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follicule-stimulating hormone (FSH)

FSH Levels: Follicle-Stimulating Hormone Function in Fertility

What is the follicle-stimulating hormone (FSH)?

There is a small gland situated inside your head, which is known as the anterior pituitary gland. The follicle-stimulating hormone (FSH) is released by this gland in response to a hormone secreted by the hypothalamus known as the gonadotropin-releasing hormone. 

In menstruators, the hypothalamus senses the fall in hormone levels towards the end of the menstrual cycle. As a result, more gonadotropin-releasing hormone is produced, which stimulates an increase in the FSH and the luteinizing hormone (LH). 

What is its function in your body?

The ovaries contain follicles that house oocytes (premature eggs), which later on grow and become a part of a process leading to ovulation. The rise in the FSH levels towards the end of your cycle stimulates the growth of these follicles, which start releasing increasing amounts of estradiol and inhibin, two other hormones which play a part in ovulation.

As the hypothalamus and pituitary gland sense the increase in these two hormones, less FSH and gonadotrophin-releasing hormone are released. But, as the follicle grows, it keeps producing more estrogen. As the estrogen levels increase, FSH levels keep declining till the egg is mature enough to be released as ovulation. After the egg is released, the FSH levels start to rise again in preparation for the next cycle.

The estrogen loop is responsible for letting your body know when the egg is mature enough to be released. As the eggs mature, the FSH levels go down, which doesn’t happen unless the eggs grow enough to release the estrogen response. In this case, the FSH levels remain heightened.

How are its levels different from that of the luteinizing hormone (LH)?

The normal menstrual cycle has different phases, two of which are the follicular phase and the luteal phase. When the follicles are growing, they are competing to claim dominance till one emerges for ovulation. When the eggs are maturing, FSH levels begin to drop coinciding with the estrogen released by the eggs. When the dominant follicle emerges, the rest of them degrade and die as a response to the FSH drop.

Usually, the FSH level and LH levels share a peak, leading to ovulation. When diagnosing PCOS, tests reveal a ratio of 3:1 LH to FSH, which is a standard diagnosis of the disorder unless the imbalance can be explained due to other reasons.

How does FSH affect fertility in women?

FSH plays a vital role in the development of the follicles and ultimately the ovulation process. Abnormal levels of FSH affect fertility in women because ovulation is directly impacted by the hormone. Apart from this, FSH also affects the menstrual cycle and plays a role in your sex drive. 

When going for infertility treatment, the FSH levels are often measured to determine how fertile you are.

FSH test - what’s that?

As discussed above, your FSH levels can be measured to determine your fertility and for that, your doctor can prescribe a blood test at a certain time of your cycle to determine your levels. Alternatively, there is also a urine test available that can detect your FSH levels.

The levels can help you understand underlying conditions affecting your reproductive system and fertility rate.

What are the normal FSH levels in your body?

For menstruators, the normal level of FSH is somewhere between 4.7 to 21.5 mIU/mL to indicate good fertility rates. However, the normal result is not always indicative of the fact that you don’t have to worry about infertility issues. 

One study found that it is possible for a woman to have normal FSH levels even when they are well beyond the age when fertility problems usually start to arise.

What happens when the FSH levels are high in your body?

High FSH levels in your body can be indicative of the following:

  • Polycystic ovarian syndrome (PCOS)
  • Primary ovarian insufficiency (premature ovarian failure)
  • Menopause or perimenopause
  • When part of a woman’s X chromosome, or the entirety of it, is missing, it results in a chromosomal abnormality, like Turner’s syndrome
  • A reduction in the quality and quantity of eggs and embryos for fertilization, which can be a result of age

With higher levels of FSH in your body, your chances of becoming pregnant are affected. However, it does not indicate that you are infertile, but only that you may need reproductive assistance like IVF in order to conceive.

And, what happens when the FSH levels are low?

Usually, in combination with your LH levels, you may sometimes find that your FSH levels are abnormally low. This can be indicative of a number of things like:

  • A problem in the function of the pituitary gland or the hypothalamus
  • Lack of egg production in the ovaries
  • A tumour that may be disrupting the brain’s ability to produce FSH

When your body produces low levels of FSH, you may not develop fully at puberty or later on suffer from ovarian problems like ovarian failure, where follicles do not develop properly to be able to release an egg. This can cause fertility problems or even infertility in women and you need to speak to your doctor to see what can be done about it.

The bottom line

Follicle-stimulating hormone is a hormone produced by the anterior pituitary gland that regulates your reproductive function. During each menstrual cycle, FSH acts in tandem with the luteinizing hormone to drive follicular development inside your ovaries, culminating in ovulation.

FSH and LH, along with other hormones, are responsible for regular menstrual cycles. During fertility testing, some women have an FSH test to see how much of this hormone they have. FSH levels that are abnormally low or high might suggest a variety of factors that contribute to infertility, such as menopause or a diminished ovarian reserve.

If you get an abnormal FSH test result, your doctor will go through the results with you and explain what they mean. Various therapies, depending on the underlying source of the problem, may be able to assist in the rectification of this imbalance.

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