simply defination as Barb says:
Free T3 - (FT3) = is the most active form of thyroid hormone
T3- TT3 - or Total T3 = is stored, bound up hormone with proteins that do nothing but set and release with no value for the body
Free T4- (FT4) = is the free- able - active conversion hormone that "should" release into the active T3.
T4-TT4 - or Total T4 - is stored and sits too.
You are finding more that doctors will utilize the Free T4 test - with the TSH - but most often - for some learned reason - they will test that FT4 - with the Total or TT3 and that is very uncomparable and difficult to find out where you should be at.
If you read about the medication you are on. Many find that the med will cause hair loss. What is the reason and how should you fix it is the issue? - Many things can be off balance - with - or without - when that med is used.
You may be not converting right with the T4 med.
You may have adrenal issues?
You may have be supplement deficient like B's - calcium - or magnesium
You may be in an estrogen dominant imbalance that is pumping off testosterone.
So many things are connecting with thyroid and digging into it with labs and knowledge is best to overcome the issues.
Plain T3 is usually total. You need to get the FREE T3, because that is the one that tells you how much of the hormone is actually available for use by the body. When you test only total, you have no way of knowing how much of that is bound up protein and can't be used.
Same goes for T4 -- you need the FREE T4, as that's the one that shows how much of the T4 is actually available for conversion to T3.
Test for TOTAL hormones are pretty much useless and obsolete now, though some doctors still order them.
Ask for FT3,FT4,these are the free hormones in your blood that your body can get use of !