If you weigh the pan and figure out how much iron it loses over a year, you could compare that to your dietary iron intake needs. If it's not over, don't worry

If you weigh the pan and figure out how much iron it loses over a year, you could compare that to your dietary iron intake needs. If it's not over, don't worry about it. If it's over, then you have to estimate how much of that iron is going down the drain during scrubbing vs how much is going into your food. My gut feeling is that, since cast iron pans seem to last quite a long time, they're probably not shedding much iron into your food, and the amount of iron the body can handle is pretty significant, on the order of tens of milligrams per day. Throw some Wheaties in a blender until they're dust, then take a magnet to the resulting pile. If we're not hearing about iron toxicity from breakfast cereals, I'm not inclined to worry about the pans.