Thyroxine from the thyroid gland stimulates the basal metabolic rate. It…

The idea

Thyroxine is a hormone secreted by the thyroid gland in the neck. It travels in the blood and raises the basal metabolic rate, the rate at which the body uses oxygen and energy while at rest. A useful method is to compare oxygen consumption or heat release in resting animals: more thyroxine means more respiration, so more oxygen is used and more heat is released. Thyroxine also controls growth and development, for example it is needed for normal bone growth and for the development of the brain and nervous system in young mammals. In amphibians such as frogs, thyroxine triggers metamorphosis from tadpole to adult. Because it affects almost every cell, thyroxine must be kept within a narrow range; too little slows metabolism and growth, while too much speeds metabolism and can cause weight loss and a rapid heartbeat.

What earns the marks

• Thyroxine is produced by the thyroid gland and transported in the blood plasma to target cells around the body.
• A higher basal metabolic rate means more oxygen is used and more energy is released as heat.
• Thyroxine is essential for normal growth and development, including bone growth and development of the brain and nervous system.
• In amphibians, thyroxine controls metamorphosis, showing its role in development.
• Thyroxine concentration must be controlled within narrow limits because it affects almost all cells.

Common mistakes and how to put them right

• Saying thyroxine is made by the pituitary gland.

  Put it right: The thyroid gland produces thyroxine, while the pituitary gland releases TSH, which stimulates the thyroid.

• Confusing thyroxine with adrenaline.

  Put it right: Adrenaline prepares the body for rapid action, whereas thyroxine raises the resting metabolic rate over a longer period.

• Stating that thyroxine only affects growth.

  Put it right: It also stimulates the basal metabolic rate in adults as well as supporting growth and development.

Exam tips

• Link thyroxine to respiration by writing that a higher basal metabolic rate means more aerobic respiration, so more oxygen is used and more heat is released.
• Use a named example, such as metamorphosis in a frog, to show that thyroxine controls development as well as metabolism.
• When a question asks why thyroxine concentration must be controlled, refer to its wide-ranging effects on cells and the need to keep metabolism and growth within normal limits.