Typically one has more iron circulating in the blood than what one has stored as ferritin in the liver and bone marrow. If your donating blood that ratio is even lower. Assume one has iron serum levels of 100 ug/dl and ferritin of 50 ng/ml. In one whole blood donation of 1 pint you could see the iron drop to 70 ug/dl or a drop of 30%. Ferritin could drop from 50 ng/ml to 20 ng/dl for a for a drop of about 60%. Being at 20 ng/dl places an individual well below the range.
How fast one depletes iron and ferritin depends of what their levels are to begin with and then how often they have donated blood. Many people on TRT have low iron and low ferritin. Low ferritin impacts the thyroid function, cortisol, hair loss, and a host of other concerns. I've read that a ferritin level below 80 starts impacting the thyroid. It inhibits T4 from converting to T3 and increase Reverse T3. Ferritin at a level of 50 or less one can experience hair loss and from what I have read it will not start to regrow until the ferritin level hits 80 (this is if low ferritin is the reason for the hair loss). One last thought......you can have normal iron levels and have low ferritin because you lose your stored iron (ferritin) before you see the circulating iron start falling.