Free T3 or Free T4: Which is More Important?

Free T3 and free T4 are incredibly important lab tests that all thyroid patients should be aware of. Having said that, is one better than the other? The answer is yes, and it will become obvious as we have this discussion. First: what are free thyroid hormones and why do they matter? Free thyroid hormones differ from other hormones that you can test because they represent the amount of hormone that is free and ready to be used by the body. Because of this, measuring the free concentration of hormones is always the most important and relevant way to measure the activity of any given hormone. Hormones can either transport through your body in the free state or the bound state and, as a thyroid patient, you want to concern yourself with the thyroid hormones which are NOT bound to a protein. Now that you understand the importance of free thyroid hormones, let's talk about free T3 and free T4. What is free T4? Free T4 is the measurement of free and active thyroxine in your bloodstream. Thyroxine, also referred to as T4, is an active thyroid hormone but it's not the MOST active thyroid hormone. T4 is not as powerful as T3 and in order to really be used by the body, your T4 must be converted into T3. Because of this, T4 is good to think about as reservoir hormone instead of an active thyroid hormone. It has the potential to do work but doesn't actually do any work until it is converted. Your thyroid produces much more T4 compared to T3 because of this. T4 is found in thyroid medications like levothyroxine so if you take levothyroxine, your free T4 level should increase. What about free T3? Free t3 represents the measurement of free and active triiodothyronine in your body. T3 is the most powerful thyroid hormone, by far, with estimates putting it as much as 200 to 300 times more biologically active compared to T4. This means if you have the same amount of both T3 and T4, the t3 is going to do 200 to 300 times the work that the same amount of T4 can do. Download my free thyroid resources here (in