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Understanding Thyroid Lab Tests

The Free T3 (FT3) Test

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

Updated May 16, 2014

Why is the Reference Range Listed Differently in Different Places?

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.

So How Do I Interpret My Free T3 Levels?

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).

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