LH stands for luteinizing hormone, a chemical messenger produced by a small gland at the base of your brain called the pituitary. Its main job is telling your reproductive organs to do their work: in women, it triggers ovulation; in men, it signals the testes to produce testosterone. When your doctor orders an LH blood test, they’re checking whether this signaling system is functioning normally.
What LH Does in Your Body
The pituitary gland releases LH in response to signals from the hypothalamus, a control center in the brain. In women, LH plays a quiet background role for most of the menstrual cycle, then surges dramatically in the middle of the cycle to trigger the release of an egg from the ovary. That surge happens about one to one and a half days before ovulation, which is why home ovulation kits work by detecting LH in urine.
In men, LH works more steadily. It binds to specialized cells in the testes and kicks off a chain reaction that converts cholesterol into testosterone. Without adequate LH, testosterone production drops, which can affect energy, muscle mass, bone density, and fertility.
Normal LH Ranges
LH levels aren’t static. They shift based on age, sex, and where a woman is in her menstrual cycle. The ranges below, measured in international units per milliliter (IU/mL), give a general picture of what’s typical.
For women of reproductive age:
- Follicular phase (the first half of the cycle, before ovulation): 1.68 to 15 IU/mL
- Midcycle peak (the ovulation surge): 21.9 to 56.6 IU/mL
- Luteal phase (the second half of the cycle, after ovulation): 0.61 to 16.3 IU/mL
Postmenopausal women typically have consistently elevated LH because the ovaries are no longer producing estrogen, and the pituitary keeps sending stronger signals trying to compensate. Men’s LH levels tend to stay in a narrower range throughout the day, and children before puberty normally have very low levels.
Keep in mind that reference ranges vary slightly between labs. Your results should always be read against the specific range printed on your lab report.
Why Your Doctor May Order This Test
An LH blood test isn’t usually ordered on its own. It’s part of a broader look at reproductive health, often paired with a test for follicle-stimulating hormone (FSH), which works alongside LH. Common reasons for testing include irregular or absent periods, difficulty getting pregnant, suspected early or delayed puberty in children, and low testosterone symptoms in men.
For women trying to conceive, the test can confirm whether ovulation is happening. A clear LH surge about two weeks after the start of a period is a reliable sign that ovulation is on track. If LH stays high throughout the month instead of spiking once, that pattern suggests the ovaries may not be responding properly.
In children, LH levels help determine whether early puberty is driven by the brain’s signaling system (central precocious puberty) or by another source. Doctors sometimes use a stimulation test, giving a synthetic hormone and measuring how LH responds, to make that distinction.
What High LH Levels May Mean
Elevated LH is the pituitary’s way of shouting louder when the reproductive organs aren’t listening. When the ovaries or testes produce too little estrogen or testosterone, the brain ramps up LH output to try to correct the problem. Persistently high LH can point to several conditions depending on your age and sex.
In women of childbearing age, high LH levels may indicate primary ovarian insufficiency, where the ovaries stop functioning normally before age 40. Genetic conditions like Turner syndrome can also cause elevated LH because the ovaries never developed properly. Thyroid diseases and adrenal gland disorders are less common but recognized causes as well.
Polycystic ovary syndrome (PCOS) is a specific case worth noting. Women with PCOS often have mildly elevated LH relative to FSH. An LH-to-FSH ratio greater than 2 generally points toward PCOS, though it’s not the sole diagnostic criterion. PCOS is one of the most common causes of irregular periods and infertility, so this ratio can be an important clue.
In men, high LH paired with low testosterone suggests the testes themselves aren’t producing enough hormone despite receiving the right signals. Klinefelter syndrome, a genetic condition where men carry an extra X chromosome, is one example. Damage to the testes from infection, injury, surgery, chemotherapy, or radiation can produce the same pattern.
What Low LH Levels May Mean
Low LH tells a different story. Instead of the reproductive organs being unresponsive, the problem is upstream: the pituitary or hypothalamus isn’t sending enough signal in the first place. This is sometimes called hypogonadotropic hypogonadism, a term that essentially means “low sex hormones because the brain isn’t asking for them.”
Pituitary tumors, head injuries, and certain genetic conditions can suppress LH production. So can extreme stress, significant weight loss, and eating disorders, all of which can shut down the brain’s reproductive signaling as a kind of survival mechanism. In men, low LH leads to low testosterone and reduced sperm production. In women, it can stop ovulation and menstrual cycles entirely.
Timing and Accuracy
Because LH fluctuates so much during the menstrual cycle, the timing of the blood draw matters. Your doctor will typically specify which day of your cycle to have the test done, depending on what they’re looking for. A test on day 3 of your cycle checks baseline function, while a test midcycle looks for the ovulation surge.
Hormonal medications, including birth control pills, can significantly alter LH levels and may need to be paused before testing. Let your provider know about any medications or supplements you’re taking.
A blood test is more precise than at-home urine ovulation kits, which detect LH reliably about 90% of the time when used correctly. Lab-based blood tests are automated, use standardized methods, and give an exact number rather than a positive-or-negative result. That precision matters when your doctor needs to diagnose a condition rather than simply predict ovulation.
LH Results in Context
A single LH number rarely tells the full story. Doctors interpret it alongside FSH, estrogen or testosterone levels, and your symptoms. For example, high LH with high FSH and low estrogen strongly suggests the ovaries are failing. High LH with a normal FSH and signs like acne, excess hair growth, and irregular periods points more toward PCOS. Low LH with low FSH and low sex hormones shifts attention to the pituitary gland or hypothalamus.
If your results come back outside the normal range, your provider will likely order follow-up tests or imaging to narrow down the cause. The LH test is a starting point, not a final answer, but it’s one of the most useful tools for understanding how your reproductive system is functioning.