Hormones are chemical messengers secreted by glands that travel through the bloodstream to regulate distant organs and tissues. They coordinate complex bodily functions, from metabolism and mood to growth and reproduction. The sex hormones govern the development of sexual characteristics and maintain reproductive health. These hormones fall into two main classes: androgens and estrogens. All biological sexes produce both, but in drastically different ratios, which dictates the primary sexual characteristics.
Primary Hormones and Production Sites
The three primary sex hormones are testosterone, estrogen, and progesterone. Testosterone is the principal androgen, promoting male characteristics. Its most potent form is dihydrotestosterone, converted from testosterone. The testes are the primary site of testosterone production in individuals assigned male at birth, though the adrenal glands contribute a small amount.
Estrogen is a collective term for several related hormones, with estradiol being the most potent form during the reproductive years. In individuals assigned female at birth, the ovaries are the major source of estrogen and progesterone. Progesterone is a steroid hormone that works closely with estrogen, particularly in the reproductive system.
In all individuals, the adrenal glands are a secondary source for small quantities of androgens and estrogens. Peripheral tissues, such as fat tissue, also convert one hormone type into another, for example, converting testosterone into estradiol via the enzyme aromatase. This peripheral conversion ensures that both androgens and estrogens are present in every body.
Defining Roles in Puberty and Development
Rising hormone levels during puberty drive the development of secondary sexual characteristics. This process is initiated by brain signals that stimulate the ovaries and testes to increase hormone output. In developing individuals assigned male at birth, androgens, predominantly testosterone, cause the vocal cords to lengthen, deepening the voice.
Testosterone promotes the development of facial hair, pubic hair growth, and stimulates the growth of the testes and penis. It also contributes to the rapid increase in muscle mass and bone growth characteristic of the male growth spurt, initiating changes necessary for fertility.
In developing individuals assigned female at birth, the surge of estrogen, particularly estradiol, initiates breast development (thelarche). Estrogen also causes the widening of the pelvis and hips, leads to a female pattern of fat distribution, and stimulates the growth of the uterus and vagina. The culmination of these changes is the onset of the menstrual cycle (menarche). Progesterone prepares the uterine lining, establishing the cyclic pattern of the mature reproductive system.
Ongoing Maintenance in Adult Physiology
These hormones maintain functions in adults, impacting systems beyond reproduction. In adult males, testosterone maintains muscle mass, bone mineral density, and red blood cell production. It also influences fat distribution, promoting lower visceral fat accumulation compared to individuals with lower levels.
A portion of circulating testosterone is converted into estradiol, which is necessary for healthy adult male physiology. This estrogen is important for maintaining bone health, preventing bone loss, and supporting healthy cardiovascular function in males. Low testosterone levels are associated with reduced libido and lower energy levels.
In adult females, estrogen protects the skeletal system by actively slowing the rate of bone resorption, thereby maintaining bone density. Estrogen also supports cardiovascular health by maintaining the elasticity of blood vessels and influencing cholesterol levels. Progesterone plays a mood-stabilizing role, and both estrogen and testosterone contribute to maintaining libido, energy, and cognitive function. Testosterone in females is a precursor to estrogen production and supports lean body mass and bone strength.
Hormonal Control and Common Fluctuations
Sex hormone production is regulated by the Hypothalamic-Pituitary-Gonadal (HPG) axis, a feedback loop involving the brain and the gonads. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These gonadotropins travel to the testes or ovaries, signaling them to produce testosterone, estrogen, and progesterone.
High levels of these sex hormones then feed back to the hypothalamus and pituitary gland, slowing the production of GnRH, LH, and FSH. This negative feedback system ensures hormone concentrations remain within a functional range. Disruptions to this axis can cause health issues.
Naturally occurring fluctuations are common throughout life. In females, the monthly menstrual cycle is defined by the rise and fall of estrogen and progesterone in response to LH and FSH. Age-related decline is notable during menopause, which involves a drop in estrogen and progesterone production by the ovaries. In males, a gradual age-related decline in testosterone production typically begins after age 30, leading to a slow decrease in circulating levels.