The FSH (serum) test measures follicle-stimulating hormone (FSH) levels in the blood, which is essential for reproductive function. In women, FSH regulates the menstrual cycle and egg development, while in men, it supports sperm production. This test is commonly used to assess fertility, evaluate ovarian function, and diagnose conditions like polycystic ovary syndrome (PCOS) or menopause, aiding in the management of hormonal imbalances.