This condition was first described by Sir "William Gull under the designation Cretinoid state. Dr. Ord recorded further cases, and suggested the term Myxcedema. It is a condition due to falling away of the secretion of the thyroid body, this being an internal secretion, as the thyroid is a ductless gland. The absence of the thyroid secretion alters the metabolism of the tissues, the principal result being an excessive production of mucin, with marked degradation of the functions of the nervous system. The relation of these symptoms to the defect of the thyroid is demonstrated by the striking improvement following the administration of extract of the thyroid glands of animals. The potency of the secretion is also shown by the fact that an overdose of the extract leads to pronounced symptoms in which mental excitement and excitement of the heart are prominent.

The disease manifests itself first as a swelling of the skin and subcutaneous tissue generally, which resembles ordinary oedema, but differs in respect that it is not influenced by gravitation, does not pit on pressure, and is obviously a more solid swelling than that of oedema. Besides the change in the skin there are pronounced symptoms pointing to the nervous system, chiefly, weakening of the intellectual powers, dulness of sensibility, muscular weakness, etc.

The condition of the skin is due mainly to the fact that mucin is present in excess, but there are also signs of irritation in the form of accumulation of round cells. The hairs are liable to atrophy and the secretion of the sweat and sebaceous glands is diminished.

The chemical examination of the skin and subcutaneous tissue by Dr. Ord showed a great excess of mucin. His examination was in a case in an early stage with the full swelling of myxcedema still present. Chemical analysis of cases at a later stage shows that in them the mucin is scarcely in excess, and the swelling of the skin is due largely to an increase of the subcutaneous fat. In the experiments in animals by Horsley, in which examinations were made in an early stage, there was found an excess of mucin not only in the skin, but also in the fibrous tissues, blood, and salivary glands. The parotid gland, which normally contains no mucin, presented that, substance in large quantity. It showed on microscopic examination the usual appearances of a mucous gland secreting mucus, namely, gland cells in the acini distended with mucin and having the characteristic appearance of goblet cells.

There are less constant changes of a similar nature elsewhere, as in the mucous membrane of the mouth, and the teeth are liable to drop out like the hairs from the skin. There are also in some cases interstitial changes in the sympathetic nerves, kidneys, liver, heart, and submaxillary glands (Virchow).

Myxoedema has been found uniformly associated with Atrophic disease of the thyroid gland. The gland is usually diminished in size, and has a yellowish or pale appearance instead of the usual dull red colour. The condition seems to depend on an interstitial inflammation of the gland. There is an infiltration of round cells between the vesicles, followed at first by proliferation of the epithelium in the vesicles, but succeeded by atrophy. In a more advanced period the gland tissue is replaced by fibrous tissue in which the remains of the proper gland tissue are represented by clumps of round cells. There is practically a cirrhosis of the gland.

The connection between myxoedema and destruction of the thyroid gland has been shown in various ways. Excision of the gland for disease in man gives rise to a condition virtually identical with myxoedema in a considerable proportion of cases. Kocher described the results of thyrectomy without being aware of the previous descriptions of myxoedema, and the two descriptions are very similar, obviously referring to the same conditions. The myxoedema following excision of the thyroid has been described under the name of Cachexia strumipriva or thyreopriva.

Extirpation of the thyroid gland in animals is followed, especially in the case of monkeys, by conditions closely resembling those of myxoedema, sometimes in an acute, sometimes in a chronic form.

The pituitary body (or hypophysis cerebri) has a structure resembling that of the thyroid gland, and its function is probably similar. In some cases of myxcedema enlargement of the pituitary body has been observed, so that it fills out and causes enlargement of the sella turcica.