Total T3 (Thyroid) Ranges

[Guest guest]

I recently spoke with Dr. , a clinical pathologist and Director of

Quality Assurance at Quest Labs in the State of Florida. I asked him how lab

ranges were determined for the thyroid labs and he had some interesting info.

I'd always thought they sampled the general population, plotted the values, made

a bell curve and

chopped off the top and bottom 2.5%, which is how you define " normal "

statistically.

Turns out they don't get a bell curve for total T3, but a negatively skewed

curve as shown here (page down to negatively skewed distribution):

http://www.mnstate.edu/wasson/ed602distshape.htm

This means the majority of the lab values are towards the upper range; it is NOT

a symmetrical distribution. If you understand mean, median, and mode, the median

and mode are to the right of the mean. What he described to me was even more

skewed than the example on this link. He said the curve from the left had a

steep ascent, peaked to the right of the midpoint, then had a much more gradual

descent to the right.

All this means is that while you can theoretically be normal at the bottom of

the range, statistically, the majority of people have values mid-range or

higher.

I would venture to guess that while this particular discussion was about total

T3 (he didn't have the FT3 curve readily available), it should apply to the

FT3. I would not expect it to apply to T4 and FT4, however, because high FT4 is

associated with higher rT3, and low FT4 is associated with hypo. I would expect

a more normal distribution for T4/FT4.

Barb