LH (Luteinizing Hormone) Blood Test

What Does This Test Measure?

Luteinising hormone is released by the pituitary gland and acts directly on the gonads. In women, a sharp mid-cycle surge of LH is what triggers ovulation, and the remnant follicle it leaves behind, the corpus luteum, produces progesterone. In men, LH acts on the Leydig cells of the testis to drive testosterone production.

It works alongside FSH, and the two are released from the same pituitary cells under the same hypothalamic signal. Sex hormones feed back to suppress both, so when the gonads fail, LH rises; when the pituitary fails, LH falls.

LH is released in pulses roughly every 60 to 90 minutes, and the amplitude varies through the day. A single measurement captures one point on a fluctuating curve, which is why borderline results are often repeated.

Why Your Doctor Ordered This Test

In women, LH is measured to investigate irregular or absent periods, infertility, and suspected polycystic ovary syndrome, where the ratio to FSH has traditionally been used as supporting evidence. It is also tracked to time ovulation in fertility treatment, and home ovulation predictor kits detect the LH surge in urine.

In men, it is ordered alongside testosterone whenever testosterone is low, because it determines whether the fault lies in the testes or the pituitary, a distinction that changes management entirely. In children it is central to the assessment of early or delayed puberty.

What High LH Means

A high LH with low sex hormones indicates primary gonadal failure. In women this is the postmenopausal pattern, or premature ovarian insufficiency when it appears before 40. In men it points to testicular failure from Klinefelter syndrome, previous mumps orchitis, chemotherapy, radiotherapy, undescended testes or injury. In this setting the pituitary is working correctly and shouting at a gonad that cannot respond.

A high or high-normal LH with normal or raised sex hormones in women, particularly with a raised LH-to-FSH ratio and clinical features such as irregular cycles, acne or excess hair growth, is suggestive of polycystic ovary syndrome. The ratio is supportive rather than diagnostic, since a substantial minority of women with PCOS have a normal ratio.

A brief, very high LH in a woman may simply be the ovulatory surge, which is a normal finding rather than an abnormality.

What Low LH Means

Low LH alongside low testosterone or low estradiol indicates hypogonadotropic hypogonadism; the pituitary signal is absent. Functional causes dominate: significant weight loss, low body fat, eating disorders, intensive endurance training, chronic illness, poor sleep and severe stress all suppress the pulse generator, and the axis generally recovers when the cause resolves.

Medication is a major and frequently undeclared cause. Testosterone therapy and anabolic steroids suppress LH profoundly, shutting down natural testosterone production and sperm formation. Opioids do the same, as do high-dose corticosteroids. A raised prolactin, whether from a pituitary tumour or from antipsychotic medication, also suppresses LH.

Structural pituitary disease (tumour, surgery, radiotherapy, head injury or infiltration) accounts for the remainder. Kallmann syndrome is a congenital form, classically accompanied by a reduced sense of smell.

How to Prepare and What Affects the Result

Fasting is not required. In women the cycle day must be recorded; baseline testing is normally on day 2 to 5, while detecting the ovulatory surge requires daily sampling around mid-cycle. In men a morning sample is preferred, since it pairs with the morning testosterone peak.

Hormonal contraception suppresses LH and usually needs to be stopped before meaningful testing. Testosterone, anabolic steroids, opioids and corticosteroids all lower results and must be declared. Biotin supplements interfere with several immunoassays. Because LH is pulsatile, a single borderline value is often repeated, and in some settings pooled samples taken 20 minutes apart are used.

Common Misinterpretations

The commonest error is reading LH without the corresponding sex hormone. Low LH with low testosterone and high LH with low testosterone mean opposite things, pituitary versus testicular, and lead to entirely different investigations.

The second is over-relying on the LH-to-FSH ratio to diagnose PCOS. Current diagnostic criteria do not require it, many women with PCOS have a normal ratio, and the ratio is distorted by oral contraceptives.

The third is mistaking the normal ovulatory surge for pathology. A very high LH drawn near mid-cycle may be entirely physiological, which is why the cycle day is essential context.

Frequently Asked Questions

What does a high LH with low testosterone mean in men?

It means the pituitary is working but the testes are not responding, primary testicular failure. The pituitary detects low testosterone and raises LH to compensate, but the testes cannot answer. Causes include Klinefelter syndrome, previous mumps infection, chemotherapy, radiotherapy or injury. This differs fundamentally from low LH with low testosterone, which points to the pituitary instead.

Is the LH to FSH ratio enough to diagnose PCOS?

No. A raised ratio supports the diagnosis but is neither required nor sufficient. A significant proportion of women with PCOS have a normal ratio, and oral contraceptives distort it. Diagnosis rests on a combination of irregular ovulation, clinical or biochemical androgen excess, and ovarian appearance on ultrasound, with other causes excluded.

How do ovulation predictor kits relate to this test?

They detect the same hormone in urine. The mid-cycle LH surge precedes ovulation by roughly 24 to 36 hours, so a positive kit indicates ovulation is imminent. A blood LH gives a precise number but is a single snapshot of a brief surge, which is why home testing across several days is often more practical for timing.

Does taking testosterone lower my LH?

Substantially. External testosterone signals the pituitary that levels are adequate, so LH output falls or stops. Without LH the testes stop producing their own testosterone and sperm production declines, often to zero. This is a common and reversible-but-slow cause of infertility, and recovery after stopping may take many months.

Why might my doctor repeat a borderline LH?

Because LH is released in pulses every 60 to 90 minutes rather than at a steady level. A single sample catches one point on a fluctuating curve, so a borderline value may reflect timing rather than a genuine abnormality. Repeating the test, sometimes with samples pooled 20 minutes apart, gives a more representative picture.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your LH result: