Eric, you do keep it interesting with your labs!
Yes, something is happening, as you have went from elevated TSH to pretty darn good! Notice also that your FT4 has gone down substantially, as it appears to be converting nicely to T3. Anything new you're taking besides the isocort? It's all just speculation at this point, but at least it's going the right direction.
Your FT4 is now at 25% of the lab value, and your FT3 is: 31.5%. Your ratio on RT3:FT3 is 16:1. You would hope to see this 20:1 or better. This IMO indicates that you are experiencing some pooling with your FT3. It's not severe, and maybe with everything else it's on the way up. It would have been good to have compared this to the previous labs, but you can go forward. As I've mentioned in other posts, pooling takes place when areas like cortisol, iron, B12 and D3 (and others) are off.
On your Isocort, you could take it just like your circadian profile, where breaking up the dosage with the largest being in the morning, then taper it down at noon and night. Licorice Root is another option that can be taken similarly. Include plenty of Vitamin C, B12, D3, and chelated magnesium. If you continue this regiment, I would run labs in another 4 weeks and compare.
You ultimately want to get your FT4 and FT3 in the 50% to 80% area of the range values, and again with RT3 >20:1. At this point, you obviously need more thyroid hormone, but just keep in mind the pooling aspect and getting that area improved. Do some research on it and you can get more information on this. If your doctor wants to jump you back on NDT, I can only suggest just going REAL slow with it, and again run the same labs next month. If your RT3 improves, and everything is staying stable, you can maybe start looking upward and onward ...
And yes, I agree with your doctor that iodine could more than likely be a major culprit here, especially seeing the huge deviation of T4 to T3 conversion. Things are going in the right direction!