Question

What is negative feedback? How does a negative feedback loop regulate hormone release? Which of the unknown hormones here showed a negative feedback effect (ACTH, LH, Testosterone, TRH, Cortisol, TSH)? How do we know this?

Answer 1

A negative feedback loop, also known as an inhibitory loop, is a loop which opposes or that counteract changes of various properties from their target values, known as set points.This mechanism results in the inhibition or slowing down of a process as opposed to positive feedback system which results in the amplification or growth of output signal, Most hormone feedback mechanisms involve negative feedback loops. Negative feedback keeps the concentration of a hormone within a narrow range.,In this feedback system, a stimulus causes the release of a substance whose effects then inhibit further release. In this way, the concentration of hormones in blood is maintained within a narrow range

Most of these hormones like ACTH, LH, testosterone, TRH, cortisol and TSH are examples of negative feedback.

The hypothalamus secretes TRH,this hormone than stimulates the pituitary gland to produce thyroid-stimulating hormone, TSH, in turn, stimulates the thyroid gland to secrete its hormones. When the level of thyroid hormones is high enough, the hormones feedback to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH.

Production of ACTH is regulated by corticotrophin-releasing hormone (CRH) from the hypothalamus and cortisol from the adrenal gland. If ACTH levels are low, the hypothalamus releases CRH, Once the CRH is discharged, it triggers the pituitary gland to secrete ACTH. High levels of ACTH are detected by the adrenal gland, which starts the production of cortisol. When cortisol levels rise in the body, the brain can sense these and the production of the CRH and ACTH decreases. Simlarily a negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and anterior pituitary to inhibit the release of GnRH, FSH, and LH. From this we can know it is a negative feedback loop because increase in level of a hormone inhibits its further release.

Positive feedback loop in release of hormones are very rare , with the exception of release of hormone like Oxytocin and Prolactin, which typically occurs at the time of birth.