When female mice can't make a protein called hepcidin, they end up with too much iron in their blood and body, and their red blood cell numbers go up — this suggests hepcidin normally keeps iron levels in check.

When the body is low on iron, the heart makes more hepcidin, which might help protect the heart from damage during iron deficiency.

In the heart, the way hepcidin responds to iron levels is different from how it works in the liver - when the body is low on iron, the heart makes more hepcidin protein even though the gene for it is less active.

Giving testosterone to mice makes more iron show up in their blood, even in mice that can't make a key iron-regulating hormone—so testosterone must be doing something else to free up iron.

When mice lose the ability to make hepcidin in their heart cells, their heart cells don't get enough iron, which causes their hearts to weaken and eventually fail, even though their overall body iron levels are normal.

When mice lose hepcidin only in their heart cells, their overall body iron levels and storage remain normal, showing that heart hepcidin doesn't affect the body's overall iron balance.