The endocrine system is composed of a network of organs called glands that produce signaling chemicals called hormones. These hormones are released by glands into the bloodstream and then travel to the other tissues and organs whose functions they regulate. When they reach their target, hormones bond to a specific receptor on cells to direct the machinery of the cell. Hormones play an important role in regulating almost all bodily functions, including digestion, respiration, sleep, stress, growth, development, reproduction, and immune response.
Much of the action of the endocrine system runs through the hypothalamus, which is highly integrated into the nervous system. The hypothalamus receives signals from the brain and in turn will release hormones that regulate both other endocrine organs and important metabolic processes. Other endocrine glands include the pineal, pituitary, thyroid, parathyroid, thymus, and adrenal glands.
Organs from other systems, including the reproductive and digestive systems, can also secrete hormones, and thus are considered part of the endocrine system. The reproductive organs in both males (testes) and females (ovaries and placenta) release important hormones, as do the pancreas, liver, and stomach.
circadian rhythms (the sleep/wake cycle)
growth, blood pressure, reabsorption of water by the kidneys, temperature, pain relief, and some reproductive functions related to pregnancy and childbirth
human growth hormone (HGH), thyroid-stimulating hormone (TSH), prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), oxytocin, antidiuretic hormone (ADH)
pituitary function and metabolic processes including body temperature, hunger, thirst, and circadian rhythms
thyrotropin-releasing hormone (TRH), dopamine, growth-hormone-releasing hormone (GHRH), gonadotropin-releasing hormone (GnRH), oxytocin, vasopressin
energy use and protein synthesis
thyroxine (T4), triiodothyronine (T3), calcitonin
calcium and phosphate levels
parathyroid hormone (PTH)
“fight or flight” response, regulation of salt and blood volume
epinephrine, norepinephrine, cortisol, androgens
blood sugar levels and metabolism
insulin, glucagon, somatostatin
maturation of sex organs, secondary sex characteristics
androgens (e.g., testosterone)
maturation of sex organs, secondary sex characteristics, pregnancy, childbirth, and lactation
gestation and childbirth
progesterone, estrogens, human chorionic gonadotropin, human placental lactogen
Disruption of hormone production in specific endocrine glands can lead to disease that affects multiple systems.