Cretinism is a very similar but more severe condition resulting from disease of the thyroid during intra-uterine life or in early childhood, usually appearing during the first year. The child does not develop, remains a dwarf, there is more or less complete loss of mind, the lips are very thick, tongue large, and the abdomen very pendulous. Frequently several members of a family are found to be suffering from it. The state is also occasionally markedly hereditary.

If the thyroid secretion be increased, there may result exophthalmic goiter, or Basedow's disease. It is characterized by enlargement of the thyroid gland, paroxysms of palpitation of the heart, bulging of the eyes, and nervous excitement. In this the administration of sheep's thyroid increases the symptoms.

The relationship between the thyroid gland and general disease is not clearly understood. There appears to be distinct bearing upon the nervous system and also upon the metabolic processes taking place within the body. The active principle seems to be "thyroidin," a substance that contains nearly 10 per cent, of iodin.