Negative Feedback: Adrenaline and Thyroxine
Build confidence in Negative Feedback: Adrenaline and Thyroxine for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) biology topic on how thyroxine is controlled by negative feedback and how adrenaline prepares the body for 'fight or flight'.
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Topic overview
Negative feedback is a control mechanism in which a change from the normal level triggers a response that reverses it. This is a Triple-only topic.
Thyroxine, from the thyroid gland, stimulates the basal metabolic rate and plays an important role in growth and development. Its level is controlled by negative feedback: if thyroxine falls below normal, the pituitary releases TSH, which stimulates the thyroid to release more thyroxine. When levels rise, TSH release is inhibited.
Adrenaline works differently. It is produced by the adrenal glands in times of fear or stress, increasing heart rate and boosting the delivery of oxygen and glucose to the brain and muscles for fight or flight. It is not controlled by negative feedback.
Revision notes
Thyroxine and negative feedback
Thyroxine from the thyroid stimulates the basal metabolic rate — the rate of chemical reactions in cells at rest.
If thyroxine falls below normal, the pituitary secretes TSH, stimulating the thyroid to release more. As levels rise, TSH secretion is inhibited. This is negative feedback.
Adrenaline and fight or flight
Adrenaline is produced by the adrenal glands in times of fear or stress.
It increases heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for fight or flight. It is not under negative feedback control.
Comparing the two
Thyroxine is maintained at a steady level by negative feedback and acts over the long term.
Adrenaline is released in response to a specific situation and acts rapidly. Knowing which is which is often the whole of a question.
Key points
- Negative feedback reverses a change from the normal level.
- Thyroxine stimulates the basal metabolic rate.
- TSH from the pituitary stimulates thyroxine release.
- Rising thyroxine inhibits TSH release.
- Adrenaline prepares the body for fight or flight.
- Adrenaline is not controlled by negative feedback.
Worked examples
Example 1
Explain how the level of thyroxine in the blood is controlled by negative feedback. [3 marks]
Model answer
If the thyroxine level falls below normal, the pituitary gland secretes TSH (1 mark). TSH stimulates the thyroid gland to release more thyroxine, raising the level (1 mark). When the level rises above normal, the secretion of TSH is inhibited, so less thyroxine is released (1 mark).
Example 2
Describe two effects of adrenaline on the body. [2 marks]
Model answer
It increases heart rate (1 mark), and it increases the delivery of oxygen and glucose to the brain and muscles (1 mark).
Example 3
Explain why adrenaline is described as preparing the body for fight or flight. [2 marks]
Model answer
It boosts the supply of oxygen and glucose to the muscles and brain (1 mark), allowing a rapid physical response such as running away or defending oneself (1 mark).
Common mistakes
Saying adrenaline is controlled by negative feedback.
It is released in response to fear or stress, not to maintain a steady level.
Confusing TSH with thyroxine.
TSH comes from the pituitary and stimulates the thyroid to release thyroxine.
Describing negative feedback as amplifying a change.
It reverses a change, returning the level towards normal.
Forgetting this is Triple-only.
Combined Science students do not need thyroxine and adrenaline in this detail.
Exam tips
- Describe negative feedback as a cycle in both directions.
- Keep TSH and thyroxine clearly distinct.
- Give at least two effects of adrenaline when asked.
- Remember this is a Triple-only topic.
Key terms
- Negative feedback
- A mechanism that reverses a change from the normal level.
- Thyroxine
- The hormone stimulating basal metabolic rate.
- TSH
- Thyroid stimulating hormone, released by the pituitary.
- Adrenaline
- The hormone preparing the body for fight or flight.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.