The physiology of the reproductive system changes dramatically with the onset of major illness. The serum testosterone concentrations fall to pre-pubertal levels secondary to a decreased secretion of gonadotropins and a decreased Leydig cell response to luteinizing hormone. At the same time, the serum oestrogen concentration rises as the result of an increased rate of peripheral aromatization. The clinical consequences of these marked changes are not yet well understood. One line of evidence argues for the administration of anabolic steroids (derivatives of testosterone) to critically ill patients to improve their catabolic state. Another line of evidence in animal models suggests that testosterone may suppress the immune system and myocardial function in critical illness. No clinical trials of oestrogen administration to critically ill patients have been reported, although two animal studies suggest that oestrogen may have a positive effect on survival. This chapter reviews changes in the physiology of the reproductive system in major illness as well as current evidence regarding the clinical effects of androgens and oestrogens in critical illness and their potential therapeutic roles.
Gonads start developing as a common primordium (an organ in the earliest stage of development), in the form of gonadal ridges ,  and only later are differentiated to male or female sex organs . The presence of the SRY gene ,  located on the Y chromosome and encoding the testis determining factor , determines male sexual differentiation. In the absence of the SRY gene from the Y chromosome, the female sex (ovaries instead of testes) will develop. The development of the gonads is a part of the development of the urinary and reproductive organs .