A relatively high percentage of iodine is the unique distinctive fact in its chemistry. Discovered by Baumann in 1895, the presence of the element has focused the intelligence of chemists upon the gland, with the consequent demonstration of arsenic also in it. It was soon manifest that the secretion of the gland was dependent upon the iodine content for its activity. Active extracts of the thyroid like thyreoglobulin and iodothyrin were proven to contain iodine, and to become inactive when the iodine was removed. Efforts to isolate the iodine containing active principle in pure form were fruitless until the work of Kendall at the Mayo Foundation. He obtained it as a white, finely crystalline, odorless and tasteless substance, heat stable, and analyzable. The free form separates as a sheaf of fine needles. Kendall at first called it the a-iodine compound, then named it thyroxin.
There are other internal secretions of the thyroid, with a function of their own, that have no iodine. But they are secondary, and obscure. Thyroxin is accepted today as the purified internal secretion of the thyroid because all the effects of the whole gland may be elicited with it. Thyroxin produces results with doses amazingly minute compared with the quantity of whole gland necessary. Moreover, a dose of thyroxin appears to last an organism in need of it over a period of time; the other has to be administered continuously.
Studies with thyroxin carried on in recent years have rounded out the whole concept of the business of the thyroid in the body economy. One may sum it up by saying that the thyroid secretion is the great controller of the speed of living. The more thyroid one has, the faster one lives; the less one has, the more slowly one lives.
That is not to imply any direct proportion between the amount of thyroid secretion in an individual, and the length of life to which he is destined. The speed of living, in the chemical sense (which is the fundamental sense), and the rate at which the chemical reactions go on that constitute the process of life, are dependent upon the thyroid. When the reactions go faster, more oxygen and food material are burned up or oxidized, more energy is liberated, the metabolic wheel rotates more quickly, the individual senses, feels, thinks and acts more quickly.
Likening one energy machine to another, the thyroid may be compared to the accelerator of an automobile. That is a rough and superficial comparison because an accelerator lets in more of the fuel to be burned up, while the thyroid makes the fuel more combustible. It thus resembles more the primer, for a rich mixture of gasoline and air burns at a greater velocity than a poor one. But the action of thyroid could really be simulated only by some substance that could be introduced into the best possible of gasoline mixtures, to increase its combustibility by a hundred per cent or more. For that is what thyroid will do to our food. Nor has it only this destructive or combustion side. Withal there is at the same time a constructive action, for the process frees energy to be used for heat, motion or other need. The thyroid, therefore, in addition to its rôle as an accelerator, acts, too, as the efficient lubricator for energy transformations. So we see it as accelerator, lubricator and transformer of our energies.
THE GLAND OF ENERGY PRODUCTION
The isolation of thyroxin has made possible the determination of the influence of the thyroid hormone upon the evolution of energy in any higher animal organism. There is, for every individual, a constant, known as the metabolic rate, or the combustion rate, a reading of the rate at which his cells are consuming material for heat. The metabolic rate is thus a gauge of the energy pressure within the organism. It may be calculated by measuring the amount of carbon dioxide gas exhaled during a unit of time, and the number of calories of heat radiated by the skin simultaneously. A simplified device has lately rendered it practicable to make actual determinations by a few five-minute readings of the rate of oxygen absorption by the lungs. Plummer, also connected with the Mayo Foundation, has shown that what would amount to less than a grain of the thyroxin would more than double the amount of energy produced in a unit of time. To be exact, one milligram of thyroxin increases the metabolic rate two per cent. That illustrates some of the power of the internal secretion of the thyroid and its importance to normal life.
THE MOBILIZATION OF ENERGY
But not only is the height of pressure of energy in the cells controlled by the thyroid. The mobility of that energy is also controlled. Without it, rapid and large fluctuations of energy output, and elasticity and flexibility of energy mobilization for any sudden mental or muscular act, let alone an emergency, become impossible. A woman suffering with myxedema, the condition described by the English physician Gull as a cretinoid state supervening in the adult life of woman, has an insufficient amount of thyroxin in her blood and tissues. She is clumsy and awkward and will stumble when endeavoring to walk upstairs. Any effort is almost paralyzed because the range of fluctuation of energy, the ability to mobilize energy, in turn dependent upon an ability to increase the metabolic rate, is limited. In slang phrase, she cannot step on it. Her existence is set to go at a rate in the neighborhood of forty per cent below the normal. By the administration of thyroxin, her metabolic rate can be raised to any desired figure, the spark can be adjusted, so to speak, to any point we like, and it can be so maintained for years.
In the normal animal, to be sure, the internal secretion of the thyroid is not absolutely essential to life. So it contrasts with the hormone of the minute parathyroids placed so closely to it, a minimum dose of which is absolutely a prerequisite for continued life. The fundamental chemical reactions within the cells occur in the complete absense of thyroxin. But they go on in a relatively fixed, rigid and unvarying way, confined within the narrow limits of a constant figure. Under such conditions, the level of energy production is bound to be low, and to remain low, and the modus of its mobilization slow and unwieldy. With thyroid is introduced the trick of catalysis, or the speeding up of the vital chemical reactions, through the agency of an intermediate which accelerates the process. It is par excellence the great catalyst of energy in the body. (A catalyst is an intermediary like the trace of water, which will bring about an explosion between dry oxygen and hydrogen that without it have stayed inert with the strongest currents of electricity.) Thus it supplies a mechanism not only for quantity output of that subtle reality we label energy, but also an apparatus for varying the available amount of it, and for permitting the maximum range in ease and rapidity of its utilization. The thyroid is still another device of life for procuring more and more variation and differentiation, its goal, as far as we can peer through the opalescent screen upon which its manifestations quiver.