. If the level is normal, we will continue the same dose and recheck again 3-4 months later. If the level is still normal at that time, we will generally just check it every 6-12 months, thereafter. If the level is abnormal, we will adjust the medication dose and recheck the level 1-2 months later, and so on.
Also, it is important to note that the timing of the blood test is important when we are monitoring response to the medication. When we are checking T4 levels on a pet that is being treated for hypothyroidism, the blood must be drawn 4-6 hours after the morning pill.