The ability to measure accurately the hormones regulating calcium homeosta sis is the fundamental first step toward understanding the roles these hormones play in health and disease. Techniques for such measurements have only been available for the past 10 years or so and remain in a state of rapid development. Sensitive parathyroid hormone (PTH) radioimmunoassays appeared in the early 1970s, and with them came a whole new appreciation for the prevalence and implications of hyperparathyroidism, primary or secondary, in the popu lation. The calcitonin (CT) radioimmunoassay came later and...