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The Free T3 (FT3) Test

Understanding Thyroid Lab Tests

By Ken Woliner, M.D., A.B.F.P.

Updated April 06, 2014

Updated April 06, 2014
by Kenneth N. Woliner, M.D., A.B.F.P.

August 2003 -- I was recently asked, “How does one interpret the normal range for Free T3?"

  • The National Academy for Clinical Biochemistry (NACB) published a consensus statement in November 2002 with guidelines stating that the normal range for Free T3 should be 3.5-7.7 pmol/L (0.2 - 0.5 ng/dL).
  • In other sources, such as a web site called Family Practice Notebook, they list a reference range for Free T3 of 230 – 619 pg/dL.
  • Meanwhile, Diagnostic Automation, Inc, one company that makes a lab test for Free T3, lists their reference range as 1.4 – 4.2 pg/mL.
A reader contacted me, asking “What’s up? Why all the different reference ranges for Free T3?"

It’s a valid question. Because sometimes the Free T3 normal range is listed in the hundreds, and other times, in the single digits with decimals points. And with these variations, how can you interpret your Free T3 levels?

Dr. Ken Woliner answers the question, taking another look at how to properly interpret thyroid laboratory tests. For a primer on how to understand thyroid laboratory tests, please see Dr. Woliner’s article, Understanding Thyroid Lab Tests.

– Mary Shomon


What is the Difference Between T3 and Free T3?

Triiodothyronine (T3) is a thyroid hormone that circulates in blood almost completely bound (]99.5%) to carrier proteins. The main transport protein is thyroxine-binding globulin (TBG). However, only the free (unbound) portion of triiodothyronine (free T3) is believed to be responsible for the biological action. Furthermore, the concentrations of the carrier proteins are altered in many clinical conditions, such as pregnancy.

In normal thyroid function, as the concentrations of the carrier proteins changes, the total triiodothyronine level also changes, so that the free triiodothyronine concentration remains constant. (In an abnormally functioning thyroid, this is not necessarily so). Measurements of free triiodothyronine (Free T3) concentrations, therefore, correlate more reliably with your clinical status than total triiodothyronine (T3) levels.

For example, the increase in total triiodothyronine levels associated with pregnancy, oral contraceptives and estrogen therapy result in higher total T3 levels while the free T3 concentration remains unchanged (in normal individuals).

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