What is FSH?

FSH stands for follicle-stimulating hormone. It is made by the pituitary, a small gland at the base of the brain. Its job is to send the ovaries a “get to work” message: it triggers eggs to develop and prepares them for ovulation (MedlinePlus, FSH test).

This system works like a thermostat. When the ovaries make enough hormones, the brain lowers FSH. When the ovaries respond less, the brain turns the message up and FSH rises. That’s why FSH rises as menopause approaches. MedlinePlus writes that high FSH is a normal sign of perimenopause and menopause.

What is AMH?

AMH stands for anti-Müllerian hormone. It is released by the small follicles, or egg sacs, in the ovary. The AMH level measured in the blood gives an idea of how many eggs are left in the ovaries. This is called ovarian reserve (MedlinePlus, AMH test).

AMH is mostly used in fertility assessments. But MedlinePlus clearly states two limits: an AMH test doesn’t show the health of the eggs, and it can’t predict whether you can get pregnant.

Why does AMH fall?

The main reason AMH falls is age. You are born with a certain number of eggs, and that number goes down over the years. As the number of eggs decreases, so does the AMH released by the follicles that surround them (MedlinePlus, AMH test).

STRAW+10 staging shows that the fall in AMH starts quite early. In the first sub-stage of the late reproductive phase, while periods are still regular and FSH hasn’t changed, AMH and antral follicle count are already low (Harlow et al., 2012). So low AMH can also be seen in someone whose periods are regular.

How do FSH and AMH change during the menopause transition?

STRAW+10 divides the path to menopause into stages based on your bleeding pattern. Along this path, hormones change like this (Harlow et al., 2012):

  • Late reproductive phase: AMH falls first. Then cycles start to get shorter, and FSH measured at the start of the cycle rises and becomes variable.
  • Early transition: A difference of 7 days or more between the lengths of consecutive cycles becomes persistent. FSH is high but variable.
  • Late transition: Gaps of 60 days or longer without a period begin. In this stage, an FSH above 25 IU/L on a random measurement is typical.
  • Early postmenopause: For about 2 years after the last period, FSH keeps rising and estradiol keeps falling; then both settle.

This picture isn’t a straight line. The word “variable” matters: during the transition, FSH can be high one month and normal the next.

Why can a single FSH or AMH measurement be misleading?

There are three reasons.

First, hormones fluctuate. FSH changes during the month. That’s why women who still have periods may be asked to take the test on a specific day of their cycle (MedlinePlus, FSH test). In perimenopause, this fluctuation becomes even more pronounced.

Second, the main criterion that defines the stages is your bleeding pattern. The STRAW+10 working group defines hormone measurements only as “supportive” criteria and stresses that they should not be considered necessary for a diagnosis (Harlow et al., 2012). The NHS also writes that there is no routine test for perimenopause and menopause, and that a doctor usually assesses you based on your age and symptoms (NHS, What are menopause and perimenopause).

Third, AMH doesn’t show when menopause will happen. Low AMH can point to menopause approaching, or to early menopause. But MedlinePlus clearly states that an AMH test can’t predict when you’ll reach menopause (MedlinePlus, AMH test).

What should your FSH level be? Why can’t one number be given?

This is the most searched question, but the answer isn’t a single number.

The 25 IU/L value in STRAW+10 was measured against a specific international standard. The working group says clinicians need to compare this value with their own laboratory’s method. For AMH, because measurement methods aren’t standardized, no numerical cut-off was given at all (Harlow et al., 2012).

That’s why the “reference range” on your test result can vary from lab to lab. You need to interpret your result not against a number you saw online, but against the range of the lab that ran the test, together with your clinician. Your clinician interprets the result alongside your age, your symptoms and your medical history (MedlinePlus, FSH test).

What do high or low FSH levels mean?

High FSH means the brain is sending the ovaries a stronger “get to work” message. In your forties, the most common reason for this is the menopause transition. But MedlinePlus notes that the FSH test is ordered not only for menopause but also to investigate irregular or stopped periods and other ovarian problems. So a high value doesn’t always mean “you’ve reached menopause”; the interpretation depends on your age and symptoms.

Low FSH raises a different question. According to MedlinePlus, low FSH in women can be linked to rapid weight loss, very low body weight or excessive exercise. Clinicians often interpret the result together with other hormones such as LH (luteinizing hormone) (MedlinePlus, FSH test).

If a high or low result has worried you, rather than trying to interpret it on your own, the best thing is to ask your clinician: “What does this result mean in my situation?”

When are FSH and AMH tests ordered?

According to MedlinePlus, at 45 and over an FSH test is usually not needed to recognize menopause; if there are no other factors, the assessment is based on symptoms (MedlinePlus, FSH test). We cover guideline recommendations on FSH over 45 separately in the menopause test article.

Situations where hormone testing is more likely to come up:

  • You’re younger than 45 and have menopausal symptoms: The NHS says a doctor may order a blood test in this situation (NHS, Early menopause).
  • You’re younger than 40 and your periods have stopped: The question here may be premature ovarian insufficiency (POI). STRAW+10 gives the POI criterion as 4 months without a period under age 40 (Harlow et al., 2012). The Menopause Society recommends that anyone younger than 40 who has missed three or more periods see a healthcare professional.
  • You want to become pregnant: AMH may be ordered to get an idea of egg reserve (MedlinePlus, AMH test).

You can find the causes of periods stopping at a young age, and what needs monitoring, in the early menopause article.

FSH and AMH compared

Hormone What it does During the menopause transition What a single measurement doesn’t show
FSH Released by the pituitary, stimulates the ovaries Rises but fluctuates Whether you’ve reached menopause
AMH Released by the follicles, reflects reserve Falls early When menopause will happen
Estradiol The ovaries’ main estrogen Fluctuates first, then falls Which stage you’re in

What should you ask when discussing your results with your clinician?

  • What was this test ordered to find out or rule out?
  • Which reference range are we using to interpret my result?
  • Could a medication or contraceptive method I’m using affect the result?
  • Does the test need to be repeated?

It’s very understandable to feel anxious while waiting for a result, especially if you want to become pregnant. Remembering that a single number doesn’t define you can help you go into the appointment feeling calmer.