Although thyroid is one of the most valuable and specific drugs used in therapeutics, its dosage is largely empiric: a given preparation is administered until the desired effect is obtained, but the dosage thus determined does not hold for other preparations and sometimes, as will be shown below, not even for another preparation of the same manufacturer, although it may bear the same label. The doses mentioned in clinical reports are usually meaningless; they seldom convey useful and sometimes convey misleading information. The same is true of the labels on most of the commercial preparations.
This state of affairs, which would be considered intolerable in the case of other important drugs, results from the fact that there is no generally accepted standard for thyroid. There are, moreover, peculiar difficulties in fixing either a chemical or physiologic standard for this drug. Present knowledge of the "active principle" or of principles of