The most important reproductive organ is the brain, where the control mechanisms reside. Follicle stimulating hormone (FSH), made deep inside the brain in the pituitary gland, is responsible for the development of ovarian follicles (the sacs within your ovaries that contain eggs).
Through an interchange of information between the ovary and the brain, the most mature eggs will ovulate while the less mature eggs will regress. This is why human beings only ovulate one egg at a time, unless they are taking fertility hormones.
When the brain detects that the ovary is struggling to make eggs, it drives the ovary harder and raises its FSH production. A healthy ovary responds to an elevated level of FSH by producing several mature eggs. When the ovary’s egg production is compromised it will produce only one egg, regardless of the level of FSH.
As a woman ages naturally, her FSH level will rise and reaches its maximum level in menopause when the egg supply is exhausted.
How Does High FSH Affect Fertility?
If you have an unusually high level of FSH at a young age, it may mean you have fewer eggs available to reach maturity. Since women with mild FSH elevation still menstruate regularly, the term is often referred to as diminished ovarian reserve (DOR). This is contrast to menopause when all eggs are exhausted. One sign of diminishing ovarian reserve may be a shortening of the menstrual cycle interval or decrease in menstrual flow.
Unlike other body parts, the ovary produces its lifetime supply of eggs before birth. Most of these eggs are stored in a hidden pool to prevent all the eggs from ovulating at once. The number of eggs hidden away is correlated with the FSH value on the second or third day of your cycle. A high FSH means that there are fewer eggs available.
FSH measurement, via blood collected from a vein, must happen on the 2nd or 3rd day of menses because it is most accurate when estrogen is at its lowest point. Elevated estrogen may artificially lower your FSH and give a false reading. Women who no longer get their cycles always have low estrogen and therefore FSH can be measured at any point.
Remember that FSH levels in your blood detects the health of the ovary and is not the cause of the problem. Strategies to lower FSH do not help the ovary recover eggs.
If you have elevated FSH levels and have not menstruated for more than one year you may be in menopause. When your ovaries fail at an earlier than expected age, it is called premature ovarian failure (POF). Since loss of ovarian function results in an absence of viable eggs, very few women with premature ovarian failure are able to get pregnant naturally.
Unfortunately, women who have high FSH levels often respond poorly to fertility medications or do not respond at all. They may also have a low chance of getting pregnant when they try in vitro fertilization to achieve a successful pregnancy. In a young patient with a modest FSH elevation, some of the eggs may still be normal.
Your reproductive endocrinologist will evaluate your ovarian reserve and determine which treatment is best for your situation.
Dr. Lawrence Grunfeld is a board-certified Reproductive Endocrinologist. He is a clinical associate professor of Obstetrics and Gynecology at the Mount Sinai School of Medicine and co-director of Reproductive Medicine Associates of New York. Dr. Grunfeld received his medical degree at Mount Sinai and his OB/GYN training at the Albert Einstein College of Medicine. He also completed fellowships in reproductive medicine at both the Albert Einstein Medical College and Yale-New Haven Hospital. Dr. Grunfeld served as Director of Fellowship Training at Mount Sinai Medical Center in New York, where he has been performing IVF procedures since 1986.