Control of metabolism

• Metabolism is the vast array of biochemical reactions occurring in an organism 
• Among the most important are cell respiration reactions which releases energy for ATP production and heat
• Thyroxine (T4) is a hormone produced by the thyroid gland and secreted into the circulatory system
• Acts on most cells to regulate metabolism by increased the basal metabolic rate (BMR)
• As a result of this, thyroxine prompts the generation of heat
• Iodine is required for the production of thyroxine and lack of iodine causes the thyroid to enlarge in a condition known as goitre
• Lack of thyroxine will cause slowing of thought and speech, lethargy, sleepiness, decreased appetite and cold intolerance
• Too much thyroxine will cause rapid heartbeat, shortness of breath, increased appetite and intolerance to heat
• The thyroid gland is found in the neck region
• Feedback loop

  1. Thyroxine levels fall

  2. Detected by receptors in the hypothalamus

  3. Hypothalamus secretes TRH

  4. Stimulates anterior pituitary to release TSH

  5. TSH travels to thyroid gland

  6. Thyroid gland releases thyroxine

  7. Increased thyroxine increases metabolic rate

  8. Stimulates energy release for ATP production and heat production
• Negative feedback

  Increasing thyroxine levels are detected by the hypothalamus and the stimulus is reversed