In secondary hyperparathyroidism the parathyroids are secreting excess parathyroid hormone in response to an initial low blood calcium level. The main causes of this are:
1. Kidney failure - the main cause of secondary hyperparathyroidism. The kidney fails to convert 25-cholecalciferol into 1,25-cholecalciferol, the active form of vitamin D. This means that the intestine absorbs calcium less readily from the diet producing a low blood calcium. The failing kidney also fails to excrete enough phosphate. Both these factors contribute to an increase in secretion of parathyroid hormone.
2. Sprue - this is a disorder where there is a generalised malabsorption of nutrients from the diet resulting in a low blood calcium and a subsequent increase in parathyroid hormone release.
3. Vitamin D and Calcium deficiency - a diet deficient in calcium or vitamin D (required to absorb calcium) leads to low blood calcium and increased parathyroid hormone release.
Secondary hyperparathyroidism is essentially treated by targeting the underlying cause of the hypocalcaemia driving the excess PTH secretion. This might be by kidney transplant in chronic renal failure, medical treatment of sprue and replacement of calcium and vitamin D for patients deficient in these. It is only when there is evidence that the glands are becoming autonomous and hypercalcaemia (raised blood calcium) begins to develop that parathyroid surgery is considered. This state is sometimes known as tertiary hyperparathyroidism, and is dealt with in its own section.