GCSE · Biology · Edexcel · Spec 1BI0

Thyroxine and negative feedback

A gland in your neck helps set the rate of every chemical reaction in your body, and its hormone level usually stays steady. How?

Biology · Hormones

Rebuild the thyroxine loop

3 stages are missing. Use the rest of the loop to work out which goes where.

Stage 1: Thyroxine level in the blood falls
The level drops below the ideal range, which is the change the body has to deal with.
Stage 3: TSH stimulates the thyroid gland
The instruction arrives at the gland in the neck.
Stage 5: Thyroxine rises back to the ideal range
The drop has been reversed and conditions are steady again.

…and stage 6 leads back to stage 1.

Exam line: Thyroxine falls, the instruction goes out. Thyroxine recovers, the instruction stops.

Biology · Hormones

Which gland?

Does each statement belong to the pituitary gland or the thyroid gland?

Still to sort

Pituitary gland (0)

The master gland that sends the instruction.

Where the line is: If the statement is about secreting TSH or making other glands secrete hormones, it is the pituitary.

Thyroid gland (0)

The gland that carries the instruction out.

Where the line is: If the statement is about thyroxine, the neck or being stimulated by TSH, it is the thyroid.

6 of 6 still to sort.

Exam line: TSH is the instruction from the pituitary gland. Thyroxine is what the thyroid gland makes when it is stimulated.

Predict, then check

You've seen the fall. Now try the rise.

Thyroxine in the blood has risen above the ideal range. What happens to TSH secretion from the pituitary gland?

Biology · Hormones

What does 'negative' mean here?

Thyroxine in the blood can drift above or below the ideal range.

Which is closest to what you think negative feedback means?

It always pushes a level down.It means something harmful is going wrong in the body.It only kicks in when a level gets too high.A change in either direction causes an action that reverses it, so the level stays within a small range.

How sure are you?

I'm sureI think soI'm guessing

It always pushes a level down.

This is a common misconception

Not quite. 'Negative' describes the action reversing the change, not the direction. A fall is reversed by an action that raises the level, and a rise by one that lowers it.

It means something harmful is going wrong in the body.

This is a common misconception

No. Negative feedback is what keeps conditions stable, so it is part of the body keeping thyroxine steady, not a sign of trouble.

It only kicks in when a level gets too high.

This is a common misconception

It also deals with decreases. The body detects both increases and decreases in a level and reverses the change.

A change in either direction causes an action that reverses it, so the level stays within a small range.

This matches the evidence

Yes. Whichever way the level moves, an action reverses the change, so there are only small variations away from the ideal range.

Negative feedback reverses a change, up or down. It is not about decreases and it is not a problem.

Biology · Hormones

Your turn to write

A person's thyroxine level in the blood falls below the ideal range. Explain how negative feedback brings it back to normal. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Pituitary gland secretes TSH (thyroid stimulating hormone). (1 mark)
TSH acts on / stimulates the thyroid gland. (1 mark)
Thyroid gland secretes more thyroxine. (1 mark)
Thyroxine level goes back up to the ideal range. (1 mark)
Pituitary gland stops secreting TSH once the level is normal. (1 mark)

Model answer

The pituitary gland secretes TSH. It acts on the thyroid gland, which secretes more thyroxine, so the level goes back up to the ideal range. Then the pituitary gland stops secreting TSH.

Keep the steps in order.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a loop between two glands keeps a hormone level steady

What you need to know

Thyroxine is a hormone secreted by the thyroid gland, which is in the neck.
Thyroxine controls the body's metabolic rate: the rate of all the chemical reactions in the body.
Have a goYour friend says, 'Metabolic rate is how fast you can run.' Rate of what, exactly? Put them right in a few words.
The rate of all the chemical reactions in the body.
Metabolic rate is about the chemical reactions going on inside the body, not how fast you move.
Thyroxine matters for growth and development, but in adults its level in the blood usually stays stable.
Negative feedback means a change in conditions causes an action that reverses the change, keeping conditions stable.
It deals with increases and decreases alike, so levels only vary a little away from the ideal range.
The pituitary gland secretes TSH (thyroid stimulating hormone), which acts on the thyroid gland and stimulates it to secrete thyroxine.
Have a goSam insists, 'The thyroid gland gives the orders and the pituitary gland does the work.' Which gland actually sends the instruction, and what is the instruction called?
The pituitary gland sends it. The instruction is TSH.
The pituitary secretes TSH, which acts on the thyroid and stimulates it to secrete thyroxine, so the thyroid carries out the job.
When thyroxine in the blood falls, the pituitary secretes TSH and the thyroid makes more thyroxine.
Have a goIn this loop, does more TSH reaching the thyroid gland lead to more thyroxine or less?
More thyroxine.
TSH stimulates the thyroid gland to secrete thyroxine, so more stimulation means more thyroxine.
Once thyroxine is back to normal, the pituitary gland stops secreting TSH.
If thyroxine rises, TSH secretion is inhibited, so the thyroid isn't stimulated and thyroxine falls back to the ideal range.
The pituitary is a master gland: its hormones, such as TSH, make other glands secrete different hormones.

The big picture

Thyroxine, a hormone from the thyroid gland, controls the body's metabolic rate. In adults its level usually stays stable because a negative feedback loop between the pituitary gland (through TSH) and the thyroid gland reverses both rises and falls.

Key points

1Thyroxine comes from the thyroid gland and controls metabolic rate, the rate of all the chemical reactions in the body.

2Negative feedback reverses any change, up or down, so conditions stay stable.

3The pituitary gland secretes TSH, which stimulates the thyroid gland to secrete thyroxine.

4A fall in thyroxine switches TSH on. A rise in thyroxine inhibits TSH secretion.

5The pituitary gland is a master gland because its hormones make other glands secrete different hormones.

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Worked example

Problem

An exam question says: 'Explain how negative feedback keeps the level of thyroxine in the blood of an adult stable.' Plan a full answer, step by step.

⚠ Watch out

Two classic slips. One: swapping the glands, so the thyroid is said to secrete TSH. It is the pituitary that secretes TSH and the thyroid that secretes thyroxine. Two: reading 'negative' as 'the level goes down'. Negative feedback reverses the change, whichever way it went.

🧠

Memory hook

The pituitary is the boss sending memos (TSH); the thyroid is the worker making thyroxine. Enough thyroxine? Memos stop. Not enough? Memos start. (TSH is a hormone, not paper: it's only a memory aid.)

✓

Check yourself

Say it out loud without peeking: which hormone does the pituitary gland secrete to act on the thyroid gland, and what does that hormone make the thyroid gland do?

Flashcards

(12)

Which gland secretes thyroxine, and where is it?

The thyroid gland, which is in the neck.

What does thyroxine control?

The metabolic rate of the body.

What is metabolic rate?

The rate of all the chemical reactions in the body.

What happens to thyroxine levels in the blood of adults?

They usually remain stable. Thyroxine also plays an important role in growth and development.

What is negative feedback?

A mechanism where changes to conditions cause an action to reverse the change, keeping conditions stable.

Does negative feedback respond to rises, falls, or both?

Both. The body detects increases and decreases in a level and reverses the change.

What does TSH stand for, and which gland secretes it?

Thyroid stimulating hormone, secreted by the pituitary gland.

What does TSH do?

It acts on the thyroid gland and stimulates it to secrete thyroxine.

Thyroxine in the blood decreases. What does the pituitary gland do?

It secretes TSH, which stimulates the thyroid gland to increase thyroxine production, bringing levels back to the ideal range.

Thyroxine is back to normal. What does the pituitary gland do?

It stops secreting TSH.

Thyroxine in the blood increases. What happens to TSH secretion?

It is inhibited, so the thyroid gland is not stimulated to make thyroxine and levels decrease back to the ideal range.

Why is the pituitary gland called a master gland?

It secretes hormones, such as TSH, that act on other glands to secrete different hormones.

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Rebuild the thyroxine loop

Which gland?

What does 'negative' mean here?

Your turn to write

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