The Endocrine System (TM)
The Endocrine System
Structure and Function of the Endocrine System
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.
Endocrine Glands
Gland | Regulates | Hormones Produced |
|---|---|---|
pineal gland | circadian rhythms (the sleep/wake cycle) | melatonin |
pituitary gland | 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) |
hypothalamus | 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 |
thyroid gland | energy use and protein synthesis | thyroxine (T4), triiodothyronine (T3), calcitonin |
parathyroid | calcium and phosphate levels | parathyroid hormone (PTH) |
adrenal glands | “fight or flight” response, regulation of salt and blood volume | epinephrine, norepinephrine, cortisol, androgens |
pancreas | blood sugar levels and metabolism | insulin, glucagon, somatostatin |
testes | maturation of sex organs, secondary sex characteristics | androgens (e.g., testosterone) |
ovaries | maturation of sex organs, secondary sex characteristics, pregnancy, childbirth, and lactation | progesterone, estrogens |
placenta | gestation and childbirth | progesterone, estrogens, human chorionic gonadotropin, human placental lactogen |
Pathologies of the Endocrine System
Disruption of hormone production in specific endocrine glands can lead to disease that affects multiple systems.
- Diabetes mellitus is a metabolic disorder that affects the body’s ability to produce and use insulin, a hormone that regulates cellular uptake of glucose (sugar). Diabetes mellitus is classified as type 1 or type 2.
- Hypothyroidism occurs when insufficient thyroxine is produced and can result in fatigue, weight gain, and cold intolerance.
- Hyperthyroidism occurs when too much thyroxine is produced and can cause anxiety, mood swings, weight loss, and palpitations. Grave’s disease is a specific cause of hyperthyroidism common in women over 40.
- Adrenal insufficiency (Addison’s disease) is the chronic underproduction of steroids.
- Cushing syndrome is caused by exposure to high cortisol levels over an extended period of time due to overproduction of cortisol from the adrenal glands.