Page 72 - The Cell in 40 Topics

The Cell in 40 Topics

exact level required for healthy human life is a miracle all by itself.
Somehow, thyroxin molecules know how much heat each cell should give
off, and how that heat is to be increased—in itself this is a miracle of cre-
ation (Figure 57).
The secretion of the thyroxin is another miracle of creation. The mo-
ment the need for the hormone is felt, the hypothalamus—in effect, the
"brain" of the hormonal system—sends a command (via the thyroid-stimu-
lating hormone or TSH) to the pituitary gland, the conductor of the nervous
system. Receivings this command, the pituitary gland realizes that the thy-
roid gland needs to go into action. And so, it immediately dispatches a
command in the form of thyrotropin (or thyroid-gland stimulating hor-
mone) to the thyroid gland. The thyroid, the final link in this chain of com-
mand, immediately produces the hormone thyroxin in accord with the
chemical instruction reaching it, and distributes it throughout the
entire body by way of the bloodstream (Figure 58).
It is not only the duty of the hormone thyroxin that is
so very important, but also the level at which it is se-
creted. But how is that level determined? How
is it that neither too much nor too little of
this hormone is secreted into the blood
(apart from in times of sickness and
fever)? What determines the level at

Figure 56.
The hormone thy-
roxin regulates the
body's tempera-
ture, just like a
thermostat.

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