FSH (Follicle Stimulating Hormone) stimulates the growth of the egg-bearing follicles in your ovaries. FSH levels vary during the menstrual cycle and are best measured at its peak or baseline (basal) level.
What is FSH ?
FSH is an acronym which stands for Follicle Stimulating Hormone. Hormones are chemicals in your body that are made in one place and are used in another. As the name suggests, FSH is responsible for stimulating the growth of the egg-bearing follicles in your ovaries. It is one of the two gonadotropins (so called because it stimulates the gonads), the other being its partner, LH, which stands for luteinising hormone. LH and FSH are secreted by the pituitary gland, a small pea-shaped gland below the base of your brain.
FSH levels vary during the menstrual cycle and will peak prior to ovulation. This is why the level of FSH in your blood is best measured on Day 3 , and this is called the baseline ( basal) level. FSH blood tests are generally performed on the second or third day of your menstrual cycle, when they provide the most accurate predictions of ovarian reserve. The FSH is often measured along with the LH level; and estradiol ( E2)
Ovarian Reserve and Follicle Stimulating Hormone
Your ovarian reserve refers to the number of eggs that you have available for fertilization. A high ovarian reserve usually indicates a good number of viable eggs present in your ovaries. A low ovarian reserve may indicate that you have fewer available eggs. In order to test ovarian reserve, we measure your FSH level, which correlates inversely with the number of eggs that you have "on reserve" in your ovaries - your ovarian reserve. Women with a poor ovarian reserve are said to have reached their oopause. You can read more about this at Click here. Women with poor ovarian reserve will have high FSH levels. Some women find it difficult to understand why FSH levels are high in women with poor quality eggs. Intuitively, more is better, so higher levels should mean better eggs, shouldn't it ? As one patient asked me, " If FSH stands for Follicle Stimulating Hormone, and I have high levels of FSH, then doesn't that mean that I have the ability to stimulate lots of follicles ? A high FSH should mean that I should have lots of eggs ! "
Let's look at the basic biology. If the ovary has many eggs, the FSH in a woman's blood is low because the body doesn't need to produce much FSH to induce normal ovulation. However, if the egg number is low, the body needs to work harder to produce ovulation, so it increases the amount of FSH in an effort to push the ovaries. A high FSH means the egg number is reduced, sometimes to levels so low that pregnancy is not possible.
What is a good level? Well that depends on each individual lab. For most Fertility centers, anything over 12 mIU/ml is considered poor . In fact, some Fertility centers will not give fertility treatment to those over 12 because the odds of pregnancy become low.
Interpreting Levels of Follicle Stimulating Hormone
A normal FSH level is usually between 3 mIU/ml - 10 mIU/ml. Levels of more than 12 mIU/ml are worrisome and suggest impaired ovarian reserve. Levels of more than 25 confirm ovarian failure and are found in menopausal women. FSH levels can also be artificially raised by ovulation inducing drugs such as clomiphene citrate ( clomid).
Low levels of FSH ( less than 2 mIU/ml) are found in a condition called hypogonadotropic hypogonadism. Women on birth control pills as well as pregnant women also have low levels of FSH.
The FSH level is best interpreted in conjunction with your estradiol level. Estradiol is one of the estrogens produced by the ovaries. Estradiol levels above 75 pg/ml on Day 3 may indicate a poor ovarian reserve. In some women , a high baseline estradiol level ( because of poor ovarian reserve) can artificially suppress the FSH level, so that it appears to be normal, thus misleading the doctor into believing that the ovarian reserve is normal. This is why it's a good idea to measure the estradiol level when checking the FSH level on Day 3. If the estradiol level is high, then even if the FSH is normal, we cannot assume that ovarian reserve is normal.
A normal FSH and estradiol level probably indicate that you have a good ovarian reserve.
It's also useful to check your FSH:LH ratio. A normal FSH:LH ratio is 1. However, if your FSH level is much higher than your LH level, then this suggests poor ovarian reserve. This can be confirmed by checking your AMH level ; and by doing a clomiphene citrate challenge test.
Elevated FSH levels may suggest impaired ovarian reserve and may imply that you might want consider using donor eggs or donor embryos.
Does a normal FSH level mean your ovarian reserve is fine ? No ! Just because your FSH level is normal does not mean your eggs are OK ! This is because a high estradiol level can artificially suppress a high FSH level into a normal range, giving you false reassurance that all is well. This is why, when you measure your FSH levels, you should also get your estradiol ( E2) level checked at the same time. A high Day 3 estradiol level is worrisome.
Sadly, some doctors will "treat" a high FSH level by giving you birth control pills or estrogen tablets. While this will temporarily suppress your high FSH levels, it will not help you grow more eggs - and will just waste even more time and give you false hope!