As a general rule, the thyroid gland is liable to few diseases, and of these diseases we are almost as ignorant as we are regarding the structure and the function of this organ.

It very frequently presents anomalies of size, being often very much enlarged. The augmentation of size is sometimes transitory and rapid, as, for instance, when it depends upon congestion or inflammation, and sometimes in the case of lymphatic goitre; or there may be a persistent gradual increase, as is observed in the more advanced stages of goitre. It either attacks the whole gland uniformly, which then retains its original shape, or one lobe only, or a small part of one may be the only portion affected, so that the pressure which the gland naturally exerts on the trachea and larynx is variously increased in extent, and may affect not only the pharynx and oesophagus, but also the great vascular and nervous trunks on both sides of the neck, and even the trachea and bronchi, and the blood-vessel within the thorax. Those forms of enlargement are rarer, but at the same time more important, in which the thyroid gland tends to surround the oesophagus like a ring, and in which the isthmus grows downwards so as to form a middle lobe, which descends along the trachea behind the manubrium sterni into the thoracic cavity; in the latter case it becomes transversely contracted when opposite the semilunar notch, but expands immediately below it (asthma thyroideum).

The diminution of size or atrophy of the thyroid gland is an affection of little interest.

Hyperemia of the thyroid gland is not unfrequently observed, and most commonly occurs when there is some mechanical impediment to the emptying of the vena cava descendens and of the right side of the heart. Under these circumstances it may be either transitory or persistent. It may be recognized by the dark color of the gland, its abundance of blood, its looseness of texture, and its swollen condition (hyperaemia, congestive turgescence). Apoplexy of this gland, when its texture is normal, is extremely rare.

Inflammation of the thyroid gland, as a primary affection, is of very rare occurrence, at least as an object of anatomical observation. But we sometimes find what are termed metastatic abscesses in it, especially when there are numerous similar deposits in other organs, consequent on puerperal uterine phlebitis. Abscesses of the thyroid gland may give rise to a deposition of pus in the mediastina, or they may open into the trachea, or, which is most commonly the case, they may enter into the oesophagus on its left side.

The most common disease of the thyroid body is that to which we apply the word struma (using the term in its strict signification), and its most striking characteristic is, as we have already mentioned, an augmentation of size. In the slighter degrees in which it usually occurs, it presents a very simple change of texture depending on a more decided development of the cellular structure of the organ. This occurs either equally through the whole gland, which then everywhere contains cells of equal size, or else we observe one, several, or very many isolated or agglomerated cells larger than the others, which are converted into roundish elongated cysts, with delicate membranous walls, and contain a gummy or glue-like, yellow, brownish or greenish matter (colloid). If this matter has attained a certain consistence, the cut surface of the gland presents a lardaceous appearance, and communicates a peculiar waxy and doughy feeling; the organ is at the same time pale and anaemic, and presents a marked increase of size without any disproportion of form.

There are certain unknown conditions under which, on the one hand, the secretion contained in the dilated cells undergoes modification either from the beginning or during the progress of the disease, or, on the other, the walls undergo a striking change. In the former case we find gelatinous or albuminous substances, of a whitish, gray, or flesh-red color, deposited in the form of concretions, whose coat may be peeled off, or they fill the interstices of an extremely delicate cellular network of new formation. In the latter case the walls of the cells increase in thickness, and the cells become developed (hypertrophied) into sero-fibrous cysts, which may contain various matters besides those already named, and which often attain an astonishing size. These changes constitute those forms of struma, which are known as struma lymphatica and struma cystica.

There can be hardly any doubt that these processes are essentially based on irritation, for repeated inflammations attack the walls of the dilated cells, and especially of the above-named cysts, during the ordinary progress of the disease, although they doubtless often pass unnoticed. Here, as on normal serous, and fibro-serous membranes, they deposit the most varied exudations, and in consequence of the newness of the tissues, these are often hemorrhagic, and accompanied by the separation of large clots of fibrin. These, together with the walls of the cyst, undergo all the same metamorphoses as occur in the exudations and the walls of normal serous sacs (see Vol. III.), even to chalky transformation and ossification. The cysts in this manner not unfrequently become perfectly obliterated by contracting around the exudation, and we then find tough, somewhat voluminous, nodular, osseo-cartilaginous, chalky concretions imbedded in the gland.

True effusion of blood not unfrequently takes place into the cavities of the dilated cells and of the cyst.

The tendency to cyst formation, exhibited by the parenchyma of the thyroid gland, extends in a remarkable manner to the adjacent cellular tissue, for in no situation do we so frequently meet with small or large cysts with serous, gelatinous, or glue-like contents, as in the neighborhood of this organ.

All other adventitious growths, excepting the above-named serous, fibrous, cartilaginous, and bony productions, are extremely rare in the thyroid gland; thus tubercles are scarcely ever found in it, and medullary cancer only very rarely.