Ferr is a universal intracellular protein that stores iron and releases it in a controlled fashion. The protein is produced by almost all living organisms, including algae, bacteria, higher plants, and animals. In humans, it acts as a buffer against iron deficiency and iron overload. Ferritin is found in most tissues as a cytosolic protein, but small amounts are secreted into the serum functioning as an iron carrier. Plasma ferritin is also an indirect marker of the total amount of iron stored in the body. Hence serum ferritin is used as a diagnostic test for iron-deficiency anemia.

25（OH）-D is a prehormone that is produced in the liver by hydroxylation of vitamin D3 (cholecalciferol) by the enzyme cholecalciferol 25-hydroxylase. Physicians worldwide measure this metabolite to determine a patient's vitamin D status.