FSH, or follicle-stimulating hormone, is known by many similar names in Chinese, but in English it has only one: Follicle-Stimulating Hormone (FSH). Together with luteinizing hormone (LH), it belongs to the group called gonadotropins. FSH's primary responsibility-exactly as its name suggests-is to stimulate follicular development. If we compare the ovaries to a fruit tree, FSH acts like the essential nutrients that help the fruits grow. In In other words, FSH is indispensable for follicle growth.

FSH and Ovarian Reserve

Before Anti-Müllerian Hormone (AMH) became widely used, FSH was the main indicator for assessing ovarian reserve. Today, although AMH is considered more reliable, FSH remains an important hormone to evaluate.

As menopause approaches, FSH levels naturally rise. This occurs because egg numbers decline with age, and the body needs to work harder to stimulate follicle development.

FSH should never be interpreted alone. It must be evaluated together with estradiol (E2)-and ideally with AMH, which provides the clearest picture of ovarian reserve.

However, FSH becomes a helpful supportive signal when paired with AMH and E2.

Crucially, low egg count is a consequence of elevated FSH, not its cause. The idea that reducing FSH increases fertility is a prevalent misunderstanding. Lowering FSH does not replenish eggs; in fact, nothing can increase the number of eggs as one ages.

How the Menstrual Cycle Affects FSH

Using the menstrual cycle as a point of reference:

▸ Day 1 (beginning of period): FSH is at its lowest

▸ FSH increases once menstruation ends, promoting the growth of follicles and the maturation of eggs

▸ Follicles start to produce estrogen (E2) as they develop

▸ The pituitary gland receives a signal to cease producing FSH from rising estrogen

▸ Ovulation is then triggered by other hormones

FSH is usually measured on Day 3 of the cycle, when readings best reflect baseline ovarian function, due to its large fluctuations.

Interpreting FSH Values: Low, Normal, and High

Recall that FSH is just one component of the ovarian reserve. It has to be interpreted using AMH and E2.

01 - Low FSH (< 3.85 mIU/mL on Day 3)

▸Normal FSH levels range from 3.85 to 8.78 mIU/mL

▸ Generally speaking, low FSH by itself does not signal an issue

02 - Slightly High or High FSH

▸ 8.78–15 mIU/mL: Slightly high

▸ >15 mIU/mL: High

Ovarian reserve is irreversible. Once egg numbers decline, they cannot increase.

Summary

▸ FSH promotes follicular growth, but fluctuates widely both within and between cycles

▸ High FSH should not cause panic-test AMH first to understand the true situation

▸ FSH should always be interpreted alongside estradiol (E2) and AMH, never alone

▸ High FSH is a result of low egg count, not the cause

▸ FSH is a valuable indicator, but AMH remains the most reliable marker of ovarian reserve