In secondary hyperparathyroidism, the causes of the disease are not abnormalities of the parathyroid glands, but disorders such as rickets, vitamin D deficiency and chronic kidney failure. These disorders lead to excessive secretion of parathyroid hormone, resulting in abnormally elevated calcium levels in the blood.

Common type of the disease: renal hyperparathyroidism

A common type of secondary hyperparathyroidism is renal hyperparathyroidism.
When the kidneys chronically fail to function (chronic kidney failure), phosphorus cannot be excreted and vitamin D3 cannot be activated in the kidneys. Lowered levels of activated vitamin D3 cause a decrease in the amount of calcium absorbed in the intestines.
In individuals with chronic kidney failure, therefore, phosphorus levels in the blood increase, while calcium levels in the blood decrease. This results in the stimulation of the parathyroid glands, inducing the secretion of parathyroid hormone. After being stimulated continuously for long periods, the parathyroid glands become enlarged and will eventually continue to secrete parathyroid hormone in spite of the calcium levels in the blood. As a result, the calcium levels in the blood become abnormally high.
This condition is called renal hyperparathyroidism.

Excessive secretion of parathyroid hormone results in the release of calcium from bones into the blood, leading to a condition called "osteitis fibrosa" in which the bones become fragile. This can be the cause of illnesses such as bone aches, bone deformation and pathologic fractures.
Elevated calcium levels in the blood caused by excessive secretion of parathyroid hormone can also result in abnormal deposits of calcium in various locations in the body (ectopic calcification). These calcium deposits, consequently, lead to disorders such as arteriosclerosis (thickening and hardening of the walls of arteries), valvular disease of the heart and arthritis (inflammation of joints).