Explain how thyroxine and negative feedback work together to maintain a stable metabolic rate in the body.

AQA GCSE Biology (8461) — 4.5.3 Hormonal coordination in humans · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Thyroxine (T4) is produced by the thyroid gland and increases the metabolic rate of cells.
When blood T4 levels rise above the optimum, the hypothalamus and pituitary sense this and reduce the release of TRH and TSH, signalling the thyroid to lower T4 production.
If T4 falls below the optimum, TRH and TSH secretion increases, stimulating the thyroid to produce more T4.
This cycle of detection and reversal is negative feedback, keeping the metabolic rate at a stable, optimum level.

Examiner tips

  • Use the term ‘negative feedback’ explicitly. Show the direction of the response (↑ T4 → ↓ TSH, ↓ T4 → ↑ TSH).
  • Mention the hypothalamus/pituitary as the signalling centres.

Mark scheme (4 marks)

  1. Thyroxine is produced by the thyroid gland and increases the metabolic rate
  2. When thyroxine levels in the blood are too high, this is detected and the thyroid gland is signalled to reduce thyroxine production
  3. When thyroxine levels fall too low, the thyroid gland is stimulated to produce more thyroxine
  4. This is negative feedback because the change is reversed / the system returns to the optimum / stable level

Key terms in this question

thyroxine · metabolic rate

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