2,3 DPG (2,3-diphosphoglycerate)

It is a  three-carbon molecule, a isomer of the glycolytic path away intermediate 1,3-bisphosphoglyceric acid. It's produced via Embden–Meyerhof pathway and present in red blood cells (5 mmol/L).

Being a highly charged anion it binds to the β chains of deoxygenated hemoglobin. This favors oxygen unloading from oxygenated hemoglobin, resulting in right shift of Oxygen Dissociation Curve of hemoglobin. 

Acidosis inhibit glycolysis leading to fall in 2,3 DPG levels.

Thyroid hormones, Growth Hormone, and Androgens increase 2,3 DPG. In addition exercise increases 2,3,DPG in 60 min.