Hormone Interactions

Hormones can circulate freely in the bloodstream or bind to carrier proteins to activate or retard a response as the body requires.

Hormones move through the body as either free-floating or bound to a protein carrier through which they can activate or retard responses in the body. Lipophilic proteins do not dissolve in water, and since the blood is composed mostly of water, they must travel bound to a protein carrier. Hydrophilic hormones can dissolve in water, so they are able to travel freely.

The amount of hormones in the blood at any given time is dependent upon how often the hormone is released from the gland and how fast it degrades. Once released, hormones have varying lifespans in the blood. Some hormones last only a few minutes due to being broken down by enzymes in their target cells. One such hormone is adrenaline, which increases heart output and produces what is called the "fight-or-flight" response. Other hormones are filtered out by the liver and kidneys. The liver and kidneys communicate to filter and eliminate toxins in the body and react to body stressors, such as dehydration. When the kidneys detect a reduction in the water/sodium balance in the blood, they produce the enzyme renin. Renin signals the liver about the imbalance and the adrenal glands produce the hormone aldosterone, which, in turn, signals the kidneys to retain both water and sodium. The length of time it takes for the amount of a particular hormone to decrease by half is called its half-life. Hydrophilic hormones have a much shorter half-life than lipophilic hormones because they are quickly flushed from the body by the kidneys.

Hormones can affect their target cells and produce a response almost immediately, or they can take hours or even days before the effects are visible. For example, the treatment of hypothyroidism (low thyroid) by hormone replacement therapy (taking synthetic thyroxine T4) can take several weeks to take effect. Hormones that increase thyroid output address a number of possible conditions, including weight gain, fatigue, dry skin, and constipation. Levels of synthetic thyroxine (T4) are measured by blood tests to determine if the synthetic hormone is functioning adequately.

The overall effects of all hormones are not completely understood, as many responses are due to the interactions of two or more hormones at the same time. Hormones interact with one another in three main ways:

  • Antagonism—In this interaction, two hormones have the opposite effects of each other. They are in competition for the same receptors in the cell. Insulin is a hormone that lowers the levels of the sugar glucose in the blood, whereas glucagon is the hormone that raises those levels.
  • Permissiveness— A hormone cannot produce the desired effect without the presence of another hormone. Thyroid hormone is needed to regulate the rate at which the reproductive hormones are released so that puberty begins at the appropriate time.
  • Synergism—This interaction involves more than one hormone producing the same response on a target cell. Because of the multiple hormones working at the same time, the target cell response is increased. Adrenaline and glucagon both cause the release of glucose into the blood by the liver. Working together, the amount of glucose is increased by more than 150 percent.