The parathyroid glands

Position:

• four small separate glands embedded in the back of the thyroid gland, two on each side.
• Hormone: parathormone (PTH). Secreted when blood calcium falls below normal.
• It raises blood calcium by removing calcium from the bones, increasing absorption of calcium in the intestine and reabsorption in the kidneys.

Parathormone and calcitonin are antagonistic and together keep blood calcium within normal levels (important for nerve and muscle activity, blood clotting, and strong bones).

• Hypersecretion (hyperparathyroidism): blood calcium rises; because the calcium comes mainly from the bones, the bones become soft and fragile, liable to bend and fracture; kidney stones may form.
• Hyposecretion (hypoparathyroidism): blood calcium falls → painful convulsions and muscle spasms (tetany), and increased excitability of the nervous system, so the person gets angry at the slightest cause.

Worked example 7. After thyroid surgery in which the parathyroids were accidentally removed, a patient has painful muscle spasms. Why? No parathormone → blood calcium falls → nerves and muscles become over-excitable → tetany. Treatment: calcium and vitamin D.