In the game of football, a team’s ultimate goal each drive is to score a touchdown, but to get to this point, there are many lines of communication that must occur smoothly. The coach relays plays to his players on the field, and those players implement the play calls to achieve their goal of scoring. The endocrine system is like a team in that it utilizes signals (play calls) from the hypothalamus (the coach). Effective communication between the players of the endocrine system can lead to balanced hormone production. Balanced hormone production is important in managing patient symptoms and ineffective communication can potentially lead to improper levels of hormones being synthesized.
Ernest Henry Starling, an MD in London, coined the term “hormone” in a series of lectures given to the Royal College of Physicians in London in 1905. Hormones are defined as “a regulatory substance produced in an organism and transported in tissue fluids such as blood or sap to stimulate specific cells or tissues into action.” Hormones are substances that are produced by the endocrine system in the body and influence the way the body grows or develops. The pituitary gland, located in the brain, is considered the “master control” of the endocrine system; however, the hypothalamus actually controls the pituitary and is the overarching control of the endocrine system.
Figure 2 (above) illustrates the top-down control of the stimulation and production of endocrine hormones. Hormone signaling and synthesis primarily begins in the hypothalamus with the hormones seen here: prolactin releasing hormone (PRH), thyrotropin releasing hormone (TRH), corticotropin releasing hormone (CRH), growth hormone releasing hormone (GHRH), and gonadotropin releasing hormone (GnRH). Each elicit the release of subsequent hormones synthesized in the anterior pituitary and have endocrine targets, with the exception of prolactin. The endocrine targets then secrete hormones that have effects in nonendocrine targets, such as the skin or heart.
In terms of the sex hormones, GnRH from the hypothalamus signals the anterior pituitary to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH), the gonadotropin hormones. FSH and LH signal to endocrine cells to secrete androgens or estrogens and progesterone for males and females respectively. FSH and LH are also able to signal directly to the germ cells of the gonads to have a nonendocrine target effect.
Just like any football team trying to win a championship, if any part of endocrine communications is not functioning optimally, the system won’t work and the end goals are not achieved. During a regular sex hormone workup, do not overlook factors such as leptin, insulin, and thyroid hormones, which are affected by hypothalamic activity. More information on the top-down control of sex hormones can be found in Module 2, Part A of the Reproductive Endocrinology Curriculum, which is available here (log in required).