Thyroxine sets metabolic rate

1. The thyroid gland is an endocrine gland in the neck that produces and secretes thyroxine.
2. Thyroxine enters the blood and is carried to cells throughout the body.
3. Thyroxine changes how quickly cells carry out chemical reactions, so it controls the body's metabolic rate.
4. The body needs thyroxine to remain near its normal concentration because reactions that run too slowly or too quickly disrupt normal cell function.

Metabolic rate changes energy use

1. Metabolic rate is the overall rate of the chemical reactions in the body.
2. A higher thyroxine concentration increases the rate of many cellular reactions, including reactions involved in respiration and energy transfer.
3. A lower thyroxine concentration slows these reactions, so the body releases and uses less energy in a given time.
4. The effect is not instant because thyroxine must be secreted, transported in the blood and act on its target cells.

The hypothalamus detects low thyroxine

1. The hypothalamus is a region of the brain that monitors internal conditions and controls hormone release from the pituitary gland.
2. When the blood thyroxine concentration falls below normal, the hypothalamus responds by producing and releasing more $\mathrm{TRH}$.
3. A low thyroxine concentration therefore begins a response that will increase thyroxine secretion and oppose the original fall.

TRH activates the pituitary

1. $\mathrm{TRH}$ is thyrotropin-releasing hormone produced by the hypothalamus.
2. $\mathrm{TRH}$ travels to the pituitary gland and stimulates it to produce and release $\mathrm{TSH}$.
3. More $\mathrm{TRH}$ therefore causes more $\mathrm{TSH}$ to enter the blood.

The pituitary releases TSH

1. The pituitary gland is an endocrine gland at the base of the brain that releases hormones which control other endocrine glands.
2. In this pathway, the pituitary is the source of $\mathrm{TSH}$ and the thyroid gland is its target.
3. The pituitary does not release thyroxine, because thyroxine is produced by the thyroid gland.

TSH stimulates the thyroid

1. $\mathrm{TSH}$ is thyroid-stimulating hormone released by the pituitary gland.
2. $\mathrm{TSH}$ travels in the blood to the thyroid gland and stimulates it to produce and secrete more thyroxine.
3. As the thyroid releases more thyroxine, the blood thyroxine concentration rises towards its normal level.
4. The higher thyroxine concentration raises metabolic rate towards its normal value.

Negative feedback reduces stimulation

1. Negative feedback is a control mechanism in which a change away from the normal level causes responses that reverse the change and restore the normal level.
2. As thyroxine returns to its normal concentration, thyroxine inhibits the hypothalamus from producing and releasing more $\mathrm{TRH}$.
3. Thyroxine also inhibits the pituitary gland from producing and releasing more $\mathrm{TSH}$.
4. Less $\mathrm{TSH}$ reaches the thyroid gland, so the thyroid receives less stimulation and secretes less thyroxine.
5. This response prevents the thyroxine concentration and metabolic rate from continuing to rise above the normal level.
6. If thyroxine later falls below normal, inhibition of the hypothalamus and pituitary weakens, so $\mathrm{TRH}$ and $\mathrm{TSH}$ secretion increase again.
7. If thyroxine rises above normal, stronger inhibition reduces $\mathrm{TRH}$ and $\mathrm{TSH}$ secretion until thyroxine falls towards normal.