CALCITONIN IS A HORMONE PRODUCED BY MAMMALIAN THYROID PARAFOLLICULAR CELLS OR THE ULTIMOBRANCHIAL GLAND IN NON-MAMMALIAN VERTEBRATES. ITS SECRETION AND BIOSYNTHESIS ARE REGULATED BY THE PLASMA-CLACIUM CONCENTRATION. IT HAS A HYPOCALCAEMIC ACTION THAT IS DUE PRIMARILY TO INHIBITION OF OSTEOCLASTIC BONE RESORPTION ; OF LESS IMPORTANCE IS A DIRECT EFFECT ON THE KIDNEYS RESULTING IN INCREASED URINARY EXCRETION OF CALCIUM AND PHOSPHORUS. CALCITONIN CONTAINS 32 AMINO ACIDS ; THE SEQUENCE VARIES ACCORDING TO THE SPECIES. SINGLE INJECTIONS OF CALCITONIN CAUSE A MARKED TRANSIENT INHIBITION OF THE ONGOING BONE RESORPTIVE PROCESS. WITH PROLONGED USE, THERE IS A PERSISTENT, SMALLER DECREASE IN THE RATE OF BONE RESORPTION. HISTOLOGICALLY, THIS IS
ASSOCIATED WITH A DECREASED NUMBER OF OSTEOCLASTS AND AN APPARENT DECREASE IN THEIR RESORPTIVE ACTIVITY. DECREASED OSTEOCYTIC RESORPTION MAY ALSO BE INVOLVED. THERE IS SOME EVIDENCE THAT INITIALLY BONE FORMATION MAY BE AUGMENTED BY CALCITONIN THROUGH INCREASED OSTEOBLASTIC ACTIVITY. HOWEVER, CALCITONIN WILL PROBABLY NOT INDUCE A LONG-TERM INCREASE IN BONE FORMATION.