This week LetsGetChecked discusses follicle stimulating hormone and your fertility.

Follicle Stimulating Hormone (FSH) 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 level.

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What Is Follicle Stimulating Hormone?

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. FSH is responsible for stimulating the growth of egg-bearing follicles in your ovaries. It is one of two gonadotropins, so called because it stimulates the gonads. LH 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, reaching their peak prior to ovulation. It is best to measure FSH on day three of your period via a blood sample. This is called the baseline (basal) level. This provides the most accurate predictions of ovarian reserve. FSH is often measured along with the LH level and estradiol.

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 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, FSH levels are measured, which correlates inversely with the number of eggs that you have “on reserve.”

Women with a poor ovarian reserve are said to have reached their “oopause". The "oopause" stands for "Poor Ovarian Response", women most commonly reach the "oopause" at the age of 51.

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?

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 trigger the release of eggs from the ovaries, so the body produces a higher amount of FSH in an effort to prompt ovulation. A high FSH means the egg number is reduced, sometimes to levels so low that pregnancy is not possible.

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 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 sometimes taken by women with polycystic ovaries.

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 oestrogens 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,even if the FSH is normal, it cannot be assumed that ovarian reserve is normal.

A normal FSH and estradiol level 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.

Does A Normal Follicle Stimulating Hormone Level Mean You Are Healthy?

Not necessarily, just because your FSH level is normal does not mean your eggs are healthy. 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 level checked at the same time.

Learn More About Follicle Stimulating Hormone With Dr. Dominic Rowley

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Written by Hannah Kingston | Approved by Medical Director Dominic Rowley