THE THYROID AND THE PARATHYROIDS.
By virtue both of its complex functions and complicated affections the thyroid is an object of surgical interest. Between it and some of the most important body changes there is intimate relation, and its effects on disposition, mentality, voice, general appearance and behavior, sexual function and the development of the sexual organs are matters of common knowledge. The latter features are abundantly illustrated by the effect upon these parts of removal of the thyroid. Embryologically it develops from the floor of the pharynx, between the upper branchial arches. Within it there forms a duct, known as the thyroglossal duct, whose glossal portion opens upon the base of the tongue, where it is of great surgical importance, because of growths of embryonic origin occurring along its path, and because downward growth of cancer of the posterior portion of the tongue usually takes the same course. In the early days of its existence it contains no iodine, at least in mankind, this lack of iodine being supplied from the mother’s milk, the babe thus receiving from its mother that which its own thyroid at first fails to supply.[49]
[49] This may afford an explanation of the unsatisfactory character of artificial foods, as well as of cows’ milk, since, unlike the babe, the calf is born with a functionating thyroid. Cows’ milk does not contain in this respect that which is found in human milk, all of which may afford a reason for adding minute amounts of thyroid extract for a short time to artificial foods for children.
That the thyroid normally produces substances of vital import in the human economy is shown both by the bad effects of their overproduction, as in tetany and certain spasmodic affections, with final clonic rigidity, and in thyroidism or the hyperthyroidism of Graves’ disease or exophthalmic goitre, with its tachycardia, mental depression, and numerous other symptoms, and by those of its underactivity, as in myxedema, cretinism, cachexia strumipriva, and certain of the toxemic neuroses.
The relations between the thyroid and the genital organs, especially in the female, are in many instances pronounced. Menstrual suppression and pregnancy are often followed by thyroidal enlargement, and nearly every woman having a goitre notes its temporary enlargement with each menstrual epoch, and its permanent enlargement with each succeeding pregnancy. The most specific constituent of the thyroglobulin, which is supposed to be the substance formed within the thyroid, is iodine, which is present in variable amounts.
In general it may be said of the thyroid: (1) That it secretes some material requisite for normal nutrition; (2) that it has much to do with the assimilation of oxygen by the tissues as well as with phosphorus metabolism; (3) that its peculiar secretion has a marked effect in lowering blood pressure and quickening the pulse, in the former respect being the direct antagonist of adrenalin.
The parathyroids have only recently assumed importance either in surgery or pathology. Their existence as separate structures with an identity of their own was first demonstrated by Sanström in 1860. Up to his time they had been assumed to be small accessory thyroids. In 1884 they were described by Horsley. Since that time they have been an object of the greatest interest to experimenters. They are of different character from the thyroid proper. Nevertheless the two are not absolutely independent of each other, for removal of either one causes changes in the other; the symptoms caused by removal of the parathyroids, especially, including tremors and various nervous symptoms, of which tachycardia and sometimes exophthalmos are the most prominent. Experimental animals will usually survive removal of the thyroid alone, but to take away all four of the parathyroids is almost invariably fatal. Anatomically they consist of two pairs of small bodies, with an average diameter of ¹⁄₂₅ inch, having color and texture much like the thyroid in gross appearance, but containing epithelioid cells, lying in man in close relation to the lateral lobes of the thyroid, behind them and to their inner sides. In minute structure they resemble that of the pituitary body. Their relatively trifling size and deep position in man have caused them to be neglected in pathology, and to be seldom recognized during operations or except by a careful dissection made for the purpose. The present trend of opinion, especially among the experimenters, ascribes to them an important role in the production of exophthalmic goitre, it being made to appear that by some neglect of duty their function of indirectly regulating the heart and controlling the sympathetic system is not properly performed.
CONGENITAL AFFECTIONS OF THE THYROID.
Congenital affections of the thyroid may assume the type either of defect or of absence of the organ, or an hypertrophy which may involve one side or both. Presumably when the thyroid is lacking its function is to some extent assumed by the thymus and perhaps by other portions of the body.
Anatomical alterations are met in the so-called supernumerary or accessory thyroids, which may be due to separate development of one of the original lobules, or they may arise independently. These vary in size, location, and importance. They may be seen as high as the base of the tongue or as low as the upper end of the sternum or behind it. Tissue of this kind has been seen in the body of the hyoid bone. These accessory masses are subject to the same type of affections as those which involve the principal thyroid, and thus tumors may develop in the anterior region of the neck, which may cause some perplexity.