Modern Thyroid Clinic Health Directory

SLOT: Full Definition

What LH measures

LH — luteinizing hormone — is a pituitary hormone that, alongside Fsh, orchestrates the menstrual cycle. The mid-cycle LH surge is the trigger that releases the egg from the dominant follicle (ovulation). After ovulation, LH supports the corpus luteum's production of progesterone. The LH blood test measures circulating LH levels and is most informative when interpreted with cycle phase, FSH, and clinical context.

At Modern Thyroid Clinic, LH is part of evaluating cycle irregularity, suspected Polycystic Ovary Syndrome, confirming ovulation, and clarifying causes of amenorrhea (absent periods).

Why it matters

LH provides crucial information when looking at hormonal patterns:

The LH-to-FSH ratio helps distinguish PCOS (often LH:FSH >2:1) from hypothalamic causes of irregular cycles
An LH surge confirms imminent ovulation — the basis of ovulation predictor kits
Persistently elevated LH in older women is consistent with menopause
Persistently low LH suggests pituitary or hypothalamic dysfunction

LH does not work in isolation. The same number can mean very different things depending on cycle day, age, and the rest of the hormonal picture.

Reference range vs. functional/optimal range

Conventional reference range:

Follicular phase: roughly 2–10 mIU/mL
Mid-cycle surge: transient peak, often 25–80 mIU/mL
Luteal phase: 1–14 mIU/mL
Postmenopause: typically 14–60+ mIU/mL

Functional/optimal target: in cycling women, the early follicular LH should generally sit at or below FSH (LH:FSH ratio ≤1). A persistent ratio greater than 2 in the early follicular phase is one supportive marker of PCOS, especially when paired with elevated androgens and oligo-ovulation.

In perimenopause and menopause, LH rises persistently as the negative feedback from ovarian estrogen weakens. In hypothalamic amenorrhea, LH falls below normal — often disproportionately to FSH.

What abnormal results suggest

Elevated LH (with elevated LH:FSH ratio) in a younger cycling woman is often associated with PCOS, particularly when paired with insulin resistance, androgen excess, or oligo-ovulation. Persistently elevated LH at high absolute levels alongside low estradiol confirms menopause.

Low LH suggests hypothalamic or pituitary suppression — common causes include hypothalamic amenorrhea (undereating, overtraining, chronic stress), high prolactin, pituitary tumors, certain medications, and severe systemic illness.

LH is also affected by hormonal contraception (suppressed) and recent hormone therapy. Like FSH, LH should be interpreted only when cycle timing, contraceptive status, and the full hormone panel — including estradiol, FSH, Prolactin, thyroid markers, and androgens — are considered together. A clinician familiar with reproductive endocrinology will look at the pattern, not the single number.

Common symptoms

Common questions

What does an LH:FSH ratio greater than 2 mean?

An early follicular (day 3) LH:FSH ratio greater than 2 is often described as supportive of PCOS, particularly when other PCOS features are present (irregular cycles, androgen excess, polycystic ovaries on ultrasound, insulin resistance). However, the ratio is not diagnostic on its own — many women with PCOS have a normal ratio, and not all women with an elevated ratio have PCOS. PCOS is diagnosed clinically using established criteria. The ratio is one piece of supporting information, not a stand-alone test, and should be interpreted alongside the rest of the workup.

Can I use LH to confirm I'm ovulating?

Yes — that is exactly what at-home ovulation predictor kits detect. They measure urinary LH and turn positive when the LH surge begins, typically 24–36 hours before ovulation. A positive surge is highly suggestive of impending ovulation, but it is not absolute confirmation that ovulation occurred. The most reliable confirmation is a luteal-phase progesterone level (5–10 ng/mL or higher) drawn about a week after the surge. Combining ovulation kits with a luteal progesterone draw gives a much clearer picture than either alone.

Why is my LH low when my periods are absent?

Low LH with absent periods often points to hypothalamic amenorrhea: the brain has down-regulated ovarian signaling in response to a perceived stressor. The most common drivers in young women are undereating, overtraining, very low body fat, chronic psychological stress, and sometimes occult illness. High prolactin, pituitary disease, and severe systemic illness can also produce low LH. The treatment is rarely hormone replacement first — it is identifying and resolving the underlying signal that is suppressing the axis. Evaluation should include FSH, estradiol, prolactin, and thyroid labs to clarify the cause.

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This content is for educational purposes only and is not medical advice. Consult a licensed clinician for diagnosis and treatment. Content on this page does not create a doctor-patient relationship with Modern Thyroid Clinic.