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Thyroxine is a hormone produced by the thyroid gland that controls basal metabolic rate. Free GCSE AQA Biology notes with key terms and 12 questions answered.
Thyroxine: Metabolism and Feedback Control is part of Hormonal coordination in humans in GCSE AQA Higher Biology.
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Thyroxine is a hormone produced by the thyroid gland that controls basal metabolic rate. Free GCSE AQA Biology notes with key terms and 12 questions answered.
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Use this lesson to stay inside Hormonal coordination in humans while connecting the detail back to the wider Homeostasis and response topic.
Read the core explanations from Thyroxine: Metabolism and Feedback Control before testing yourself from memory.
Right now, your body is burning energy just to keep you alive. Your heart beats, your brain thinks, your cells repair themselves. This constant energy use has a name — your basal metabolic rate. But what controls how fast or slow this energy burning happens?
Meet thyroxine — the hormone that acts like a thermostat for your entire body's energy system. Too little, and everything slows down. Too much, and your body goes into overdrive.
Trap: Many students think thyroxine only affects weight. Actually, it controls the speed of nearly every chemical reaction in your body — from how fast your heart beats to how quickly your hair grows.
Think of thyroxine as your body's accelerator pedal. When thyroxine levels are normal, your metabolism runs at the right speed. But what happens when levels go wrong?
High thyroxine levels: Everything speeds up. Heart rate increases, body temperature rises, weight loss occurs even with normal eating, and people feel restless or anxious.
Low thyroxine levels: Everything slows down. Heart rate decreases, body temperature drops, weight gain occurs, and people feel tired and sluggish.
But thyroxine doesn't just control energy use. It's crucial for growth and development, especially in children. Without enough thyroxine, children don't grow properly and brain development is affected.
Your body needs to keep thyroxine levels just right — not too high, not too low. This is where negative feedback comes in. It's like a thermostat that automatically adjusts to maintain the perfect temperature.
Worked example: How Thyroxine Negative Feedback Works
Trace what happens when thyroxine levels drop below normal
Answer: Thyroxine levels return to normal through negative feedback control
The key word here is 'negative' — when thyroxine goes up, it switches off the signals that made it go up in the first place. This prevents thyroxine levels from getting too high.
Sometimes the thyroid system breaks down. Understanding these conditions helps you see why the feedback system is so important.
Hyperthyroidism: Thyroid produces too much thyroxine. The negative feedback can't keep up. Symptoms include rapid heartbeat, weight loss, and feeling hot all the time.
Hypothyroidism: Thyroid produces too little thyroxine. Even maximum stimulation isn't enough. Symptoms include slow heartbeat, weight gain, and feeling cold.
Both conditions show why the feedback system is crucial — without it, your metabolism would be completely out of control.
Thyroxine questions often test your understanding of negative feedback and the effects on metabolism. Here's how to nail them every time.
Exam depth extension: In thyroxine: metabolism and feedback control, connect the master controller of your metabolism to what thyroxine actually does using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Common exam mistake: Students often confuse positive and negative feedback. Remember — in negative feedback, the output REDUCES the input. High thyroxine REDUCES further hormone release.
Master the thyroxine feedback loop and you'll find these questions straightforward. The key is understanding that your body is constantly adjusting hormone levels to maintain perfect balance.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Basal metabolic rate (BMR) is the minimum rate of energy expenditure needed to maintain basic body functions at rest, measured in kilojoules per day. It is controlled by the hormone thyroxine.
Thyroxine is a hormone produced by the thyroid gland that controls basal metabolic rate and is important for growth and development. It controls the speed of nearly every chemical reaction in the body, and without enough thyroxine children do not grow properly and brain development is affected.
Thyroxine is made by the thyroid gland. It controls basal metabolic rate, the minimum rate of energy use needed at rest, and is important for growth and development.
When thyroxine levels fall, the pituitary gland detects the change and releases a stimulating hormone that travels in the blood to the thyroid gland, which produces more thyroxine. As thyroxine rises, it reduces further stimulating hormone release, returning thyroxine levels to normal.
In negative feedback, the output of a system reduces the processes that led to it, maintaining homeostasis; for example, high thyroxine reduces further stimulating hormone release. Confusing positive and negative feedback is a common exam mistake.
In hyperthyroidism the thyroid produces too much thyroxine, causing rapid heartbeat, weight loss and feeling hot all the time. In hypothyroidism it produces too little, causing slow heartbeat, weight gain and feeling cold.
When thyroxine levels are low, everything slows down: heart rate decreases, body temperature drops, weight gain occurs, and people feel tired and sluggish. In children, too little thyroxine affects growth and brain development.
If thyroxine levels are too high, everything speeds up: heart rate increases, body temperature rises, weight loss occurs even with normal eating, and people feel restless or anxious. Producing too much thyroxine is called hyperthyroidism.
Thyroxine is important for children because it is crucial for growth and development. Without enough thyroxine, children do not grow properly and brain development is affected.
Thyroxine does not only affect weight. It controls the speed of nearly every chemical reaction in the body, from how fast the heart beats to how quickly hair grows.
In thyroxine control, the pituitary gland detects when thyroxine levels fall and releases a stimulating hormone, which travels in the blood to the thyroid gland and tells it to produce more thyroxine. High thyroxine then reduces further release of the stimulating hormone.
Hypothyroidism is when the thyroid produces too little thyroxine, so even maximum stimulation is not enough. Symptoms include a slow heartbeat, weight gain and feeling cold.