What is the difference between T3 and FT3 and T4 and FT4? I am 5 weeks out from Total Thyroidectomy due to nodules which, thankfully, were benign. I'm trying to understand the meaning of the various lab tests.
Hi and welcome: you asked the million dollar question and not so easy to answer. The best I can recommend is to do some research by books and net as it is fairly involved. In a nutshell, the free Ts (FT4, FT3) are hormones that your body uses to function properly. If these are off, for some people, by just a bit, that person gets hyper symptoms or hypo symtpoms. For propper testing of thyroid function a good doctor tests FT3, FT4, TSH and also goes by symptoms of the patient. Here is a good book that I would recommend: Living Well with Hypothyroidism by Mary Shomon. There is a lot of general info. about the thyroid as well.
I'll take a stab at it.
The thyroid hormones are T4 and T3. (There are others, like T1 and T2, but those are minute quantities which are never measured.) Most of each hormone is bound to a protein which transports it through the bloodstream till it reaches the receptor sites on the cells. The protein then must be stripped from the hormone to "free" it before it can enter the cells. The hormone in its bound form is T4 (or T3), and unbound is called free T4 or FT4 (free T3 - FT3).
When the total T4 or total T3 is measured by testing, the result is the sum of both bound and free hormone, and there is no way to know how much of it is free and therefore usable by the cells. That's why the best thyroid doctors consider the free tests the more revealing of the two kinds.
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Explain T3, T4, FT3, FT4 
