16. Calcium-Phosphate Homeostasis: Parathyroid Hormone, Calcitonin, Vitamin D – Physiological Effects and Control of Secretion. Impairment of Calcium and Phosphate Homeostasis

Physiological Effects and Hormonal Regulation

(parathyroid hormone and calcitonin)

1. Parathyroid Hormone (PTH)

• Secreted by the principal (chief) cells of the parathyroid glands.

• Actions:

  • Kidneys: Increases calcium reabsorption and promotes phosphate excretion to avoid calcium-phosphate precipitation and kidney stones.

  • Bone: Stimulates osteoclast activity to break down bone matrix, releasing calcium and phosphate into the bloodstream.

  • Vitamin D: Stimulates the formation of active vitamin D (calcitriol) in the kidneys, enhancing intestinal calcium absorption.

• Control of Secretion:

  • Low blood calcium levels stimulate PTH secretion.

  • High blood calcium levels inhibit PTH secretion via negative feedback.

  • Vitamin D also inhibits PTH production.

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2. Calcitonin

• Produced by the parafollicular (C) cells of the thyroid gland.

• Actions:

  • Lowers blood calcium by inhibiting osteoclasts, reducing bone resorption.

• Control of Secretion:

  • Increased blood calcium stimulates calcitonin release.

  • Low calcium levels inhibit calcitonin secretion.

Physiological Effects and Hormonal Regulation

calcitriol

• Produced mainly in the kidneys (activated form).

• Actions:

  • Increases intestinal absorption of calcium and phosphate.

  • Supports bone mineralization.

• Control of Secretion:

  • Stimulated by PTH.

  • High levels of vitamin D inhibit PTH secretion (negative feedback).

  • Elevated phosphate levels inhibit vitamin D activation.

Impairment of Calcium and Phosphate Homeostasis

impairment of Calcium and Phosphate Homeostasis

• Normal levels:

  • Calcium (Ca²⁺): 2.12–2.62 mmol/L

  • Phosphate (P): 0.7–1.7 mmol/L

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1. Hypocalcaemia (Low blood calcium)

• Causes:

  • Hypoparathyroidism (low PTH secretion)

  • Vitamin D deficiency

  • Renal diseases (reduced calcitriol synthesis)

  • Hyperphosphatemia (high phosphate binds calcium, lowering free calcium)

• Symptoms:

  • Muscle spasms (tetany)

  • Urinary dysfunction

  • Increased bone fragility and fractures

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2. Hypercalcaemia (High blood calcium)

• Causes:

  • Hyperparathyroidism (excess PTH)

  • Vitamin D intoxication (excessive vitamin D)

  • Malignant tumors producing PTH-like peptides (paraneoplastic syndrome)

• Symptoms:

  • Kidney stones (due to calcium precipitation)

  • Bone pain (due to increased resorption)

  • Muscle weakness

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