Hormone feedback loops help the body keep internal conditions stable even when the outside environment changes. In the endocrine system, glands release chemical messengers called hormones into the bloodstream to control processes such as growth, metabolism, stress response, and blood sugar. Negative feedback is especially important because it reduces or shuts off a response once the body has enough of a hormone or has corrected a change.
This prevents hormone levels from becoming too high or too low.
Key Facts
- Negative feedback means the output of a system reduces the original stimulus.
- Hypothalamus signals pituitary gland, pituitary gland signals target endocrine gland, target gland releases final hormone.
- In the thyroid axis: TRH stimulates TSH release, and TSH stimulates thyroid hormone release.
- High thyroid hormone levels inhibit TRH and TSH release.
- Blood glucose control uses insulin to lower glucose and glucagon to raise glucose.
- Homeostasis depends on sensors, control centers, signals, effectors, and feedback.
Vocabulary
- Hormone
- A hormone is a chemical messenger released by an endocrine gland that travels through the blood to affect target cells.
- Negative feedback
- Negative feedback is a control process in which a change triggers responses that reduce or reverse that change.
- Hypothalamus
- The hypothalamus is a brain region that links the nervous system to the endocrine system and helps control the pituitary gland.
- Pituitary gland
- The pituitary gland is an endocrine gland at the base of the brain that releases hormones controlling many other glands.
- Target gland
- A target gland is an endocrine gland that responds to a hormone signal by releasing its own hormone.
Common Mistakes to Avoid
- Confusing negative feedback with a harmful effect. Negative feedback does not mean bad, it means the response reduces the original change.
- Drawing hormone arrows as one-way activation only. Many endocrine axes also include return signals where high final hormone levels inhibit earlier glands.
- Forgetting the bloodstream step. Endocrine hormones usually travel through blood, so they do not act like nerve signals moving directly across synapses.
- Mixing up insulin and glucagon. Insulin lowers blood glucose by helping cells take up glucose, while glucagon raises blood glucose by signaling glucose release.
Practice Questions
- 1 A person's blood glucose rises from 90 mg/dL to 150 mg/dL after a meal. Which hormone should increase, and what effect should it have on blood glucose?
- 2 In a thyroid feedback loop, TSH is 4.0 units and then thyroid hormone rises above normal. If negative feedback reduces TSH by 60%, what is the new TSH level?
- 3 A patient has high thyroid hormone levels but low TRH and low TSH. Explain how this pattern fits a negative feedback loop.