Along with the follicle stimulating hormone, LH increases estrogen levels produced by the follicle cells in the ovaries in women and stimulates the production of testosterone in the testicles of men. In women, it is primarily responsible for ovulation: a spike of LH in the blood—called the LH surge—triggers ovulation. LH also plays a role in egg maturation and the menstrual cycle. 

The LH blood test measures the blood levels of LH, which is produced by the pituitary gland. This information can be used to help determine if problems with the menstrual cycle are related to this hormone. Similarly, women who are having a hard time conceiving should measure their LH levels to see if abnormal hormone levels are affecting ovulation, fertility and conception. Men who are experiencing abnormal levels of testosterone may also find this test helpful, as LH is linked to testosterone production.

LH is usually tested in conjunction with the follicle stimulating hormone because they are both produced by the pituitary gland and have similar functions. Abnormal levels of either can indicate developmental or reproductive problems. It is rare to have a problem with one and not the other, and both should be tested, especially when hormone replacement therapy is desired.