ctory in the body, its appropriation of a greater amount of iodine
for itself is understandable.

When thyroxin is administered in a single dose, there is a distinct
lag in the absorption of it by the tissues. A single dose does not
generate its maximum effect until the tenth day. This effect continues
for about ten days. Then there is a gradual decrease in the intensity
of reaction for another ten days. So that the length of time a single
administration of thyroxin functions within the body is about three
weeks. Again we have occasion to notice a protective device of the
cells. Since the presence of thyroxin in the tissues determines the
rate at which they burn themselves up, it is obvious that if there
were no mechanism for retarding its action, and at need varying it,
they really would set fire to themselves. That is to say, if the
tissues held a maximum of the thyroid internal secretion, and had to
take up more and more as it was fed out to them by the thyroid through
the blood, the pressure of energy production would attain the state of
a boiler without a safety valve. Even if self-destruction were avoided
by the ingestion of the largest quantities of energy-bearing foods,
rest for the cells would be difficult, if not impossible.

The thyroxin in the tissues diminishes after a period of great
exertion, the thyroxin probably being carried back to the thyroid
gland and kept there as reserve until further demand. So it has been
discovered that during the winter months, the thyroid glands of beef,
sheep and hogs all contain much less iodine than during the summer
months. During the winter months, manifestly, more energy is required
to maintain body temperature, hence the gland surrenders more of its
secretion to the tissues and so keeps less of it itself. There must
be, too, a certain wearing out of the potency of the iodine with time.
Even dead inorganic catalysts, made of simple elements, wear out after
having been used time and time again.

Though the thyroid is the supreme energizer, life is incompatible with
a certain excess of it. Death can be produced by successive daily
injections of its internal secretion. But it has, besides the
energizing effect, certain formative and nervous influences equally
marvelous. As illustrations, there are the cases of thyroid
deprivation in human beings, cretinism and myxedema, as well as
those in which it is believed there occurs an excess of the
thyroid secretion in the blood