Downregulation of carbonic anhydrases (CA) CA1, CA2 and CA4 was a common feature of thyroid tumours, which may be related to lower erythrocyte abundance since we found other erythrocyte proteins to be downregulated, including bisphosphoglycerate mutase; spectrin beta chain and haemoglobin subunits HBA, HBB and HBD. Here, CA4 is linked to thyroid tumor.