THYROID GLAND, DISEASES OF.—The thyroid gland is an organ situated in the neck, and belonging to the group of what are called the ductless glands (see p. 143). These are structures which, although resembling other glands (like the pancreas or salivary glands) in their general make-up, differ from these in not having any apparent outlet through which to pour out the products they manufacture. Other glands belonging to this class are the thymus gland, the suprarenal capsules, and the spleen. Although it has been very difficult to find out much about the functions of these organs, and even now a great deal remains to be learned concerning them, they seem, through the agency of the blood-stream that circulates through them, to play a considerable part in controlling the complicated chemical and other processes that are constantly going on in the living body.
The thyroid gland consists of two halves, which are situated on either side of the trachea, or windpipe, near the level of the larynx (Adam's apple), and are connected by a bridge of gland-tissue passing in front of the wind pipe, and called the isthmus of the gland. The entire organ usually weighs about one and one-half ounces, and is proportionately larger in women than in men. It is composed of a very large number of minute cavities lined with a single layer of cells, and filled with a glairy, mucous-like substance ; and it has a very rich blood-supply. Although it was long ago recognised that the swelling in the neck called goitre was due to enlargement of the thyroid gland, it is only comparatively recently that any real understanding of the function of the organ in relation to the general working of the body has been gained. Surgeons observed that, if in performing operations for the cure of goitre, the precaution was not taken of leaving behind a small portion of the gland, very serious disturbances to the health of the patient resulted. The experiment was then made of removing the gland completely from animals, such as dogs or cats, and it was discovered that this always resulted in the death of the animal from general weakness and inability to absorb nourishment, within about four weeks. It was furthermore found, by experimenting on animals, that if the gland was completely removed from the neck, but a small portion was transplanted to the interior of the abdominal cavity, death did not follow. The very great importance of the thyroid gland in preserving the general health was thus demonstrated, but it has not been possible to find out in just what way the gland exerts its influence. Two main theories have been suggested : (t) that the gland manufactures some product which enters the blood, and has an effect on the body processes; and (2) that the gland in some way neutralises or alters harmful substances carried to it by the blood-stream. While the question is not yet settled,
most authorities incline to the former view, though it is not impossible that, to some extent at least, the gland may perform both offices.
The diseases of the thyroid gland may be divided into three main groups : (t) Tumours of the thyroid gland, or goitre ; (2) exophthalmic goitre, which is the result of over-activity of the gland ; (3) myxcedema and cretinism, which are conditions due to insufficient activity or absence of the gland.
• Goitre.—Like all other glands, the thyroid is sometimes the seat of cancer, but the disease in this situation does not differ in any important feature from cancer in other parts of the body, and it is mentioned here only to emphasise the necessity for obtaining medical advice at once when any swelling in the neck is noticed, as only prompt treatment can save life if cancer is present. The most frequent disease of the thyroid gland is simple goitre. This consists of an enlargement of one or both halves of the gland, which may be so slight as to cause only moderate thickening of the neck, or may be so extreme that a large tumour results. At the age of puberty a slight enlargement of the thyroid gland is frequently seen in girls, and some women at each menstrual period manifest a noticeable increase in the circumference of the neck ; but this is due only to congestion of the gland, and usually does not lead to any permanent change. There are some parts of the world (for instance, certain regions in Switzerland, Italy, France and England) in which goitre is very common, and the growths attain great size. In the United States it is said to be most common about the Great Lakes and in the mountainous regions of Pennsylvania ; but most cases of goitre are simply scattered instances of the disease, and do not have any relation to a particular locality. The cause of goitre is still unexplained, though in a general way it is known that some peculiarity of the drinking-water is largely responsible for the frequency with which the disease occurs in the special districts just mentioned. Blows or injuries to the front of the neck, tight neckbands, and the practice of carrying loads on the head are said to favour the development of goitre ; but these are probably not very active causes. The affection is much more common in women than in men, and is a disease of early or middle life.