Unlike some blood tests whose single results can aid the physician in diagnosing your condition, there are several thyroid blood test levels that must be examined and compared. Understanding the basic process of the thyroid gland will increase your understanding of your test results.
Function
The basic function of the thyroid is to take in iodine and use it to create the thyroid hormones triiodothyronine (T3) and thyroxine (T4), as explained by Endocrine Web. The thyroid gland is under the direct control of the pituitary gland in the brain, which secretes thyroid stimulating hormone (TSH), in essence to direct the thyroid gland in its hormone production.
Importance
The thyroid gland is the only gland in the body that produces T3 and T4, the hormones that regulate the metabolism of every cell in your body.
Thyroid Blood Testing
No single blood test can lead to an accurate diagnosis, but two or more tests will yield the physician the information he needs to understand the function status of your thyroid gland.
Basic Tests
The T4 test by RIA (radioimmunoassay) is the test most frequently used. This test indicates the overall function of the thyroid. This test is sometimes also called the T7, because special testing of T3 serum is also done.
The T3 by RIA is another common blood test. There are situations where the thyroid may be overproducing T3, but T4 is being produced at normal levels. This test, along with the T4, gives the physician a more accurate picture of thyroid function.
Other Blood Tests
Thyroid Binding Globulin (TBG) blood test may be performed if either or both the T3 and T4 levels are abnormal. If the TBG is abnormal, it explains the abnormal measurements of T3 and/or T4. While not a health problem in itself, too much or too little TBG can falsely effect the measurements of T3 and T4, but not their hormonal activity.
In a person exhibiting symptoms of hyperthyroidism or hypothyroidism, but whose T3, T4 and TBG test results are within normal limits, the physician will likely order a TSH level. In the first stages of abnormal thyroid function, the pituitary will send out increasingly high levels of TSH in an effort to maintain normal levels of T3 and T4, meaning the TSH level will be elevated before the T3 and T4 levels become abnormal.
