Progesterone

Progesterone is a steroid hormone involved in the menstrual cycle, implantation, and pregnancy. It is synthesized from cholesterol and is produced mainly by the corpus luteum of the ovary after ovulation. During pregnancy, the placenta becomes a major source of progesterone. Smaller amounts are also produced in tissues such as the adrenal glands and testes .^1,2

During the luteal phase of the menstrual cycle, progesterone acts on the endometrium that has previously been stimulated by estrogen. It promotes transformation of the proliferative lining into a secretory endometrium prepared for possible implantation. If pregnancy does not occur, the corpus luteum regresses and progesterone concentrations fall. This hormone withdrawal contributes to breakdown and shedding of the endometrium during menstruation .^1,2

Progesterone exerts many of its effects by binding to intracellular progesterone receptors, principally the PR-A and PR-B isoforms. These receptors belong to the nuclear receptor family and regulate gene expression after activation by progesterone. Progesterone signaling affects reproductive tissues including the uterus and mammary gland and contributes to physiological changes involved in establishing and maintaining pregnancy .^2,3

Progesterone should be distinguished from progestins. Progesterone is a naturally occurring hormone with a specific molecular structure, whereas progestins are compounds, commonly synthetic and used clinically, that produce progesterone-like effects through progesterone receptors. Progestins can differ from progesterone in their receptor activity and other biological properties .³