Glands that regulate the function of the ovary and the testis are present in the brain. The hypothalamus produces gonadotrophin-releasing hormone (GnRH) which stimulates the pituitary to produce follicle stimulating hormone (FSH) and luteinising hormone (LH). Both FSH and LH are called gonadotrophins.
Both hormones stimulate the gonads (ovary and testis) to produce female and male hormones as well as orchestrate the production gametes (eggs and sperms).
FSH is measured in the first few days (2-5) of the menstrual cycle. FSH levels vary from cycle to cycle and it is the highest level that matters. That is why your doctor may repeat measuring FSH. In deed it should be measured yearly in women under infertility treatment. Normal FSH with high LH suggests polycystic ovarian disease. Low level of FSH, LH, and estradiol indicate a decrease of the production of these hormones by the pituitary gland.
High level of FSH points to the possibility of premature ovarian insufficiency that occur when the number of the eggs inside the ovaries is declining. Women under the age of 40 with high FSH and low Estradiol may have premature ovarian failure and usually experience early menopause. Premature ovarian failure is one of the most serious causes of absence of menses in reproductive age women. It can be due to chromosomal abnormalities, damage to the ovary by infection, tumor, surgery, or radiation. Autoimmune diseases can lead to body glands including the gonads to malfunction. In some women the cause of premature ovarian failure is not known. There is a possibility of spontaneous recovery in some women with resumption of menses and occurrence of pregnancy. Consultation and follow up with reproductive endocrinologist and infertility specialist is mandatory.