I have bought and read the book Thyroid for Dummies but I'm still confused about the healthy range for T3 and T4. Also, what's the difference between Free T3 and Free T4?

Thyroid textbooks state that healthy people have FreeT4/T3 levels above mid-range and often near the high ends of their ranges.

Logic follows that doctors treating patients with hypothyroidism should try to achieve the same types of levels for their patients that healthy people would have.

Following that same line of thought, we can do a little math to figure out what 60-80% of range would be for each level. We can use those results as a treatment goal during the meds titration process.

Since different labs use different ranges for their FreeT4 and FreeT3 tests, each of us needs to interpret our results based upon our lab's ranges.

For example, I use LabCorp and their ranges for FreeT4 are .82-1.76....for FreeT3, they are 2.0-4.4

Therefore, if a patient's lab uses the same ranges as mine, it would be appropriate to aim for a FreeT4 level of 1.4-1.6 and FreeT3 level of 3.4-3.8 during the meds titration process.

These values represent levels that are 60-80% of range - the types of levels healthy people have. (just as healthy people have TSH 1.0 ish)

Once those types of levels are reached, it's appropriate to ask the doctor to adjust our dose (up or down) to address any residual symptoms.




T4 is our storage hormone and the amount that is available for the body's use is measured by the FreeT4 test.

Our bodies convert our T4 storage hormone into T3, the active hormone. The FreeT3 test measures T3 that is available for the body's use.

Any other type of T4 or T3 test is either a calculation (vs actual measurement) or measures hormone that is bound to proteins and not usable by the body.

I think the part that confuses me is that apparently there is no "industry standard" (for lack of a better word). Why do we have to interpret it according to our lab's range? Why aren't the ranges the same regardless of the lab where the blood was drawn? I consider myself to be a fairly intelligient person so please forgive me if this is a stupid question!

Different labs use different testing kits and that's why the ranges are different.

I couldn't agree more about the challenges of interpreting our results within the context of different lab ranges.

That's why I recommend always using the same lab each time.(although LabCorp slightly changed their FreeT4 range within the past 5 years).