11. Feedback mechanisms of hormones

The endocrine system relies on feedback mechanisms to regulate hormone production and secretion.

The majority of endocrine glands are controlled by negative feedback to maintain homoeostasis or avoid deviation from an ideal value.
The regulation of the thyroid hormone thyroxine, which governs multiple essential metabolic processes, is an excellent example of a negative feedback system.

 

Neurons secrete the thyroid-releasing hormone in the hypothalamus.  This hormone stimulates cells in the anterior pituitary to generate thyroid-stimulating hormone. The thyroid-stimulating hormone then causes the thyroid gland to release thyroxine.

 

At first, the sensory neurons sense the increase in blood concentrations of thyroxine over the optimal amounts. Then, the hypothalamus is signalled to stop the secretion of the thyroid-releasing hormone.

 

This stoppage will lower thyroxine levels in the blood.  The hypothalamus is signalled to start secreting thyroid-releasing hormone again when they drop below the optimal value.

 

The positive feedback stimulates the secretion or production of the hormone. Positive feedback is considered insignificant to maintain homeostatic functioning, in contrast to negative feedback, which keeps hormone levels within limited ranges. A good example is when a child is born.

 

The posterior pituitary produces the hormone oxytocin, which promotes and increases contractions during labour. Pressure receptors in the cervix signal the hypothalamus to activate the pituitary to secrete oxytocin as the infant passes through the birth canal.

 

Oxytocin travels through the bloodstream to the uterus, stimulating the contraction of the uterine wall muscles, which increases the activation of the pressure receptors and encourages more oxytocin release.

 

Muscle contractions become stronger until the baby is born.  After the baby is born, the stimulation of the pressure receptors is removed, stopping the production of oxytocin.