Final answer:
Hyperparathyroidism is an endocrine condition caused by the overproduction of PTH by the parathyroid glands, leading to high blood calcium levels and potential bone density reduction.
Step-by-step explanation:
The condition characterized by sustained secretion of parathyroid hormone (PTH) when calcium levels are normal is known as hyperparathyroidism. This endocrine disorder occurs when the parathyroid glands produce excessive amounts of PTH despite adequate or even high levels of blood calcium. The increased PTH leads to excessive calcium reabsorption from the bones and increased calcium absorption from the diet, which can reduce bone density and increase the risk of fractures.