Iron / Transferrin: In iron deficiency anemia, serum iron is low while transferrin (and total iron-binding capacity) is elevated as a compensatory response, since the liver upregulates transferrin production to try to capture whatever iron is available. Transferrin saturation is characteristically reduced.

Vitamin D deficiency: This commonly coexists with iron deficiency anemia because they share overlapping causes - malabsorption states (e.g., celiac disease, inflammatory bowel disease, bariatric surgery), poor dietary intake, and chronic disease. Vitamin D also plays a role in iron regulation by suppressing hepcidin and inflammatory cytokines; low vitamin D can worsen functional iron deficiency. Studies confirm a bidirectional relationship where low vitamin D status increases the risk of iron deficiency anemia, and this association is most pronounced in women (per Mendelian randomization and cross-sectional data).

Serotonin: Iron is a required cofactor for tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis. When iron stores are depleted, serotonin production falls, which is thought to contribute to some of the mood disturbances, fatigue, and even restless-legs-type symptoms seen in iron-deficient patients.