What the LH Cycle Is and Why It Matters
The LH cycle refers to the natural fluctuation of luteinizing hormone across the menstrual cycle. LH is produced by the pituitary gland and plays a central role in triggering ovulation and supporting early pregnancy. For people tracking fertility, understanding the LH cycle offers a window into when the body is preparing to release an egg. The hormone does not stay at one level; it rises and falls in a predictable pattern that correlates with the phases of the menstrual cycle.
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In a typical 28-day cycle, the LH cycle begins at a low baseline, then climbs sharply in the middle of the cycle. This surge is the signal that ovulation is imminent. After ovulation, LH drops back toward baseline and stays relatively low until the next cycle begins. The pattern repeats, which makes it measurable and trackable.
How the LH Cycle Connects to Ovulation
The LH surge is the direct trigger for ovulation. Without a sufficient surge, the mature follicle may not release the egg. This is why the LH cycle is one of the most watched fertility markers.
Here is a simplified breakdown of what happens during a typical LH cycle:
- Early follicular phase: LH is low, but the pituitary is preparing to stimulate follicle growth.
- Late follicular phase: Rising estrogen from the maturing follicle signals the pituitary to increase LH production.
- LH surge: LH spikes sharply, usually over 24 to 48 hours, and triggers the release of the egg.
- Post-ovulatory phase: LH falls quickly and remains low while the corpus luteum forms and produces progesterone.
Ovulation predictor kits work by detecting this surge in urine, giving a practical way to follow the LH cycle at home.
Normal LH Levels Across the LH Cycle
LH levels change meaningfully depending on where someone is in the cycle. The following table shows typical ranges, though individual variation is common and lab reference ranges can differ.
| Cycle Phase | Approximate LH Range (IU/L) | Context |
|---|---|---|
| Early follicular | 1.9 – 12.5 | Baseline; low compared to mid-cycle |
| Pre-ovulatory | 8.7 – 76.3 | Rising as the LH surge begins |
| LH surge | 14.0 – 95.6 | Peak; triggers ovulation |
| Luteal phase | 0.6 – 16.6 | Falls after ovulation; stays low |
| Pregnancy | Very low or undetectable | Placenta takes over hormone support |
Because the LH cycle is so dynamic, a single blood test is rarely as useful as tracking the pattern over multiple days. One elevated reading does not necessarily mean a problem; context matters.
What Disrupts the LH Cycle
Several factors can alter the shape or timing of the LH cycle. Stress, significant weight change, intense exercise, thyroid disorders, and polycystic ovary syndrome are among the more common influences.
In PCOS, LH levels may be elevated at baseline, and the ratio of LH to FSH can be skewed. This can interfere with regular follicle development and ovulation, making the LH cycle less predictable. Thyroid dysfunction, both hypo- and hyperthyroidism, can suppress or exaggerate the LH surge. High prolactin (hyperprolactinemia) can also dampen the LH cycle, which is why doctors sometimes check multiple hormones when fertility questions arise.
Tracking the LH Cycle for Fertility
People trying to conceive often track the LH cycle using ovulation predictor kits, basal body temperature charting, or fertility tracking apps. OPKs detect the LH surge in urine and typically turn positive one to two days before ovulation.
Combining LH tracking with other signs, such as changes in cervical mucus or a sustained rise in basal body temperature after ovulation, can give a clearer picture of where someone is in the LH cycle. For those with irregular cycles, serial blood tests ordered by a clinician can map the LH cycle more precisely.
The LH cycle is not a single number but a pattern. Recognizing that pattern helps people understand their fertility window, plan or avoid pregnancy, and spot potential hormonal issues early.