7.3 - Thyroxine and feedback

7.3 - Higher: thyroxine and negative feedback

Higher only (7.3). The thyroid gland produces thyroxine, a hormone that controls metabolic rate, the rate at which chemical reactions occur in cells. More thyroxine generally raises metabolic rate and less thyroxine lowers it, so the concentration of thyroxine in the blood needs to be controlled.

This control uses negative feedback: a change away from the normal level causes responses that oppose the change and return the level towards normal.

When thyroxine is low, the sequence is:

1. A low thyroxine concentration stimulates the hypothalamus in the brain to produce and release TRH (thyrotropin-releasing hormone).
2. TRH causes the pituitary gland to release TSH (thyroid-stimulating hormone) into the blood.
3. TSH travels to and acts on the thyroid gland, stimulating it to produce and release more thyroxine.
4. As thyroxine returns to its normal level, it inhibits TRH release from the hypothalamus and inhibits TSH production by the pituitary. The thyroid then receives less stimulation, preventing thyroxine from continuing to rise.