PCOS and Hyperinsulinemia

Dr. Jason Fung
5 min readNov 8, 2018

Normal Follicular Development

The most iconic feature of PCOS and what the syndrome is named for — is the presence of the multiple cysts on the ovary. Many women have a few cysts on their ovary, but the sheer number of cysts distinguishes this syndrome from virtually all others. Almost no other human disease causes polycystic ovaries. These cysts develop from a group of cells known as follicles.

Normal ovaries are endowed at birth with a finite pool of primordial follicles, which have the potential to develop into a mature egg. Primordial follicles remain quiescent for many years until recruited for growth into a primary follicle during the menstrual cycle. From birth to menarche, all primordial follicles remain dormant. Follicle numbers gradually decrease with age as used follicles are never replaced. When the follicles are fully depleted, women undergo menopause and can no longer have children.

At the start of each menstrual cycle, a few primordial follicles grow into primary and then secondary follicles. Only one of these follicles is selected to become the dominant follicle and the rest simply shrivel away (involute) and are reabsorbed by the body. The dominant follicle alone continues to grow. At ovulation, in response to a surge in luteinizing hormone (LH), the egg is expelled into the respective fallopian tube which carries it to the uterus. The rest of the dominant follicle becomes the corpus luteum, which secretes estrogen and progesterone to support the hoped-for pregnancy.

Primary follicles are generally detectable by ultrasound when they reach 2mm in size. The key growth signal for the primary follicle is testosterone. Insulin is important, too, but acts indirectly by increasing testosterone. The dominant follicle typically measures 22–24 mm.

While the follicle is growing, the lining of the uterus thickens in preparation for egg since it must supply all the necessary nutrients to await arrival of the sperm. If the sperm makes contact and fertilization occurs (yeah, baby), then the uterus continues to increase its growth to sustain the growing fetus. This is the missed period which often signals a normal pregnancy. If the sperm (working late at the office) never arrives, then the egg cannot wait forever. The lining of the uterus is shed, along with the discarded egg…

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Dr. Jason Fung

Nephrologist. New York Times best selling author. Interest in type 2 diabetes reversal and intermittent fasting. Founder www.TheFastingMethod.com.