Follicle-stimulating hormone (FSH) is secreted by the basophil cells of the adenohypophysis in response to stimulation by hypothalamic gona-dotropin-releasing hormone (GnRH), which also is called luteinizing hormone-releasing hormone (LHRH) and follicle-stimulating hormone-releasing factor. FSH affects gonadal function in both men and women. In women, FSH promotes maturation of the graafian (germinal) follicle, causing estrogen secretion and allowing the ovum to mature. In men, FSH partially controls spermatogenesis, but the presence of testosterone also is necessary. GnRH secretion, which in turn stimulates FSH secretion, is stimulated by decreased estrogen and testosterone levels. Isolated FSH elevation also may occur when there is failure to produce spermatozoa, even though testosterone production is normal. FSH production is inhibited by rising estrogen and testosterone levels.

FSH levels are normally low during childhood but begin to rise as puberty approaches. Surges of FSH occur initially during sleep but, as puberty advances, daytime levels also rise. During child-bearing years, FSH levels in women vary according to the menstrual cycle. Decreased FSH levels after puberty are associated with male and female infertility. After the reproductive years, estrogen and testosterone levels decline, causing FSH levels to rise in response to normal feedback mechanisms. A 24-hour urine specimen also can be collected and tested for FSH.