Blood Tests

One of the most important things your doctor will do is order blood tests to figure out if you have hypothyroidism. Blood tests are critical in determining whether your thyroid is the cause of your symptoms. It's also a relatively simple process. What does get tricky is making sense of the alphabet soup that comes with the lab results.

TSH Levels

For most doctors, the most telling measure of all is the level of TSH in your blood. As you might recall, TSH is the hormone released by the pituitary gland that tells the thyroid gland to release more hormones. Higher-than-normal levels of TSH reveal that your pituitary is trying to stimulate the thyroid gland to release more thyroid hormone.

The American College of Clinical Endocrinologists currently considers normal levels of TSH to be in the range of 0.3 milliunits per deciliter (mIU/dL) to 3.0 mIU/dL — a change made in November 2002, when the range was narrowed from the previous range of 0.5 mIU/dL to 5.0 mIU/dL. The new, more stringent definition of normal has paved the way for more diagnoses of hypo- and hyperthyroidism. Anything above that range might be considered hypothyroidism, depending on other factors. Measures below it might be hyperthyroidism.

Although TSH levels are considered the best gauge of thyroid function, there are times when this measure is less than ideal. For instance, measuring TSH assumes that the pituitary gland is healthy and functioning properly. But if the pituitary gland has a tumor, it may be incapable of producing enough TSH. Deficiencies in TSH may also be caused by damage to the hypothalamus brought on by an injury, tumor, or stroke. In these cases, the TSH level may be inappropriately normal, even when levels of thyroid hormone are low.

In addition, the time of day that your blood is drawn makes a difference. TSH tends to be in the midrange in the early morning, drops at noon, then rises at night.

Total T4 and Free T4

Total T4 is just that — a measure of all the T4 in your blood, including the T4 that is bound to protein and unavailable to body cells for use. In fact, the bulk of the T4 released by your thyroid is bound to proteins.

Alert

A good thyroid doctor will rely on more than just the TSH test as a way to diagnose hypo- or hyperthyroidism. The TSH test, after all, reveals only part of the picture; that is, how much the pituitary gland is egging on the thyroid gland for more hormone. More comprehensive testing will involve tests for free T3 and free T4 — the actual amount of available thyroid hormone.

Although testing for total T4 has been used in the past to diagnose hypothyroidism, today it is considered a less useful tool. That's because the amount of total T4 can be affected by the amount of binding proteins in the blood. The amount of protein is influenced by medical conditions such as certain types of liver and kidney diseases as well as by pregnancy. Nonetheless, low levels of total T4 can often suggest that you have hypothyroidism.

A more useful diagnostic tool is the free T4 test. This test measures the unbound T4 in the blood, which is called free thyroxine. Too little of it is a sign of hypothyroidism.

If your doctor decides to test free T4, he should use a lab that uses the equilibrium dialysis method, which involves separating the free hormone from the bound version. Free T4 is the thyroid hormone available to enter body cells, where it can be converted into T3, the active form of thyroid hormone.

In patients who have hypothyroidism, free T4 levels are rarely lower than normal. Patients with hypothyroidism, especially mild hypothyroidism, usually have an elevated TSH with a low-normal free T4 because the TSH goes up before the free T4 does. Your doctor needs to consider the relationship between free T4 and TSH to see if there is a discrepancy that may suggest a problem.

Total T3 and Free T3

T3, also called triiodothyronine, is the active form of thyroid hormone. The total T3 in your blood is generally not an accurate measure of hypothyroidism, but may be used to diagnose hyperthyroidism.

Another test that is occasionally helpful for diagnosing hypothyroidism is one that measures the amount of free T3 in your body. Free T3 is the active form of triiodothyronine, the unbound version of the hormone that circulates in the blood. It is the active part of thyroid hormone produced by the conversion of T4 to T3. Again, if your doctor decides to measure free T3, he should use a lab that uses the equilibrium dialysis method for a more accurate measurement.

People who have hypothyroidism rarely have low levels of free T3. Most of the time, even in hypothyroidism, the body increases the enzymes that convert T4 to T3, so the free T3 remains normal.

An old way to indirectly measure free T4 and free T3 involves the resin T3 uptake test (RT 3U), an indirect measurement that estimates the amount of free T4 and free T3. This test tells you whether you have a lot of TBG, the protein that carries most of the T3 and T4 in your blood. RT3U is inversely proportional to TBG levels; the more TBG you have, the lower your RT3U levels will be.

High levels of TBG means there are more proteins available to bind T4 and T3, leaving less free T4 and free T3 in the blood. Elevated levels of TBG can be caused by pregnancy, estrogen, and oral contraceptives. Patients with altered levels of TBG do not have symptoms of hypo- or hyperthyroidism. In fact, if your doctor measures free T4 and free T3, the TBG alterations won't even be detected. For these reasons and others, the resin T3 uptake test is rarely used now and has been replaced by tests that directly measure free T4 and T3.

Thyroid Autoantibodies

When people get sick, the immune system produces antibodies to ward off the invading virus or bacteria. But in people who have an autoimmune condition, autoantibodies are produced to fight the patient's own body tissues, which for mysterious reasons are being treated as harmful invaders.

Unfortunately, the thyroid can be the target of an autoimmune attack. In the process, several antibodies may be produced. These substances are easily measured in the blood and serve as markers, or physical evidence, of a disease process.

An autoimmune attack on the thyroid gland often triggers the production of antithyroid peroxidase (TPO) antibodies. Anti-TPO antibodies wreak havoc by attacking parts of the thyroid cells that produce thyroid hormone. Testing for these anti-TPO antibodies has become the gold standard for detecting Hashimoto's disease. More than 80 percent of people with Hashimoto's will test positive for anti TPO antibodies. Some doctors will test for anti-TPO antibodies even if other tests appear normal. If your anti-TPO antibodies are elevated, your hypothyroidism is probably caused by Hashimoto's thyroiditis.

Antithyroglobulin antibodies can also be found in Hashimoto's thyroiditis but are not as helpful as identifying the presence of anti TPO antibodies and are not necessary to diagnose Hashimoto's thyroiditis. Similarly, some doctors may still order a test for antimicrosomal antibodies, which is a less specific test than the anti-TPO test. If you're already getting an anti-TPO test, then you do not need to be tested for antimicrosomal antibodies.

  1. Home
  2. Thyroid Disease
  3. Hypothyroidism
  4. Blood Tests
Visit other About.com sites: