Secondary Hyperparathyroidism
Definition
The parathyroids are four glands in the neck that produce parathyroid hormone to help control calcium metabolism.
Excessive production of this hormone caused by increased activity of these glands is known as hyperparathyroidism. When this occurs in response to low blood calcium caused by another condition, the condition is called secondary hyperparathyroidism.
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Overview, Causes, & Risk Factors
Parathyroid hormone helps maintain blood calcium by regulating bone turnover, absorption of calcium from the gut, and excretion of calcium in the urine.
Many disorders may lead to secondary hyperparathyroidism by causing hypocalcemia (low levels of calcium in the blood), disordered phosphate balance, or both. These include the following:
- Disorders of vitamin D
- osteomalacia (rickets)
- vitamin D deficiency
- vitamin D malabsorption
- abnormal vitamin D metabolism induced by drugs
- Disorders of phosphate metabolism
- malnutrition
- malabsorption
- aluminum toxicity
- kidney disease
- cancers
- phosphate depletion (may also cause osteomalacia)
- Calcium deficiency
- not enough calcium in the diet
- too much calcium loss in the urine
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Chronic renal failure
Chronic renal failure is an important cause of secondary hyperparathyroidism. The disorder is complex:
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Phosphate clearance is impaired.
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Phosphate is released from bone.
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Vitamin D is not produced.
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Intestinal calcium absorption is low.
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Blood levels of calcium are lowered.
A positive feedback loop leads to increased bone resorption (bone is broken down in an attempt to regulate abnormal levels of these chemicals) and hyperphosphatemia (high levels of phosphates in the blood), which causes further secondary hyperparathyroidism.
At-risk populations include children with malnutrition and elderly people with little sun exposure.
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