LH Hormone Guide: The Luteinizing Hormone, LH Surge, and Ovulation

What Is Luteinizing Hormone (LH)?

LH - you've probably seen it on ovulation test strips and fertility apps, but what actually is it? I explain LH to my clients all the time, and the lightbulb moment usually comes when they realise this one little hormone is basically the starting gun for ovulation. Once you understand how it works, a lot of other fertility pieces fall into place.

Luteinizing hormone (LH) is a gonadotropin - a reproductive hormone produced by the anterior pituitary gland (a pea-sized structure at the base of your brain). In women, LH has two primary jobs:

1. Triggering ovulation: The LH surge causes the dominant follicle in the ovary to rupture and release a mature egg. This is ovulation.
2. Supporting the corpus luteum: After ovulation, LH helps the ruptured follicle transform into the corpus luteum, which produces progesterone to support the uterine lining for potential implantation.

LH doesn't work alone - it works alongside FSH (follicle-stimulating hormone) in a carefully coordinated dance that drives your menstrual cycle. Understanding how LH behaves throughout your cycle is one of the most useful things you can learn for fertility tracking.

The LH Surge and Ovulation

The LH surge is one of the most significant hormonal events in your menstrual cycle, and for women trying to conceive, it's the single most important signal to pay attention to.

What Triggers the LH Surge?

As the dominant follicle in your ovary grows during the follicular phase, it produces increasing amounts of oestradiol (oestrogen). When oestradiol reaches a critical threshold - typically around 200 pg/mL sustained for about 50 hours - it triggers a positive feedback loop in the hypothalamus and pituitary gland. This causes a dramatic, rapid rise in LH - the LH surge.

Hoff et al. (1983) demonstrated that the LH surge is a remarkably consistent event: it begins approximately 34-36 hours before ovulation and lasts for 12-24 hours. The actual ovulation (release of the egg) occurs roughly 24-36 hours after the surge begins.

How Long After the LH Surge Do You Ovulate?

This is one of the most common questions I see, and the answer is important for timing intercourse:

• The LH surge begins - ovulation will occur in approximately 24-36 hours
• Peak LH - ovulation will occur in approximately 12-24 hours
• LH declining - ovulation has likely already occurred

Park et al. (2007) studied the precise timing and found that 90% of women ovulated between 24 and 36 hours after the LH surge onset. The most fertile days are the day of the LH surge and the day after - which aligns with the known survival times of sperm (up to 5 days) and egg (12-24 hours).

What Does the LH Surge Feel Like?

Honestly? Most women don't feel anything specific. Some women report mild cramping on one side around ovulation (Mittelschmerz), a change in cervical mucus (becoming clear, stretchy, and slippery - like egg white), or a slight increase in libido. But these signs are subtle and inconsistent - which is why LH testing is so valuable as an objective measure.

Normal LH Levels Throughout Your Cycle

LH levels are not static - they fluctuate significantly throughout the menstrual cycle. Understanding what's "normal" at each stage helps you interpret your own results.

Follicular Phase (Days 1-13)

LH levels are low during most of the follicular phase, typically ranging from 2.4-12.6 mIU/mL. The pituitary releases small amounts of LH (pulsatile secretion) that work alongside FSH to stimulate follicle growth.

Ovulation (Around Day 14)

The LH surge causes levels to spike dramatically - typically reaching 14-95 mIU/mL or higher. The exact peak varies between women and between cycles, which is why OPKs use a threshold rather than an absolute number.

Luteal Phase (Days 15-28)

After ovulation, LH levels drop back to baseline, typically ranging from 1.0-11.4 mIU/mL. LH continues to play a supporting role by maintaining the corpus luteum and its progesterone production.

These ranges come from Stricker et al. (2006) and are based on standard laboratory reference values. Individual labs may have slightly different reference ranges, so always compare your results to the reference range provided on your report.

LH Levels and Fertility Problems

Abnormal LH levels can provide clues about underlying fertility issues. Here's what different patterns may indicate:

High LH Levels

Elevated baseline LH levels (particularly during the follicular phase) can be a sign of:

• PCOS (Polycystic Ovary Syndrome): Women with PCOS often have elevated LH levels with a high LH:FSH ratio (greater than 2:1). This means LH is disproportionately high compared to FSH, which disrupts normal follicle development and ovulation (Jonard and Dewailly, 2004).
• Ovarian insufficiency (diminished ovarian reserve): As the ovaries produce less oestrogen, the pituitary gland increases LH (and FSH) production in an attempt to stimulate the ovaries. This is why high LH in the early follicular phase may suggest declining ovarian function.
• Premature ovarian failure: Persistently high LH and FSH levels (particularly FSH >25 mIU/mL on day 3) can indicate premature ovarian insufficiency.

Low LH Levels

Low or absent LH levels may indicate:

• Hypothalamic amenorrhoea: When the hypothalamus doesn't produce enough GnRH (gonadotropin-releasing hormone), LH levels remain low and ovulation doesn't occur. This is common in women who are underweight, over-exercising, or under severe stress.
• Pituitary disorders: Conditions affecting the pituitary gland (tumours, Sheehan syndrome, radiation damage) can reduce LH production.
• Kallmann syndrome: A rare genetic condition characterised by absent or delayed puberty and low LH levels.

No LH Surge

If you're testing with OPKs and consistently not detecting an LH surge, it may mean:

• You're not ovulating (anovulation)
• Your surge is very short and you're missing it (try testing twice daily)
• You have PCOS, which can cause elevated baseline LH that doesn't show a clear surge
• Your urine is too diluted (test with concentrated urine, usually mid-morning)

Using OPKs (Ovulation Predictor Kits)

OPKs are the most accessible way to detect your LH surge at home. They work by measuring LH concentration in your urine, similar to how a pregnancy test measures hCG.

How to Use OPKs Effectively

• When to start testing: Begin testing around day 10 of a 28-day cycle (or adjust based on your cycle length - start about 4 days before you expect to ovulate)
• Test twice daily: The LH surge can be brief (as short as 10-12 hours in some women). Testing once daily risks missing it. I recommend testing at around 11am and again at 5pm.
• Reduce fluid intake: Avoid drinking large amounts of water for 2-4 hours before testing to ensure concentrated urine.
• Don't use first morning urine: Unlike pregnancy tests, OPKs work better with mid-morning urine because LH is typically produced in the early morning and takes several hours to appear in urine.
• Read within the time window: Results read after the recommended time (usually 5-10 minutes) may be unreliable.

Interpreting OPK Results

• Test line is lighter than control line: Negative - no LH surge detected yet. Keep testing.
• Test line is as dark or darker than control line: Positive - LH surge detected. Ovulation will likely occur within 24-36 hours.
• Consistently dark test lines: May indicate PCOS or perimenopause. The elevated baseline LH can cause OPKs to show a "positive" reading without a true surge.

OPK Limitations

While OPKs are useful, they're not perfect. Miller and Soules (1996) found that OPKs correctly identified the LH surge in about 80-90% of cycles, but had limitations:

• They predict ovulation timing (24-36 hours) but don't confirm ovulation actually occurred
• Women with PCOS may get false positives due to chronically elevated LH
• Very short surges may be missed if testing frequency is too low
• They don't account for the variation in time between surge onset and actual ovulation

For confirmation that ovulation actually occurred, combine OPKs with basal body temperature (BBT) charting or progesterone blood tests on day 21 (7 days post-expected ovulation).

LH vs. FSH: Understanding the Ratio

LH and FSH are often measured together, and their ratio provides useful diagnostic information:

• Normal ratio (LH:FSH approximately 1:1): Indicates balanced hormonal function
• Elevated ratio (>2:1 LH:FSH): Classic PCOS pattern - Jonard and Dewailly (2004) found this ratio was present in approximately 60-70% of PCOS patients
• Low LH with normal/low FSH: May suggest hypothalamic dysfunction or pituitary issues
• Both LH and FSH elevated: May suggest diminished ovarian reserve or menopause

LH in Men

While most fertility hormone discussions focus on women, LH is equally important in men. In the male body, LH stimulates the Leydig cells in the testes to produce testosterone. Without adequate LH, testosterone production falls, which can affect sperm production and libido.

McLachlan et al. (2002) reviewed the role of LH in male fertility and found that both very low and very elevated LH levels could impair sperm production. Men with low LH may benefit from LH-stimulating medications (such as hCG injections), while elevated LH with low testosterone may indicate primary testicular failure.

LH Blood Tests: What to Ask Your Doctor

If you're having trouble conceiving or have irregular cycles, your doctor may order LH blood tests. Here's what to know:

• Day 3 LH: Drawn on cycle day 3 (with FSH and oestradiol) to assess baseline hormonal function. Normal range: 2.4-12.6 mIU/mL.
• Day 21 progesterone: While this tests progesterone (not LH), a low day-21 progesterone suggests you didn't ovulate, which may be related to LH problems.
• Random LH: A single LH level outside of the surge window isn't very informative. If possible, ask for timed blood draws around expected ovulation.

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Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. If you are concerned about your hormone levels or fertility, please consult a qualified healthcare professional for personalised testing and interpretation.