Secretion of hormones is regulated by a system of sensing elements possessing the means to detect need for both increased and decreased secretion. The particular sensing network, feedback elements, and network of control responses are unique for each hormone. Hormonal pathways maintain homeostasis, and adjustments in secretion usually result in changes that will help maintain the status quo. In addition, secretion and activity of a particular hormone may be adjusted upward or downward in response to challenges such as chronic stress, disease, or change in nutritional status.
The concept of negative feedback and its relationship to control of hormonal pathways is important in understanding pathway regulation and evaluation of endocrine function tests. For example, insulin is released in response to an increase in glucose concentration bathing the β-cells in the pancreatic islets of Langerhans. One of insulin’s actions is to lower glucose concentrations in extracellular fluid by enhancing its uptake in target tissues. This decline in glucose leads to reductions in insulin secretion. In patients suspected of having an insulin-secreting tumor (insulinoma), this feedback relationship is lost, and the finding of a low blood glucose concentration (hypoglycemia) together with an increased insulin concentration demonstrates inappropriate feedback, characteristic of such a tumor. In another example, patients with high blood calcium concentrations should have low levels of PTH in circulation. Measurement of high PTH levels in patients with hypercalcemia indicates a malfunction at the level of the parathyroid, most often associated with parathyroid adenoma.
Secretory patterns of hormones vary tremendously. The thyroid hormones tend to have less variability than the steroid hormones and show only moderate daily or weekly variation. In contrast, blood levels of the adrenal steroid cortisol show much more fluctuation, with occasional bursts of secretion followed by periods of low activity (low blood levels) occurring throughout the day.