6-keto Prostaglandin F1α-d4 (6-keto PGF1α-d4) contains four deuterium atoms at the 3, 3', 4, and 4' positions. It is intended for use as an internal standard for the quantification of PGF1α by GC- or LC-mass spectrometry.6-keto prostaglandin F1α (6-keto PGF1α) is the inactive, non-enzymatic hydrolysis product of PGI2. 6-keto PGF1α serves as a useful marker of PGI2 biosynthesis in vivo. When [3H]-PGI2 is injected into healthy human males, 6.6% of the radioactivity is recovered from urine as [3H]-6-keto PGF1α.