• Produced by the parafollicular or C cells in the thyroid gland, but it is also synthesized in a wide variety of other tissues, including the lung and intestinal tract.

• 32 amino acid peptide cleaved from a larger prohormone.

Physiologic Effects

• calcitonin has the ability to decrease blood calcium levels at least in part by effects on two well-studied target organs:

• Calcitonin suppresses resorption of bone by inhibiting the activity of osteoclasts
• inhibits tubular reabsorption of calcium and phosphorus, leading to increased rates of their loss in urine.

Compared to Parathyroid hormone, Calcitonin seems toplay a minor role in Calcium and Phosphorus balance.