Hormone regulation depends upon the concentration of the specific hormone and the rate at which that hormone is secreted. Since hormones are not released from cells at a constant rate, mechanisms need to be in place to make sure the appropriate amounts of hormones reach their target cells when needed. The first kind of regulatory mechanism is called negative feedback control. The concentrations of hormones in the blood are maintained at a certain level. When this level decreases or increases away from its normal levels, negative feedback, the mechanism used by the body to counteract a change, initiates the release of more of the hormone into the blood. A body experiences a fright, such as a threatening attack by a dog, and the reaction is immediate. The body automatically secretes extra adrenaline, heart rate increases, blood flow increases, and additional oxygen levels move through the body. Once the threat is over, adrenaline production returns to normal ranges.
Changing the number of receptors is another method of changing the response to a particular hormone. There are specific control mechanisms that add or remove receptors to the target cells, which allows for more or less of the hormone to attach at a given time. A method called down regulation, which is the cellular reduction in the number of hormone receptors to a molecule, lessens the cell's reaction to the molecule and occurs when too much of a hormone is constantly present in the blood. Over time, the amount of the hormone is reduced by the number of receptors being gradually eliminated as the need for the hormone is reduced. For example, women reaching menopause produce much less estrogen and progesterone because they no longer can become pregnant. The number of receptors for these hormones are reduced as the need for estrogen and progesterone are reduced.
Many hormone levels change during the day as a result of a circadian rhythm, fluctuations that repeat multiple times due to a set change in time, such as the change from light to dark. Controlled by the central nervous system, this cycle causes the levels of hormones to rise and fall as the amount of light changes. For example, cortisol levels are their lowest right before bed, when it is dark outside. They rise during the night and reach their peak just prior to a person waking up in the morning. The brain creates a set point, which is the normal level of the hormone, and then negative feedback maintains the levels for the particular time of day. In the brain, a structure called the suprachiasmatic nucleus (SCN) is found in the hypothalamus. The SCN works like a master clock by which all cycles of the body run.Removing hormones from the blood impacts their effectiveness by reducing the responses initiated by the hormone. All hormones eventually become inactive because they have been acted upon by enzymes from the liver or kidneys. Hydrophilic hormones are most commonly inactivated by the breakdown of the peptide bonds that hold the molecules together. Sometimes the hormone is taken through endocytosis into the cell and digested from within. Lipophilic hormones are deactivated when a change in the active portion of the molecule occurs through chemical reactions. Once these hormones are deactivated, the liver converts them into molecules that are water soluble and can be removed from the body with other liquid wastes in the urine.