Thyroxine (Edexcel GCSE Biology: Combined Science)

Higher tier only

Negative feedback mechanisms

• Negative feedback mechanisms in homeostasis help to maintain conditions in the body within an optimal narrow range; any movement away from ideal conditions results in changes occurring which bring them back
• This involves detecting that the level of a substance or a condition has gone above or below normal levels, which triggers a response to bring the level back to normal again
• Thyroxine and control of the basal metabolic rate is an example of negative feedback

A negative feedback cycle

• Thyroxine is a hormone that is released from the thyroid gland (which is located in the neck)
• Thyroxine has a number of important roles in the body
  • It stimulates the basal metabolic rate (BMR); this is the speed at which chemical reactions occur in the body when it is at rest

• Thyroxine levels are also controlled by negative feedback; with levels of TSH (thyroid-stimulating hormone) released from the pituitary gland responsible for maintaining normal levels in the bloodstream
• If the level of thyroxine is normal or too high
  • TRH production is inhibited in the hypothalamus
  • The release of TSH from the pituitary is therefore also inhibited
  • So less thyroxine is released from the thyroid gland

• If the level of thyroxine falls below a normal level
  • TRH is released in the hypothalamus
  • This increases the release of TSH from the pituitary gland
  • TSH stimulates the thyroid to release more thyroxine

Conditions of the thyroid

• Two conditions related to the thyroid gland are:
  • Hyperthyroidism, caused by an overactive thyroid gland secreting too much thyroxine into the bloodstream which causes an increase in BMR and protein synthesis
  • Hypothyroidism caused by an underactive thyroid gland secreting too little thyroxine into the bloodstream which can lead to heart and nerve problems, and death