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While positive feedback does exist, negative feedback in the endocrine system is much more common. The negative feedback process is sometimes compared to a home heating system, where the temperature above a certain level is determined by a thermostat. This switches off the heating until the temperature drops below the minimum threshold, when the heating is switched on again. Negative feedback turns hormone production on and off, creating impulses for hormone secretion. This means that the level of hormones in the blood rises and falls cyclically within a relatively narrow normal range.
An example of negative feedback in the endocrine system can be seen in the regulation of thyroid hormones. It starts with the hypothalamus of the brain, which produces the thyroid hormone (TRH). This hormone travels to the nearby pituitary gland, causing the production of thyroid-stimulating hormone (TSH), which is then released into the bloodstream. Once it reaches the thyroid gland, TSH stimulates the cells to release thyroid hormones. These are important hormones that affect the physiology of virtually all cells in the body.
When the level of thyroid hormones in the blood reaches an upper threshold, cells in the hypothalamus that produce and secrete TRH are affected. As a result, levels of TRH, TSH and finally thyroid hormones drop. As soon as the level of thyroid hormones in the blood falls below the lower threshold, the negative feedback ends. This means that TRH is produced by the hypothalamus again, which leads to the resumption of TSH secretion and the synthesis and release of thyroid hormones.
The production of hormones by many endocrine glands is associated with negative feedback control. Besides feedback loops, other factors can influence hormone secretion. One example of this is the increase in TRH secretion seen in a young child who is in a cold environment. This mechanism is believed to be due to the ability of thyroid hormones to regulate body temperature.
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