The Thyroid in Relation to Other Endocrine Glands.

In this series of articles on the diseases of the thyroid gland I have been asked to write an article dealing with the relationship of the thyroid gland to the rest of the Endocrine System, showing how it may affect other ductless glands, and in its turn be affected by them. It would seem that an article of this kind should be written by a physician or an experimental endocrinologist rather than by a general surgeon, whose time and energies are devoted more to the operative treatment of the fully established stages of thyroid disease. Intercurrent disturbances of a polyglandular nature are often incidental in these cases, and perhaps the only advantage enjoyed by the surgeon is their study under the stress and strain of operative procedure. These primary and secondary interactions are extremely difficult to interpret or assess. Indeed, the same is true of the whole subject of Endocrinology where, owing to the limitations imposed on clinical science, medical research has had to resort more and more to animal experimentation. This wide and growing field has added much to our knowledge, and has been the chief source not only of our observations on interglandular reactions, but has given us useful tests, such as the Aschheim Zondek test for pregnancy and the Capon test for male sex hormone, which are directly applicable to clinical advantage. On the other hand this mode of investigation has the inherent defect that it deals with species in different stages of evolution, and the results are apt to be confusing and conflicting. For instance, if tadpoles are fed on thyroid it hastens their metamorphosis, but produces dwarf frogs; whereas removal of the thyroid gland in young goats and puppies retards their growth, a condition which corresponds to cretinism in children. Before proceeding, let us for a moment consider the position of the thyroid gland from the evolutionary point of view. There is much to be said in favour of the increasing r6le of importance played by the thyroid in the rise of man from the lower animals. Crile made an important observation in correlating the relative size and weight of the thyroid and adrenal glands in different species of animals. For instance, the size of the adrenal to the thyroid is greater in aggressive types of animal, such. as the lion, than it is in the more defensive types, such as the antelope; whereas in the sluggish types, like the alligator, there is little or no difference. This ratio is entirely upset in man, where, except in the foetal stage, the thyroid is much bigger than the adrenals. Consequently in losing many of his animal attributes, such as muscular strength, energy, agility and endurance; by the dwindling in acuity of his special senses, as that of hearing and smelling; by contraction of his visual fields, although this is compensated for by binocular vision; by a diminution of the aural prima, and by a loss in the quality of his teeth, man might have become a sluggish animal, had it not been for those attributes that we associate with the thyroid gland, which have also been accompanied by the rise of the cerebral cortex. These attributes of the thyroid are correlated with a restless activity and a heightened degree and range of metabolism. Another feature in the function of the thyroid gland is foreshadowed by its anatomical development from the buccal cavity, a feature which it also shares with the anterior lobe of the pituitary gland. Their common origin suggests that they were primarily designed for digestive purposes. It is therefore not surprising that these two glands together should have become primarily responsible for the promotion of normal growth and development.