It depends on the lab that did the testing and which value was tested. I know with Antech, the range for a T4 (not a free T4) test was 0.8 to 4.0. That only applies to Antech though, as Idexx and other labs will have their own normal ranges. Also, the upper end of the range is only normal in younger cats. Senior cats are often suspected of being hyperthyroid if their T4 is above 2.5 (again, that's just the value for Antech and it varies with other labs). I would highly recommend that you ask your vet for a copy of the results. My kitty Spot was hyperthyroid, and I found it very useful to track his results myself.
With Spot, we had him on medication for a while before I was able to afford radioiodine therapy. If I could have done so, I would have had the radioiodine treatment done very early--his T4 was already very high (at 10) when I found him. It's pretty common for vets to recommend a trial run with medication before doing radioiodine therapy. High T4 levels can suppress kidney failure on blood tests--the high thyroid hormone levels increase blood pressure and make the kidneys work harder. As a result, they are filtering more so the kidney values that typically indicate kidney failure are low. In that situation, if the thyroid values are brought back down to normal, less blood flows through the kidneys and if the cat is suffering from renal disease, it shows up in the blood tests and urinalysis. This is often referred to as unmasking kidney failure. It's not that the medication caused the kidney problems, it just keeps the kidneys from overworking so that you can see their true state. Since kidney disease is fairly common in senior cats, it's not a bad idea to put the cat on medication for a few weeks (to bring the T4 level to the normal range) and then check kidney function.
Keep in mind that slight kidney impairment does not always rule out the possibility of radioiodine treatment. Having high thyroid levels ultimately taxes the organs, including the heart, the liver, and the kidneys. Keeping the thyroid level high makes the kidney values look good, but doing so may also damage the kidneys over time. It's a balancing act. I knew of a cat who (as of a few years ago) was still alive four years after radioiodine. He had renal disease before his treatment, but it was mild, and his owner was pleased that she had gone ahead with the treatment.
I would talk to your vet and to Radiocat about whether it is best in your kitty's situation to do a trial run with medication. The trial run usually only takes a couple of weeks.