Secondary hyperparathyroidism

Definition

The parathyroids are four glands in the neck that produce parathyroid hormone (PTH) to help control calcium use by the body.

Too much production of PTH caused by increased activity of these glands is known as hyperparathyroidism. When this occurs in response to low blood calcium that is caused by another condition, the hyperparathyroidism condition is called secondary hyperparathyroidism.

Causes

Parathyroid hormone helps maintain blood calcium by regulating bone turnover, absorption of calcium from the gut, and release of calcium in the urine.

Many disorders may lead to secondary hyperparathyroidism by causing hypocalcemia (low levels of calcium in the blood), a phosphate imbalance, or both. These include the following:

• Disorders of vitamin D
  • Rickets (osteomalacia)
  • Vitamin D deficiency
  • Vitamin D malabsorption
  • Abnormal vitamin D metabolism, induced by drugs

At-risk populations include children with malnutrition and elderly people with little sun exposure.

• Disorders of phosphate metabolism
  • Malnutrition
  • Malabsorption
  • Aluminum toxicity
  • Kidney disease
  • Some types of cancer
  • Phosphate depletion (may also cause osteomalacia)
• Calcium deficiency
  • Not enough calcium in the diet
  • Too much calcium loss in the urine
• Chronic renal failure

Chronic renal failure is an important cause of secondary hyperparathyroidism. The disorder is complex:

• Not enough phosphate is cleared from the body.
• Phosphate is released from bone.
• Vitamin D is not produced.
• Absorption of calcium in the gut is low.
• Blood levels of calcium are lowered.

Bone is broken down in an attempt to regulate abnormal levels of the above chemicals, and the high levels of phosphates in the blood rise higher. These conditions cause further secondary hyperparathyroidism.