Percent through Range

What is Percent through Range and how can we use it to help understand our thyroid meds?

Like many lab tests, our thyroid lab results have a set of expected values — the laboratory reference range — that most doctors consider normal.
A reference range, or interval, is a set of values that includes upper and lower limits of a lab test based on a group of otherwise healthy people… These intervals are thought of as “normal ranges or limits.”    

It might be surprising to know how few samples may determine a reference range for a test.
NCCLS recommends testing at least 120 patient samples for establishment of a statistically significant reference interval. Other experts recommend a minimum of 200 samples to ensure stable lower and upper reference limits.
Ranges can vary from lab to lab or may be updated for variety of reasons which is why it’s important to include the range when discussing test results.

While test results falling within the expected range are usually considered normal, that does not imply that the result is optimal for an individual’s health or situation.  Results need to be viewed in the context of related test results and  clinical health or symptoms. While thyroid replacement hormone meds are dosed in small amounts — micrograms — the reference ranges are quite broad and a result at the low or high end of a range may be a far cry from “normal”. 

An example of a typical situation where “normal” is not meaningful for thyroid cancer survivors is the need for suppression — keeping the TSH level below the reference range to discourage recurrence of thyroid cancer. 

While the established lab interval for TSH is fairly consistent globally (from as low as 0.35mIU/L up to 4.75mIU/L) , the reference intervals for thyroid hormones (T4, T3, FT4, FT3) can vary a lot. Some hospitals and smaller labs have established their own ranges and different labs and countries use different units of measure.  A patient may have test results from a doctor’s office or clinic which look quite different from their results done at a commercial lab. 

Percent through Range: comparing apples and oranges.

Looking at the Percent through Range allows us to compare results that use different reference ranges.  For example (with the reference range shown in parentheses):
Lab #1:  Free T4 1.53 (0.82 – 1.77) = 75% through range
Lab #2:  Free T4 1.53 (0.7 – 1.5) = 104% through range
So even though the Free T4 value appears the same in both sets of results, they do not mean the same thing because the ranges are different. Lab #2 is indicating a much higher circulating level of FT4.

Looking at Percent through Range also allows us to see where we actually fit within that range and if we have a basis for increasing or decreasing our medication.  For example:
Free T4 1.62 (0.82 – 1.77) = 84% through range
Free T3 2.11 (2.0 – 4.4) = 5% through range
Depending on clinical signs and symptoms and TSH level (if suppression needed) T4 dosing could be lowered and T3 added or increased for an improved balance.  

As everyone is an individual, the “correct” balance of FT4 and FT3 is also individual. An example of a “fifty-fifty” balance might be FT4 1.29 (0.82 – 1.77) and FT3 3.21 (2.0 – 4.4) — both close to 50% through range.  People often ask for specific target levels of FT4 or FT3; this is difficult to answer because what is right for me may not work at all well for you.  Many thyroid cancer patients after thyroidectomy report feeling best when their Free T4 is in the lower 25 to 40% of the range and Free T3 at 50% or above while some do best at higher or lower levels. 

The Math for finding Percent Through Range

(Your result minus the low end of the range)
divided by
(high end of the range minus low end of the range)
So if your result for Free T4 is 1.62 and the range is (0.82 – 1.77)
then (1.62-0.82) / (1.77-0.82) = 0.84 or 84% through range

New Online Calculator!
This is an easy to use online calculator for thyroid levels and more with a printable summary from a thyroid advocate in the UK .
Online Thyroid Calculator

Or download a spreadsheets you can keep in you own files.
Thyroid Percent Calcs

Regarding “units” – when calculating “percent through range”, the “units” do not figure into the calculations so there is no need to convert. If your “units” do not match what is in the online calculator you can simply ignore them.
You’ll notice that using “percent” not only lets us compare lab results with different reference ranges but also compare results which use different units of measure, i.e. comparing pmol/L to ng/dL.

Further Reading:

About Reference Ranges

For more on establishing reference intervals