Negative Feedback

• Thyroxine levels are controlled by negative feedback
• Hormones are chemicals released directly into the bloodstream by glands and cause an effect in specific target organs. Two of these are:

  1. Adrenal glands
  2. Thyroid gland
• The adrenal glands are on top of the kidneys
• During times of fear or stress, the adrenal glands release the hormone adrenaline into the blood
• Adrenaline increases the heart rate, because that means more oxygen and glucose are delivered to the brain and to the muscles.
• The thyroid gland is found in the base of the neck and produces the hormone thyroxine.
• Thyroxine had two main effects:

  1. Stimulates the body's basal metabolic rate
  2. Important role in growth and development
• Metabolism is the sum of all reactions in a body or cell.
• Thyroxine makes the body's chemical reactions take place at a faster rate
• Level of thyroxine in the blood is controlled by negative feedback
• • The level of thyroxine in the blood is monitored by the brain.
  • When the thyroxine level falls, the pituitary gland releases the hormone TSH into the bloodstream.
  • TSH triggers the thyroid gland to release more thyroxine into the blood
  • The increased level is detected by the brain
  • This prevents the pituitary gland from releasing more TSH
  • This means that the thyroid releases less thyroxine
• Negative feedback cycles include:

  • Levels of blood glucose
  • Menstrual cycle