AQA GCSE Biology: Adrenaline, Thyroxine and Negative Feedback

This whole guide is Higher tier only. Two more hormones, adrenaline and thyroxine, show hormones in action, and thyroxine is a clear example of negative feedback. This guide explains what each hormone does and how thyroxine levels are kept steady. The contrast to notice is that adrenaline is not controlled by negative feedback, but thyroxine is.

Adrenaline

Adrenaline is produced by the adrenal glands in times of fear or stress. It prepares the body for fight or flight.

• It increases heart rate (and breathing rate, and the breakdown of glycogen in the liver).
• This delivers more oxygen and glucose to the brain and muscles.
• So more energy can be released by respiration in these tissues.

Read that as a chain rather than three separate facts, because the marks are for the links. More oxygen and glucose delivered is not the end point; releasing more energy by respiration is, and an answer that stops at "increases heart rate" has not explained anything.

Adrenaline is also the exception to the rule that hormones are slow. It is a hormone, carried in the blood like any other, but the effect arrives in seconds. That is worth holding alongside the usual nervous-versus-hormonal contrast: hormonal responses are generally slower and longer lasting, and adrenaline is the one that is not.

Adrenaline does not involve negative feedback: the adrenal glands simply stop producing it once the danger or stress has passed.

Thyroxine

Thyroxine is released from the thyroid gland. It stimulates the basal metabolic rate (BMR), the rate at which the body's chemical reactions happen at rest, including respiration and the building of glycogen, lipids and proteins. Thyroxine is also important in growth and development.

Where adrenaline is an emergency response, thyroxine is a background setting. It is not answering an event; it is holding the body's whole rate of working at a steady level, day in and day out. Too little leaves someone tired and cold because their reactions are running slowly; too much leaves them running hot.

That difference in job is exactly why only one of them is under negative feedback. A setting has to be held at a level, so it needs a loop that detects drift and corrects it. An emergency response does not: it needs to switch on hard and then stop.

Negative feedback

Negative feedback keeps a level steady by opposing any change.

• Receptors detect that a level has risen above or fallen below normal.
• Effectors respond to counteract the change.
• The level is restored to normal.
• Once normal, the corrective response is reduced or stopped.

All four steps earn credit, and the fourth is the one most often missed. A loop that never switched off would overshoot and drive the level past normal in the other direction.

Negative feedback control of thyroxine

Thyroxine is controlled by negative feedback involving the pituitary gland.

• When thyroxine falls below normal, the pituitary secretes more TSH (thyroid stimulating hormone), which stimulates the thyroid to release more thyroxine, so the level rises back to normal.
• When thyroxine is normal or above, the high level inhibits the pituitary, so it secretes less TSH, the thyroid releases less thyroxine, and the level falls back to normal.

Inhibits is the word to use for the second arm. Thyroxine does not merely fail to stimulate the pituitary; a high level actively suppresses TSH release, and that active suppression is what makes the loop a loop rather than a one-way switch.

Three glands, so keep them straight: the pituitary gives the order (TSH), the thyroid carries it out (thyroxine), and the thyroxine then reports back to the pituitary.

How this fits together

Adrenaline, thyroxine and negative feedback complete the human part of Topic 5. In Triple award biology, the topic ends with how plants use hormones to respond and grow, covered in the plant hormones guide.