I have used Ferrous bisglycinate chelate (Vitacost) and Carbonyl iron (Sundown Naturals). These were the only two kinds I tried that did not have intestinal side effects. With any iron supplement, it is recommended to take it with vitamin C for greater absorption, and some say with lysine as well. Do not take at the same time as calcium or thyroid meds-- space apart by 4 hours or more. However, if your thyroid levels are not optimal you may find it continually difficult to raise ferritin.
I'm a little conflicted about this one. I feel that the ferritin levels are affecting the thyroid. I feel that if I am at an optimal level my thyroid operates better.
Sure, it could go both ways. I am just saying that to adjust one without paying attention to the other may make it more difficult to reach a good level.