Understanding the Free T3 Thyroid Test - FT3 - Free Triiodothyronine

Kenneth Woliner

I am regularly asked, “How does one interpret the normal range for Free T3?"

  • The National Academy for Clinical Biochemistry (NACB) consensus statement states 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?

    Kenneth N. Woliner, M.D., A.B.F.P. -- a board-certified family physician in private practice at Holistic Family Medicine in Boca Raton, Florida -- helps us understand in this Q&A interview. 

    Q: What is the Difference Between T3 and Free T3?

    Dr. Woliner: 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).

    Q: Why is the Reference Range Listed Differently in Different Places?

    Dr. Woliner: As you may have noticed, the reference range for the T3 tests listed above has been recorded in different units: pmol/L, ng/dL, pg/dL, and pg/mL. Nanograms (ng) and picograms (pg) are different units on the metric system similar to centimeters (cm) and millimeters (mm). The same thing is true for liters (L), deciliters (dL) and milliliters (mL). To give a standard measure: 0.2 ng/dL is equivalent to 2 pg/mL or 200 pg/dL.

    Picomoles (pmol) and picograms (pg) are units used in different measurement systems. Very much like centimeters (cm) are used in the metric system and inches (in) are used in the English system there is not a simple conversion of multiplying or dividing by factors of ten.

    The conversion factor is based upon Avogadro’s Number (6.022 x 1023) and the number of soluble particles a compound breaks into when placed in solution (such as water), and is different depending upon the substance you are measuring. For example, table salt (composed of sodium chloride (NaCl)), has 2 particles, but Dead Sea Salt (which contains high concentrations of magnesium chloride (MgCl2)), has 3 particles, yielding a different conversion factor.

    Your head might be spinning and you probably do not want to touch this issue with a 10-foot pole, but consider this: A Canadian (or anyone using the metric system living outside of the United States of America) doesn’t want to touch this issue with a 3-meter pole! This gets pretty complicated, even for someone such as myself that received a degree in Nutrition Biochemistry from Cornell University. Needless to say, there will be differences in reporting values for a test if you use different units of measurement.

    But there are other reasons more significant for discrepancies in “normal ranges." It is important to keep in mind that establishment of a range of values which can be expected to be found by a given method for a population of “normal" persons is dependent upon a number of factors:

    • the specificity of the method,
    • the population tested, and
    • the precision of the method in the hands of the analyst.

    For these reasons, each laboratory should depend upon the range of expected values established by the Manufacturer (such as Diagnostic Automation, Inc.) only until an in-house range can be determined by the analysts using the method with a population indigenous to the area in which the laboratory is located. A reliable lab will set a reference range based on how they do the test and which population they do the test on.

    Q: So How Do I Interpret My Free T3 Levels? 

    Dr. Woliner: To keep things simple, I am going to use pg/dL as a common unit of measurement for the rest of this article. Diagnostic Automation, Inc, one company that makes a lab test for Free T3, lists their reference range as 140 – 420 pg/dL. The NACB is composed of international members as well as members from the United States. They published a consensus statement, and a majority of their committee members decided to list a reference range in a little bit higher: 200 – 500 pg/dL. The Family Practice Notebook may be using different laboratories, have a different population, or wanted to be even more “sensitive" at finding people to be hypothyroid. They prefer to use a range of 230 – 619 pg/dL. Lastly, the reference range for Quest Diagnostics is narrower: 230-420 pg/dL. Quest uses a premier endocrinology laboratory, the Nichols Institute in San Juan Capistrano, CA, as their reference laboratory, and it is the laboratory that I currently send most of my specimens to. (Please note: I own no stock in Quest or the Nichols Institute, and I do not receive speaking or other consulting fees from these laboratories).

    So what is normal? When specimens are sent to Quest, normal is 230-420 pg/dL. If you are lower than 230 or above 420, you are “abnormal". That doesn’t tell the whole story, however. There is a difference between “normal" and “optimal". Someone can be “low normal" with a Free T3 of 231 and still feel crummy.

    Some of my patients, have many hypothyroid symptoms, but have normal lab tests. When they have “low normal" values, I often will empirically try thyroid medication by starting low and going slow. If my patients feel better with medication, I interpret this situation as one where the laboratory tests do not tell the whole story, and that in the future I would make medication adjustments based upon their symptoms and physical exam. If they feel worse with less thyroid medication (such as gaining weight, being more fatigued), I give them more. If they feel like they are getting too much thyroid medication (heart palpitations, anxiety), I give them less. As an aside – giving thyroid hormone to patients who are not truly hypothyroid does not cause these patients to lose weight. They wind up only getting the side effects (such as palpitations), but no benefits (there will be no weight loss if thyroid is not the true problem).

    It is important to realize that if a patient has thyroid antibodies, all bets are off. All thyroid blood tests from that point on are thrown off, including the Free T3. In that case, one definitely has to go by symptoms to decide the correct level of thyroid medication. 

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