midwest1

I found some answers to your questions at the site of the National Assn of Clinical Biologists. This is a PDF file; if you can't read it on your computer, you can download the Word document from the site. The relevant information is found on pages 8 and 9 of the PDF file. ~
http://www.nacb.org/lmpg/thyroid/2_thyroid.pdf

Anyway... what is the lab range for the FT4? Where your result falls within the range is important, because what's "normal" is proportional. The NACB report says that an abnormal FT4 result in a hospitalized patient should be confirmed by a T (total) T4 measurement. If both TT4 and FT4 are abnormal (in the same direction) a thyroid condition may be present. When TT4 and FT4 don't agree, the FT4 abnormality is unlikely due to thyroid dysfunction and more likely a result of the illness or medications.

The main gist of the NACB information agrees with what your doctor has told you. Severe non-thyroidal illness skews thyroid results. TSH often elevates, and either or both FT4 and TT4 can look abnormal as well.

It seems reasonable to take a wait and see approach; but as you know, the neck radiation has put you at high risk for hypoT, so it's very wise to keep an eye on things and test regularly.