Thyroxine and metabolic rate

1. The thyroid gland lies in the neck, just below the voice box, and secretes thyroxine into the blood.
2. Thyroxine molecules contain iodine, which the body can only obtain from the diet.
3. Thyroxine sets the basal metabolic rate, which is the rate at which the body's chemical reactions run while at rest.
4. It raises the rate of aerobic respiration in cells, so more energy is transferred from glucose every second.
5. A high concentration of thyroxine speeds up these reactions, raising the heart rate, increasing body heat and causing weight loss.
6. A low concentration slows the reactions, causing tiredness, weight gain and a feeling of being cold.
7. Thyroxine is also needed for normal growth and for the development of the brain during childhood.

Negative feedback control

1. The concentration of thyroxine in the blood is held close to a set point by negative feedback.
2. The pituitary gland secretes thyroid stimulating hormone, known as TSH, into the blood.
3. TSH travels to the thyroid gland and stimulates it to secrete thyroxine.
4. When the thyroxine concentration rises above the set point, thyroxine acts on the pituitary and inhibits the release of TSH.
5. Less TSH reaches the thyroid, so the thyroid secretes less thyroxine and the concentration falls back towards the set point.
6. When the thyroxine concentration drops below the set point, that inhibition is lifted and the pituitary secretes more TSH.
7. More TSH stimulates the thyroid, so thyroxine output rises and the concentration climbs back towards the set point.
8. The result is a concentration that fluctuates slightly around the set point rather than drifting away from it.

Adrenaline and fight or flight

1. The adrenal glands sit one on top of each kidney and secrete adrenaline into the blood.
2. Release is triggered by nervous impulses sent from the brain when it detects fear, danger or stress.
3. Adrenaline increases the heart rate and the force of each beat, so blood is pumped around the body faster.
4. It increases the breathing rate and the depth of each breath, so more oxygen enters the blood.
5. Together these changes deliver oxygen and glucose to the muscles and the brain more quickly.
6. Adrenaline makes the liver break down stored glycogen into glucose, raising the blood glucose concentration.
7. Blood vessels supplying the muscles widen while those supplying the gut narrow, diverting blood to where it is needed.
8. The pupils widen, letting more light into the eye so that vision improves.
9. All of these changes together prepare the body either to fight the danger or to run from it.
10. Adrenaline is not controlled by negative feedback, because it is released as a response to a stimulus and stops once the danger has passed.

Comparing the two hormones

1. Thyroxine is secreted continuously and its concentration is held steady, while adrenaline is secreted in short bursts.
2. Thyroxine release is controlled by negative feedback from the pituitary, while adrenaline release is triggered by nervous impulses from the brain.
3. Thyroxine controls a long-term process, the overall rate of the body's reactions, over months and years.
4. Adrenaline controls a short-term response that lasts minutes, preparing the body for immediate action.
5. Both hormones are secreted into the blood and both raise the rate at which energy is transferred, so the difference lies in the timing and the trigger.