FSH is the pituitary hormone that controls the gonad in both men (testicles) and women (ovaries). We say that it controls because by increasing its value, it stimulates the ovary/testes and once these are stimulated, they exert a retrograde action to decrease FSH and thus lower the gonadal stimulus.
FSH in the female promotes follicular development to achieve follicle growth. In men, it promotes spermiogenesis, i.e. the production of germ cells: spermatozoa.
The follicular cycle in women is shorter, lasting only 14 days until ovulation occurs, as the germ cell - the egg cell - is already present in the follicle. This egg cell only has to grow to ensure an adequate corpus luteum to support early embryonic development in the woman's body.
In the case of the male, his germ cells are not present, they have to be produced and therefore the cycle is longer (2-3 months) and, in addition, nothing has to be prepared to receive the embryo.
In the male, FSH tells us about spermiogenesis, indicating testicular function. In this case, the sample can be taken at any time.
FSH in women indicates follicular development and should always be taken at the beginning of the menstrual cycle from the third or fifth day after the start of the period.