LH (Luteinizing Hormone)

The hormone that triggers the release of an egg (ovulation).

What is Luteinizing Hormone (LH)?

Luteinizing Hormone (LH) is a vital reproductive hormone produced by the anterior pituitary gland, a small, pea-sized structure located at the base of the brain.

While it is present in small amounts throughout your entire menstrual cycle, LH is famous for one very specific, dramatic event: The LH Surge. This surge is the biological trigger that causes your ovary to release a mature egg, an event known as ovulation.

The Role of LH in the Menstrual Cycle

LH acts in close partnership with Follicle-Stimulating Hormone (FSH) and estrogen to orchestrate the menstrual cycle. Here is how it works:

1. The Build-Up: During the first half of your cycle (the Follicular Phase), FSH stimulates ovarian follicles to grow. As the dominant follicle matures, it produces rapidly increasing amounts of estrogen.
2. The Tipping Point: For most of this phase, LH levels are low and steady. However, when the dominant follicle is fully mature, it pumps out a massive, peak amount of estrogen. The brain monitors this rising estrogen. When the estrogen hits a critical threshold and stays high for roughly 2 days, the brain responds by drastically altering its output.
3. The Surge: The pituitary gland dumps a massive amount of Luteinizing Hormone into the bloodstream in a very short period. This is the LH Surge.
4. The Climax (Ovulation): The LH surge travels to the ovary and acts like a chemical key. It causes the dominant follicle to undergo its final maturation steps, swell, weaken its outer wall, and finally rupture, releasing the egg into the fallopian tube. Ovulation occurs approximately 24 to 36 hours after the LH surge begins.
5. The Aftermath: After ovulation, LH has one more job. It is responsible for transforming the empty, ruptured follicle into the Corpus Luteum (hence the name luteinizing hormone). The Corpus Luteum then produces progesterone to support a potential pregnancy. LH levels quickly drop back down to baseline for the remainder of the cycle.

How to Test and Track LH

Because the LH surge is the most reliable predictor that ovulation is imminent, tracking LH is a cornerstone of trying to conceive (or trying to avoid pregnancy).

Ovulation Predictor Kits (OPKs)

The most common way to track LH is at home using Ovulation Predictor Kits. These are inexpensive urine test strips (similar to pregnancy tests) that detect the presence of LH in your urine.

• How They Work: You urinate on the strip daily as you approach your fertile window. The test has a control line and a test line.
• Reading the Result: Unlike a pregnancy test where any line means positive, an OPK is only positive if the test line is as dark as or darker than the control line. A faint line is a negative result, representing the low baseline LH always present in your system.
• Timing: Once you get a true positive OPK, you are generally in your most fertile window, and ovulation is likely to occur within the next 24 to 36 hours.

Blood Tests

If you are undergoing fertility treatments or diagnostics, a doctor may order a serum LH blood test.

• Testing LH on Day 3 of your cycle (alongside FSH and Estradiol) helps doctors assess your basic ovarian function and evaluate conditions like PCOS.
• In a normal Day 3 panel, the ratio of LH to FSH is usually about 1:1.

Abnormal LH Levels

LH levels outside the normal range can indicate underlying conditions impacting fertility.

High LH Levels

If LH is chronically high (and not just spiking for ovulation), it can indicate:

• Polycystic Ovary Syndrome (PCOS): Women with PCOS often have an LH-to-FSH ratio of 2:1 or even 3:1. High baseline LH causes the ovaries to produce excess testosterone, which halts follicle development and prevents ovulation.
• Premature Ovarian Insufficiency (POI) / Menopause: If the ovaries are failing and failing to produce estrogen, the brain frantically pumps out massive amounts of LH and FSH to try and stimulate them, resulting in sky-high numbers.

Low LH Levels

Abnormally low LH can prevent ovulation, a condition called anovulation. This is often caused by:

• Hypothalamic Amenorrhea: Severe stress, extreme weight loss, eating disorders, or over-exercising can cause the brain to shut down the reproductive system to conserve energy, halting LH production.
• Pituitary gland disorders.

The Bottom Line

Luteinizing Hormone is the crucial trigger-pull of the menstrual cycle. Taking the guesswork out of your cycle by tracking your LH surges with OPKs and logging those positive tests into the Bloom App provides a precise roadmap to your unique fertile window.