Why Are LH and FSH Compared?
Understanding Why Fertility Specialists Look at These Hormones Together
"LH and FSH work as a team. Looking at one without the other is like trying to understand a conversation by hearing only one person speak."
Why This Matters
If you've had fertility blood tests, you may notice that your healthcare professional doesn't just look at your LH or FSH result individually.
Instead, they're often compared.
You might wonder:
Why do these hormones need to be compared?
What does the comparison tell my doctor?
Is one hormone more important than the other?
The answer is that LH and FSH have different jobs, but they work together throughout your menstrual cycle. Looking at both hormones helps healthcare professionals understand how well your brain and ovaries are communicating.
What Does FSH Do?
FSH (Follicle Stimulating Hormone) is responsible for starting the process.
At the beginning of each menstrual cycle, it encourages several follicles in the ovaries to begin growing.
Each follicle contains an immature egg.
You can think of FSH as the hormone that gets things moving.
What Does LH Do?
LH (Luteinising Hormone) takes over later in the cycle.
As one follicle becomes dominant and oestrogen levels rise, the brain releases a surge of LH.
This LH surge triggers ovulation, causing the mature egg to be released.
After ovulation, LH also helps support the production of progesterone.
Why Compare Them?
Looking at LH and FSH together can give important clues about how your reproductive hormones are working.
Your healthcare professional may use the results to help answer questions such as:
Is ovulation likely to be occurring?
Are the ovaries responding normally?
Could there be signs of a hormonal imbalance?
Are further investigations needed?
Neither hormone provides the full picture on its own.
Can the LH:FSH Ratio Help Diagnose PCOS?
Sometimes.
In the past, healthcare professionals often looked at the LH:FSH ratio when assessing for Polycystic Ovary Syndrome (PCOS).
Some people with PCOS have a higher LH level compared with FSH.
However, this ratio is not reliable enough to diagnose PCOS on its own.
Many people with PCOS have a normal LH:FSH ratio, while some people without PCOS may have an increased ratio.
Today, healthcare professionals diagnose PCOS by considering your symptoms, medical history, blood tests and often an ultrasound scan—not the LH:FSH ratio alone.
Why Are Other Hormones Also Important?
LH and FSH are only part of the story.
Your healthcare professional may also consider:
AMH – to estimate ovarian reserve.
Oestradiol – to understand ovarian activity.
Progesterone – to confirm whether ovulation has occurred.
Thyroid hormones – as thyroid problems can affect fertility.
Prolactin – as raised levels may interfere with ovulation.
Putting all these results together gives a much more complete understanding of your fertility.
Does an Abnormal LH or FSH Mean I Can't Get Pregnant?
No.
An unusual result simply suggests that further assessment may be helpful.
Many people with abnormal hormone levels go on to conceive naturally or with fertility treatment.
Your healthcare professional will always interpret your results alongside your age, symptoms, ultrasound findings and medical history.
My Story
During my fertility investigations, I remember feeling overwhelmed by the number of hormone tests.
Every result seemed important, but nobody explained how they fitted together.
Looking back, I realise they weren't looking for one "good" or "bad" number.
They were looking at how all the hormones interacted.
That understanding helped me stop seeing each blood test as a separate hurdle and start seeing them as pieces of the same puzzle.
Questions Worth Asking
If you've had LH and FSH blood tests, you may wish to ask:
Are my LH and FSH levels what you'd expect for my age?
What do they suggest about ovulation?
Do they indicate any hormonal imbalance?
Should I have additional hormone tests?
What do my ultrasound findings show?
What are the next steps?
I Wish Someone Had Explained...
I wish someone had explained that fertility hormones work as a team.
It's easy to focus on one result and assume it holds all the answers.
But fertility isn't like that.
FSH starts the process.
LH helps trigger ovulation.
Progesterone prepares the womb.
Each hormone has its own role, and it's the way they work together that gives your healthcare professional the clearest picture.
Key Takeaways
✔ FSH helps follicles grow at the start of your cycle.
✔ LH triggers ovulation later in the cycle.
✔ Comparing LH and FSH helps healthcare professionals understand how your reproductive hormones are working together.
✔ An abnormal LH:FSH ratio may provide a clue, but it cannot diagnose PCOS on its own.
✔ LH and FSH are always interpreted alongside other fertility tests and your medical history.