The big picture: LH (luteinizing hormone) is a hormone that stimulates the ovaries to ovulate and helps produce progesterone, a hormone that prepares the uterus for pregnancy. Measurements are useful when investigating when ovulation occurs (time you’re most likely to get pregnant), menstrual irregularities, fertility issues and confirming the start of menopause.

LH is a hormone made by your pituitary gland. It works closely with a similar hormone called FSH (follicle-stimulating hormone) to control the function of your reproductive organs.

LH levels change throughout the menstrual cycle, with a quick rise just before ovulation..

In weeks one to two of the cycle, LH is required to stimulate the ovarian follicles in the ovary to produce the female sex hormone, oestradiol.
Around day 14 of the cycle, a surge in LH levels causes the ovarian follicle to rupture and release a mature egg from the ovary (ovulation).
For the remainder of the cycle (weeks three to four), the remnants of the ovarian follicle form a corpus luteum. LH stimulates the corpus luteum to produce progesterone, which is required to support the early stages of pregnancy, if fertilization occurs.

Abnormally high LH can be an indication of infertility:

High levels of LH can indicate decreased sex steroid production from the ovaries.
Polycystic ovary syndrome is a common condition in women associated with high levels of LH and reduced fertility. In this condition, an imbalance between LH and FSH can stimulate inappropriate production of testosterone.
That said, as a woman ages and menopause approaches, ovarian function wanes and eventually ceases. As this occurs, LH and FSH levels rise.

Abnormally low LH values is also an indication of infertility. The lack of LH means that ovulation does not occur and menstrual periods may not occur regularly.