This article summarizes our main findings concerning calcitonin (CT) physiology and pathology. We have devised a simple extraction method of circulating CT that much improves assay sensitivity and specificity for the measurement of the CT monomer. Combining this extraction method with newer immunometric assays permits a sensitive and exclusive measurement of monomeric CT. CT secretory capacity is 4-5 times lower in women than in men, but there is no significant fall in basal or calcium-stimulated CT levels with age. We also found no evidence for CT deficiency in postmenopausal osteoporosis. On the contrary, changes in circulating CT levels appear to be secondary to the changes in bone turnover, whether in postmenopausal osteoporosis, after estrogen replacement therapy or in renal stone formers. We found a marked CT deficiency after thyroidectomy, radioactive iodine treatment and in patients with congenital hypothyroidism or autoimmune primary hypothyroidism. The deleterious effects of CT deficiency on bone mass remain, however, essentially speculative. Lastly, we have found specific and high-affinity CT receptors on normal circulating T-lymphocytes. Their meaning remains to be demonstrated but we have recently found that the osteolytic cytokines IL-1 and IL-6 are able to markedly reduce the number of CT receptors on T-lymphocytes without changing their affinity.