Ok I went back and pieced this together:

As I shared above, Labcorp direct FT test is correlated but not absolute measure of fT. Quest dialysis fT test is.

Maybe a picture will help:

Note the slope of the line on the left panel of the figure. Note that slope (approx 0.15) is very different than the slope of the right panel (~1.2).

This confusion is from folks not being clear about units. The 20 and 30-50 numbers referenced are for an equilibrium dialysis FT measurement in ng/dl not pg/ml.

Note: 10 pg/ml = 1 ng/dl

Careful with units.

To convert Labcorp direct result multiply by 7 so

Labcorp result converted to absolute comparator = 7*10 = 70 pg/ml = 7 ng/dl

Quest ED FT = 85 pg/ml = 8.5 ng/dl

Vermeulen FT calculator gives 9-10 what? 9-10 ng/dl

As mentioned above, Vermeulen FT calculator may be 20% high compared to Labcorp FT by ED. 80%*10 ng/dl = 8 ng/dl which compares well with your Quest fT by ED result and transformed Labcorp direct FT result.

In summary:

Transformed Labcorp direct FT method: 7 ng/dl

Quest FT ED method: 8.5 ng/dl

Vermeulen calculated FT = 9-10 ng/dl

Transform of Vermeulen calculated free T to ED result = 0.8 * (9-10) = 7.2-8 ng/dl

So there we have it, a decent tight range of FT estimate using the data from multiple measurement assays and calculation option (7-8.5 ng/dl).

Until all the FT methods get harmonized why not just use the Vermeulen calculator for everyday work? Or pay for ED at Labcorp or Quest. Or know how to convert and work between the various options.

As shown here, a reasonable physiologic range for FT is 5-28 ng/dl based on Labcorp FT ED method. What is confusing is Quest has a range of 3.5 to 15.5 ng/dl. So cross lab harmonization still needs to be done.

The takeaway is TOT channel gives the impression FT of 30-50 ng/dl is reasonable place to shoot for “symptom resolution” as per the experience of their members. 30-50 ng/dl is above physiologic range. 50 ng/dl is really high for a human (statistical improbably and reckless to be throwing out this range to people on the internet).

See this discussion (this thread and below) for more context about absolute fT measurements by ED above 30 ng/dl:

Please guys, I realize everyone is not a trained scientist but you have to keep track of units and assay type for this FT stuff to make any sense in any self-consistent fashion. Otherwise just look at your results with respect to reported reference range or just trust your provider given his/her/their experience. Of course you should do the later and make sure you have someone you can count on for your health care guidance. Not some weirdo on the internet.

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