The more intimate study of the composition of the blood has revealed the most astonishing parallelism between it and the compounds of sea water.  The blood is sea water, to which has been added hemoglobin as a pigment for carrying oxygen to the cells not in direct contact with the atmosphere, nutrients to take the place of the prey our marine ancestors gobbled up frankly and directly, and white cells to act as the first line of defense.  To keep the concentration of iodine in the blood a constant, the thyroid evolved, since there is no iodine in most foods and very little in those which do contain it.

That a minimum amount of iodine in the food is necessary to health is shown by the existence of goitre regions.  Around some of the Great Lakes in the United States, for instance, the water does not contain enough iodine.  As a result, numerous cases of goitre occur.  Iodine in the form of sodium iodide in small doses will act as a prophylactic.  The amount of iodine in the blood is about one or two parts to ten millions, and that of the liver is about three or four parts to ten millions.  Since the liver is the most complex and active chemical factory 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.