Why is high calcium in the bloodstream bad?

Normally, about 99% of calcium is found in bone tissue as part of hydroxyapatite, while only about 1% of calcium circulates in the bloodstream. The body tightly regulates this level, maintaining it even with insufficient calcium intake from food.

If there is not enough calcium in the diet, the body "maintains the norm" in the blood by breaking down bones.

When blood tests show calcium levels above the average norm, it may indicate:

• Active resorption of bone tissue. Parathyroid hormones (PTH) stimulate osteoclasts, the cells that break down bone tissue, to release calcium and maintain its level in the blood. This is a typical mechanism in osteoporosis or hyperparathyroidism.
• Calcium deficiency in tissues with "normal" or high blood levels. In the bones, there is depletion of reserves, while in the blood, there is a "normal" or elevated value due to the mobilization of calcium from deposits. This is similar to a situation where the body withdraws money from a savings account to keep the balance on a card unchanged.
• Increased breakdown of bone tissue (osteoporosis, osteomalacia, postmenopausal period). In these conditions, bone minerals enter the bloodstream, masking the actual calcium deficiency in the skeleton.
• Disorders of vitamin D metabolism. A deficiency of the active form of vitamin D (calcitriol) hinders the normal absorption of calcium from food, forcing the body to use bone calcium.

Thus, high calcium levels in the blood are often a signal of bone calcium loss, which over time leads to bone thinning and osteoporosis.