A patient that is experiencing depression or other problems that may be related to thyroid hormone production should ask a local doctor to perform a free T3 test to determine if this is the underlying problem. T3, or triiodothyronine, is a hormone found in the blood and produced within the thyroid. The difference between regular T3 and free T3 is that the free T3 hormones are unbound to proteins. It is thought that the free hormones control biological action. Certain conditions, such as pregnancy or using birth control pills, may alter the regular T3 levels, but the amount of free T3 hormone generally remains the same.
There is some controversy over what constitutes normal free T3 levels. Different sources advise different levels. As the role of the thyroid is not completely understood, it is difficult to determine which sources are accurate. According to the National Academy for Clinical Biochemistry, the normal range of free T3 levels in a healthy person should be between 3.5 and 7.7 pmol/L (0.2-0.5 ng/dL). However, one company that produces laboratory tests for free T3, Diagnostic Automation Inc., estimates that a person should have 1.4-4.2 pg/mL. Yet another source, the Family Practice Notebook website, provides a different opinion. They state that normal thyroid function should include a free T3 range of 230-619 pg/dL. Depending on the testing method used, a laboratory may establish another set of ranges for a normal person.
One of the leading laboratories used for evaluating free T3 is Quest Diagnostics. They have a slightly more specific normal range of 230-420 pg/dL. Therefore any amount less than 230 pg/dL is considered low free T3 and anything higher than 420 pg/dL is considered high. There is a direct correlation between decreased free T3 levels and the condition of depression.
The test utilized in determining thyroid hormone levels consists of a sample of blood that is analyzed by an endocrinology lab. The test will result in accurate measurements of a patient’s T3, T4, FT4, FTI, and TSH levels. This will include both free T3 and T4 levels as well as those bound to proteins within the blood. A total combination of these hormone levels can be an accurate indicator of both hyperthyroidism and hypothyroidism if they appear in increased or depressed amounts. The presence of TSH, or thyroid stimulating hormone, is what tells the thyroid gland to increase or decrease the production of T3 and T4 hormones. T4, or thyroxin, is split into free and regular types. The overwhelming majority of T4 is bound to plasma proteins, but approximately 5% is used in metabolism when it is free. It is considered by many experts to be the best determining factor of abnormal thyroid function. In addition to these levels, a total free thyroxine index, or FTI, will be computed based on the results of each hormone level. This index is commonly used as well as the free T3 test to diagnose thyroid problems.