One of the most important things your doctor will do is order blood tests to figure out if you have hyperthyroidism. Blood tests are critical in determining whether your thyroid is causing your bothersome symptoms. It's a relatively simple process, but interpreting the results can be tricky.
Like hypothyroidism, hyperthyroidism can be measured with a simple test to determine your TSH levels. The TSH test is considered by most doctors to be the single best screening tool for hyperthyroidism.
As you might recall, TSH is the hormone secreted by the pituitary gland that tells the thyroid gland to release more T4. The American College of Clinical Endocrinologists currently considers normal levels of TSH to be in the range of 0.3 to 3.0 mIU/dL. When your body produces enough thyroid hormone, your TSH levels stabilize in the normal range. But when your body has too much thyroid hormone, it automatically shuts down the release of TSH.
Sometimes, a TSH test is not enough. TSH levels may be low in both hyperthyroidism and hypothyroidism due to a pituitary disorder (central hypopituitarism). In that case, other tests are needed to help your doctor determine if you have hyperthyroidism.
Some groups recommend regular screening for thyroid disease. The American Thyroid Association (ATA) for instance, recommends that adults older than 35 be screened with a TSH test every five years. The TSH is used to detect both hyper- and hypothyroidism. But the U.S. Preventive Services Task Force recommends against any routine screening. Some groups suggest screening women before and after pregnancy, whereas others recommend screening adults over age 50 or 60.
Total T3 and Free T3
When it comes to diagnosing hyperthyroidism, levels of total T3 are generally an accurate tool. Total T3 reveals the amount of the active form of thyroid hormone, both the form that binds to proteins and the form that gets into body cells. Too much of it indicates that you have an overactive thyroid. In some patients, especially the elderly, the test can reveal T3 toxicosis, a condition in which too much T3, but normal amounts of T4, is being released from the thyroid and causing hyperthyroid symptoms.
Although knowing your total T3, along with your TSH, is often enough to determine whether you have hyperthyroidism, a free T3 test is sometimes taken, especially if your free T4 results are normal and you're still having symptoms of hyperthyroidism.
Total T4 and Free T4
Total T4 is a measure of all the T4 in your body, including the T4 that is bound to protein and unavailable to body cells for use. Since the bulk of the T4 released by your thyroid is bound to proteins, it is usually a less than accurate measure of hyperthyroidism. Abnormal levels of total T4 may reveal problems with protein binding and not thyroid problems. Still, it may be one more clue that you have hyperthyroidism, and your doctor may request it as an additional piece of information.
A more reliable measure is free T4. This test measures the remaining T4, or free thyroxine, in the blood. Free T4 is the thyroid hormone available to enter body cells, where it can be converted into T3, the active part of thyroid hormone. In patients who have hyperthyroidism, free T4 levels can be higher than normal.
When viruses or bacteria invade your body, the immune system releases antibodies that attack the invaders. But in people who have an autoimmune condition, the body mistakenly attacks healthy tissue with destructive autoantibodies. When the thyroid gland suffers an autoimmune attack, the body produces several antibodies. Lab tests to detect these substances offer further evidence that your thyroid disease is the result of an autoimmune reaction.
Some people who have other autoimmune diseases such as rheumatoid arthritis, Sjogren's syndrome, lupus, and pernicious anemia may also have thyroid-stimulating immunoglobulin (TSI) antibodies in their blood. The odds of having these antibodies increase with age, and they are more common in women.
Testing for TSI can be done to determine if the hyperthyroidism is caused by Graves' disease. In healthy people, TSH from the pituitary gland will bind to TSH receptors on the thyroid cell, triggering it to produce thyroid hormone. In people with Graves' disease, TSI will take over the action of TSH, bind to the TSH receptor, and trigger the release of T4 and T3. But when thyroid hormone levels become too high, TSI, unlike TSH, doesn't stop stimulating the production of thyroid hormone, causing levels to soar to unhealthy heights.