Thyroxine and adrenaline - Higher Thyroxine Thyroxine is produced from the thyroid gland . Thyroxine stimulates the basal metabolic rate - the speed at which oxygen and food products react to release energy for the body to use, and other chemical reactions that occur in the body. Thyroxine also plays an important role in growth and development. Thyroxine levels are controlled by negative feedback . Negative feedback mechanism that controls the secretion of thyroxine The pituitary gland has an important role in detecting and controlling thyroxine levels. High thyroxine levels in the bloodstream prevent the release of TSH from the pituitary gland, so normal blood levels are restored. Low thyroxine levels in the bloodstream stimulate the pituitary gland to release TSH so the thyroid releases more thyroxine. So, blood levels return to normal. Adrenaline Adrenaline is produced by the adrenal glands in times of fear or stress. Adrenaline increases the heart rate, boosting the delivery of oxygen and glucose to the brain and muscles, preparing the body for 'fight or flight'. When adrenaline is released into the bloodstream it has multiple effects: it increases breathing rate, heart rate, blood flow to the brain and muscles and conversion of glycogen to glucose so more energy can be released in the muscles as we have only a set volume of blood in our bodies, it diverts blood away from areas such as the digestive system and skin towards the muscles - this is why people go pale when they have a fright The effects of adrenaline allow the body to prepare for action in situations where a quick response may be essential.