The iron lost to (normal) menstruation is
tiny.
This question is kinda like eGFR on a CMP.
Lemme bring you up to speed.
GFR is glomerular filtration rate, how quickly fluid is passing from your bloodstream into your kidneys. (Nearly all of this filtrate is reabsorbed, the rest becomes urine.) The "e" means "estimated." And a CMP is a comprehensive metabolic panel, a frequently-used set of blood tests.
GFR can be measured by injecting inulin into the blood, waiting, and measuring how much is left in the blood vs how much ends up in urine. This works because inulin (a non-toxic plant product) isn't reabsorbed by the kidneys. But, this test is invasive (two blood draws, an injection, and urine collection) and requires waiting.
It can be estimated with one sample by comparing creatinine (not reabsorbed) to urea (reabsorbed) and guessing how quickly the patient's body produces both. This guess depends on age, sex, and race. The lab order specifies age and sex, but not race, so the report usually gives both African and non-African estimates. Mixed-race? Lol-who-knows.
(Not only that, but eGFR doesn't give meaningful numbers for healthy kidneys, just a result of "healthy, probably.")
Medicine is a really inexact science. Sometimes a patient's sex is informative. Sometimes, it's misleading. People with intersex conditions, or who have started HRT, or both? There just isn't the data to say.
