Calcitonin

Overview

Calcitonin (CT) is a 32-amino acid linear peptide hormone synthesized and secreted by the parafollicular C cells of the thyroid gland. Discovered in 1961 by Copp and Cameron, calcitonin was initially characterized as a calcium-lowering hormone antagonistic to parathyroid hormone (PTH). In humans, calcitonin plays a relatively modest role in calcium homeostasis under physiological conditions, though its pharmacological properties have proven therapeutically valuable.

Structure

Calcitonin contains 32 amino acids arranged in a single polypeptide chain with a seven-residue intramolecular disulfide loop between cysteine residues at positions 1 and 7. The C-terminal proline amide is essential for full biological activity. The sequence is highly conserved across species, with particularly strong homology in the N-terminal region. Salmon calcitonin (sCT) shares 50% sequence identity with human calcitonin and exhibits 40-50 times greater potency at the human calcitonin receptor, a property exploited therapeutically.

The calcitonin peptide adopts a predominantly alpha-helical conformation in membrane-mimetic environments, with the N-terminal disulfide loop and C-terminal amidation both contributing to receptor binding affinity and efficacy.

Calcitonin Receptor

Calcitonin signals through the calcitonin receptor (CTR), a class B G-protein-coupled receptor. The CTR exists in two principal isoforms generated by alternative RNA splicing: the C1a and C1b variants, which differ in the presence or absence of a 16-amino acid insert in the first intracellular loop. This insert modifies G-protein coupling specificity, with C1a preferring Gs and C1b coupling to both Gs and Gq pathways.

The CTR is expressed on osteoclasts (mediating bone resorption inhibition), osteoblasts (modulating bone formation), renal tubular cells, and neurons. Receptor activity-modifying proteins (RAMPs) interact with the CTR to create amylin receptors, demonstrating the multifunctional nature of this receptor system.

Physiological Functions

Calcitonin inhibits osteoclastic bone resorption by binding to CTRs on mature osteoclasts, reducing their ruffled border formation and resorptive activity. This action lowers serum calcium and phosphate concentrations. In the kidney, calcitonin inhibits calcium and phosphate reabsorption in the distal tubule and thick ascending limb.

The physiological significance of calcitonin in humans remains debated. Patients who have undergone total thyroidectomy do not develop clinically significant hypercalcemia, suggesting that PTH is the primary regulator of calcium homeostasis. Calcitonin may function more importantly during periods of increased calcium demand, such as pregnancy and lactation.

Clinical Applications

Salmon calcitonin (Miacalcin) is approved for the treatment of Paget disease of bone, hypercalcemia of malignancy, and postmenopausal osteoporosis. The nasal formulation provides convenient administration with reduced gastrointestinal side effects compared to the injectable preparation. Calcitonin also serves as a tumor marker for medullary thyroid carcinoma, where elevated levels reflect C-cell hyperplasia or neoplasia.