Giving testosterone to mice makes their developing red blood cells contain more hemoglobin, even in mice that can't make a key iron-regulating hormone — meaning testosterone helps use iron to make blood better, no matter what.

Giving testosterone to female mice makes their blood carry more oxygen by increasing certain blood cells, even when a key regulatory protein called hepcidin is missing or when the liver can't respond to testosterone—meaning testosterone might boost blood cell production on its own.

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 young men take a high dose of testosterone while eating very few calories, their body starts making more iron available to produce red blood cells, but their metabolism and how they burn food don’t change.

Even when mice can't use the usual testosterone signal in their liver, testosterone still lowers a protein called hepcidin—suggesting it’s using a different trick, maybe by boosting red blood cell production, to do this.

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.