Calcium Physiology

Calcium is the most abundant mineral in the body and is essential for processes including muscle contraction, blood coagulation, and enzymatic activity. Because only a narrow range of serum ionized calcium (9.0-10.2 mg/100ml) is compatible with life, the body employs a sophisticated endocrine feedback system to maintain balance. This system relies on three primary target tissues: the intestines (absorption), the kidneys (excretion/reabsorption), and the bone (storage/resorption).

Three primary hormones govern calcium and phosphate levels:

• Parathyroid Hormone (PTH): Secreted by the parathyroid glands in response to low serum calcium, PTH increases calcium levels by stimulating bone resorption, enhancing renal calcium reabsorption, and promoting the renal synthesis of active vitamin D.
• Calcitonin (CT): Secreted by the thyroid gland in response to high serum calcium, CT acts to lower blood calcium levels by inhibiting bone resorption and promoting renal calcium excretion.
• Active Vitamin D (1,25-(OH)2D3): Produced in the kidneys, this hormone acts on the intestines to increase calcium absorption and works synergistically with PTH to regulate bone remodeling.

Disruptions in this hormonal axis lead to significant clinical disorders. Hypoparathyroidism results in hypocalcemia and hyperphosphatemia, often causing muscle tetany. Conversely, hyperparathyroidism—frequently caused by benign adenomas—leads to elevated serum calcium, bone loss, and renal stones. Vitamin D deficiency manifests as rickets in children and osteomalacia in adults, characterized by the failure of the bone matrix to calcify. Osteoporosis, the most common bone disorder, involves a state of insufficiently calcified bone, often linked to dietary deficiency, hormonal changes, or chronic immobilization.