Taking iron supplements, my ferritin is very low (5), my tranferrin and iron are normal. My TIBC and % transferrin saturation are high. Cause?

Partially treated. Ferritin reflects body storage iron, while serum iron and transferrin saturation reflect current iron circulating in plasma. When you take oral iron supplement, the amount of iron in circulation (absorbed from gut on its way to storage sites) is transiently high. Your data suggest you have iron deficiency, partially treated, with blood tests taken within a day or two of your most recent iron pill.
Low ferretin. ferritin is an indicator of the iron stores of the body. low ferritin means the iron stores are depleted due to blood loss..most common blood loss from#1. menses in females who are menstruating Or #2 from GI.(gastro intestinal tract.