Diseases of the thyroid gland, particularly the hyperthyroid states, have caused so much confusion among clinicians and pathologists that it is often difficult to place in the same classification identical diseases of this gland with different nomenclature. It is improbable that the great number of terms which can be found in the literature to designate the various diseases of the thyroid gland actually represents such a variety of diseases. This extensive terminology no doubt suggests the multiplicity of interpretations which have been made by those who are concerned with diseases of the thyroid gland. In order to understand the diseased thyroid, it is important to have a clear concept of the variations which may occur in the normal gland during a life span. An examination of some of the textbooks dealing with histology, pathology and diseases of the thyroid reveals that the normal thyroid organ is a gland situated in