Explain how the hormone thyroxine is regulated in the body using a negative feedback cycle.

AQA GCSE Biology (8461) — 4.5.1 Homeostasis · 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 is produced by the thyroid gland and raises the metabolic rate.
When blood thyroxine is high, the hypothalamus/pituitary sense this and lower the release of TSH.
Reduced TSH means the thyroid produces less thyroxine.
Thus thyroxine falls back towards the optimum – a negative feedback loop.

Examiner tips

Use the term ‘negative feedback’ explicitly.
Show the sequence: high thyroxine → ↓TSH → ↓thyroxine.
Mention the hypothalamus/pituitary as the detecting organs.
Keep the answer concise – 4 points only.

Common mistakes

Confusing TSH with thyroxine itself.
Forgetting to mention the hypothalamus/pituitary.
Using vague terms like ‘the body’ instead of specific organs.

Mark scheme (4 marks)

Thyroxine is produced by the thyroid gland and increases the metabolic rate
When thyroxine levels in the blood are high, the brain/pituitary gland detects this and reduces the release of the hormone that stimulates thyroxine production
This causes the thyroid gland to produce less thyroxine, so levels fall back towards the optimum
This is a negative feedback cycle because the change (high thyroxine) is reversed / the system reverses the original change to maintain optimum conditions

Key terms in this question

thyroxine · negative feedback cycle