Iron!

The world could learn from the body's recycling abilities!
Once an organism has "filled the tank" with iron during childhood, it essentially needs no more during it's life unless it bleeds. Of course, in humans means women need refills, but men do not.
So, if a male shows up iron deficient it means he's bleeding, usually from the GI tract. Just one of the many clues we use to assess homeostasis!
On the other hand, genetic errors of protein synthesis can cause the body to "mishandle" iron, which, in excess, is toxic to our cells. A number of conditions can lead to iron overload (which is what is pictured in the liver microphoto above--iron staining blue), which can be fatal.
One of the reasons medicine and biology are so endlessly fascinating is this warp and woof of homeostasis--following the clues to diagnosis, resolution and hopefully better health!
Easy does it on the iron--an elegant sufficiency, as my Granny used to say, is all that's needed!