Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
Explain how thyroxine controls metabolic rate as an example of negative…
Full specification wording
Topic 7 – Animal coordination, control and homeostasis: 7.3 Explain how thyroxine controls metabolic rate as an example of negative feedback, including: a low levels of thyroxine stimulates production of TRH in hypothalamus b this causes release of TSH from the pituitary gland c TSH acts on the thyroid to produce thyroxine d when thyroxine levels are normal thyroxine inhibits the release of TRH and the production of TSH
Thyroxine is a hormone made by the thyroid gland that raises metabolic rate.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 3 of 19
Study notes
From the checked teaching notes for this specification point.
The idea
Thyroxine is a hormone made by the thyroid gland that raises metabolic rate. Its level is held steady by negative feedback along a hypothalamus–pituitary–thyroid axis. When thyroxine falls below normal, the hypothalamus produces more TRH (thyrotrophin-releasing hormone). TRH travels to the pituitary gland and causes it to release more TSH (thyroid-stimulating hormone). TSH acts on the thyroid gland, which produces more thyroxine, so metabolic rate rises back towards normal. When thyroxine returns to normal, the higher thyroxine level inhibits the hypothalamus so less TRH is released, and inhibits the pituitary so less TSH is produced. With less TSH the thyroid is stimulated less, so thyroxine output falls. This is negative feedback: the response cancels the original change.
What earns the marks
- Low thyroxine stimulates the hypothalamus to produce more TRH.
- TRH causes the pituitary gland to release TSH.
- TSH acts on the thyroid gland, which produces thyroxine.
- Raised thyroxine inhibits release of TRH from the hypothalamus and production of TSH from the pituitary.
- The loop is negative feedback because the rise in thyroxine reduces the stimulus that caused it, returning the level towards normal.
Common mistakes and how to put them right
Naming the wrong gland as the source of TRH.
Put it right by linking each hormone to its gland.
Reversing the direction of the loop, saying thyroxine stimulates TRH when levels are normal.
Put it right: Normal thyroxine inhibits TRH and TSH release.
Confusing TSH with thyroxine.
Put it right: TSH is a pituitary hormone that stimulates the thyroid; thyroxine is the thyroid hormone acting on metabolic rate.
Exam tips
- Write the sequence in order with arrows: low thyroxine → hypothalamus TRH → pituitary TSH → thyroid thyroxine.
- State clearly that the final step is inhibition of TRH and TSH, and name negative feedback explicitly.
- Link thyroxine to its effect on metabolic rate so the purpose of the loop is clear.
Try the lesson
The start of the Topic 7 – Animal coordination, control and homeostasis: 7.3 Explain how thyroxine controls metabolic rate as an example of negative feedback, including: a low levels of thyroxine stimulates production of TRH in hypothalamus b this causes release of TSH from the pituitary gland c TSH acts on the thyroid to produce thyroxine d when thyroxine levels are normal thyroxine inhibits the release of TRH and the production of TSH lesson, made from checked teaching notes for this specification point.
Have a go first
When thyroxine levels in the blood fall below normal, which gland increases its production of TRH?
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Topic 7 – Animal coordination, control and homeostasis: 7.3 Explain how thyroxine controls metabolic rate as an example of negative feedback, including: a low levels of thyroxine stimulates production of TRH in hypothalamus b this causes release of TSH from the pituitary gland c TSH acts on the thyroid to produce thyroxine d when thyroxine levels are normal thyroxine inhibits the release of TRH and the production of TSH
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge