MODULE SUPPLEMENT: ENDOCRINE SYSTEM
Circadian Rhythms

Another important point to keep in mind when reading research related to endocrine changes with age is that hormone levels tend to fluctuate over each twenty four hour period (diurnal variation) with varying rhythmicities; some are pulsitile in nature, some peak in the early morning, and some peak later in the day. They can also vary in their secretory patterns depending on the time of year. Thus, when assessing levels of a given hormone, we need to know when it was measured and what the norms are for that particular time of day or season. In addition, aging can alter the circadian rhythm of a hormone with minimal or no change occurring in the absolute amount measured. For example, the circadian variation in cortisol flattens, as discussed later.

A third key point is that, as more research is accomplished, we are finding that gender makes a difference. Hormonal changes with age are not always the same between men and women. So the target population of any study has to be considered when evaluating the findings.

Finally, another important point is that much of the research accomplished on changes in endocrine function has been accomplished on animals and these findings, while helpful, do not always correlate with what is found in humans. As much as possible, the data provided here are from research using humans.