Source of Calcitonin

Calcitonin secreted by-

Correct Answer: Thyroid

Calcitonin is secreted by the Parafollicular cells (C cells) of the Thyroid gland. It acts to lower blood calcium by promoting Ca²⁺ deposition in bone.

Two Different Cell Types of Thyroid

• Follicular cells (majority): Secrete Thyroxin (T₄) and Tri-iodothyronine (T₃) → increase Body Metabolic Rate (BMR)
• Parafollicular cells / 'C' cells: Secrete Calcitonin → decrease blood Ca²⁺

Calcitonin vs. PTH — Antagonistic Pair

• Calcitonin (thyroid C-cells): ↓ blood Ca²⁺ → promotes bone deposition, inhibits osteoclasts, decreases renal Ca²⁺ reabsorption
• PTH / Parathormone (parathyroid glands — 4 glands): ↑ blood Ca²⁺ → increases renal Ca²⁺ reabsorption + phosphorus excretion, activates osteoclasts, stimulates Vitamin D activation

Endocrine Gland Sizes

• Largest endocrine gland: Thyroid
• Smallest endocrine gland: Pineal
• Largest endocrine organ: GIT
• Largest gland overall: Liver
• Largest organ overall: Skin