Ferritin is a hollow globular protein of mass 474 kDa and comprising 24 subunits.  Typically it has internal and external diameters of about 8 and 12 nm, respectively. The nature of these subunits varies by class of organism:
* In vertebrates, the subunits are of two types, light (L) and heavy (H), which have apparent molecular mass of 19 kDa and 21 kDa, respectively; their sequences are homologous (about 50% identical).
* Amphibians have an additional ("M") type of ferritin.
* Plants and bacteria have a single ferritin; it most closely resembles the vertebrate H-type.
* In the gastropods of the genus Lymnaea, two types have been recovered, from somatic cells and the yolk, respectively (see below).
* In the pearl oyster Pinctada fucata, an additional subunit resembling Lymnaea soma ferritin is associated with shell formation.
* In the parasite Schistosoma, two types are present: one in males, the other in females.
All the aforementioned ferritins are similar, in terms of their primary sequence, with the vertebrate H-type.  In E. coli, a 20% similarity to human H-ferritin is observed.
Some ferritin complexes in vertebrates are hetero-oligomers of two highly related gene products with slightly different physiological properties. The ratio of the two homologous proteins in the complex depends on the relative expression levels of the two genes.
Inside the ferritin shell, iron ions form crystallites together with phosphate and hydroxide ions. The resulting particle is similar to ferrihydrite. Each ferritin complex can store about 4500 iron (Fe) ions.
A human mitochondrial ferritin, MtF, was found to express as a pro-protein. When a mitochondrion takes it up, it processes it into a mature protein similar to the ferritins found in the cytoplasm, which it assembles to form functional ferritin shells. Unlike other human ferritins, it appears to have no introns in its genetic code. An X-ray diffraction study has revealed that its diameter is 1.70 angstroms (0.17 nm), it contains 182 residues, and is 67% helical. The mitochondrial ferritin's Ramachandran plot shows its structure to be mainly alpha helical with a low prevalence of beta sheets.