Normal Blood Calcium & Factors That Affect Calcium Balance

Calcium is the most abundant mineral in the body. It is essential for healthy bones, but we also need it for the proper function of our nerves and muscles. Read on to understand more about testing calcium and what important factors affect calcium levels in the body. We’ll also discuss a common, yet often missed cause of high calcium levels.

Why is Calcium Important?

Calcium is the most abundant mineral in the human body. It has many important functions, including [1]:

• Keeping our bones and teeth strong
• Enabling nerve and muscle function
• Helping in blood clotting
• Ensuring proper enzyme function

Approximately 99% of the calcium in our bodies is in our bones and teeth. The remaining 1% of calcium circulates in the blood [1].

Calcium Blood Test

A calcium test measures your calcium levels in the blood. Most commonly, the test estimates your total calcium levels, which include [2, 3, 4]:

• Calcium bound to proteins, mainly albumin (about 40 – 45%)
• Calcium complexed with phosphate and citrate (about 10%)
• Ionized calcium, also known as free or active calcium (about 45 – 50%)

You don’t need to prepare for this test or fast beforehand. A healthcare professional will simply collect a blood sample from your vein that will be analyzed.

Your doctor may also order an ionized (free) calcium test.

Ionized calcium is the body’s active form of calcium. It is assumed to be around 45 – 50% of the total blood calcium, although the exact percentage may vary. Ionized calcium levels are often just an estimate based on total blood calcium and albumin levels. Testing ionized calcium is far more precise, but it requires special equipment and is more expensive [2].

How It’s Used

The calcium blood test is often done as part of a routine metabolic panel. The basic metabolic panel (BMP) checks blood sugar, calcium, and electrolytes, as well as kidney function. The comprehensive metabolic panel (CMP) also includes protein levels and markers of liver function [5].

Your doctor may order a calcium test if you have:

• Symptoms of very high or very low calcium
• Kidney disease (including kidney stones)
• Thyroid disorder
• Parathyroid disease
• Malabsorption
• Cancer

Normal Range

The normal ranges for total calcium in adults are around 8.5 to 10.5 mg/dL or 2.12 – 2.62 mmol/L. These values can vary slightly from lab to lab, depending on the techniques, equipment, and chemicals used [1, 6].

Hypocalcemia (“hypo” = low, “calcemia” = calcium in the blood) is when calcium blood levels drop below 8.5 mg/dL. You can read more about the symptoms and causes of hypocalcemia in our posts about calcium deficiency.

Hypercalcemia (“hyper” = high, “calcemia” = calcium in the blood) is when calcium blood levels rise above 10.5 mg/dL. We detail all the causes of high blood calcium here. However, one cause often goes unnoticed, especially in older people: hyperparathyroidism (see more below).

Factors Affecting Calcium Levels

Minerals and Hormones

Calcium blood levels depend on several important factors, including [6]:

• Parathyroid hormone (PTH)
• Vitamin D
• Phosphorous
• Magnesium

PTH

Calcium blood levels are mainly controlled by parathyroid hormone (PTH). Cells in the parathyroid gland – located in the neck just behind the thyroid – release PTH when they sense a drop in calcium blood levels [7].

PTH raises blood calcium levels by increasing the absorption of calcium from the gut and kidneys. This hormone can also mobilize calcium and phosphate from the bones: that is, it can break down and free the mineral content of bones to compensate for low calcium levels. This process is known as bone resorption [7].

Vitamin D

PTH also increases the amount of vitamin D that is converted to its active form, calcitriol. Active vitamin D increases calcium absorption in the gut and limits the amount of calcium lost via the kidneys [7].

Phosphorus

Phosphorus binds to calcium. When phosphorus rises in the blood, calcium levels tend to drop. Conversely, if phosphorus becomes depleted, calcium levels rise. This can sometimes lead to hypercalcemia (high calcium) [6].

Magnesium

Last, but not least, calcium levels are dependent on magnesium. When magnesium levels are normal, calcium is not affected. But if magnesium levels are low, it can suppress parathyroid hormone (PTH) release. This leads to a drop in calcium levels. That’s why magnesium deficiency can be a cause of hypocalcemia (calcium deficiency) [6].

Hyperparathyroidism: An Underdiagnosed Condition?

Hyperparathyroidism or an overactive parathyroid gland is the most common cause of hypercalcemia (high calcium). An enlargement of one or more of the parathyroid glands (either benignly or due to cancer) will cause this condition [8, 9].

Some studies suggest that hyperparathyroidism may often go unrecognized. According to one study, for example, elevated calcium levels were missed or ignored in 58% of cases. Even when both calcium and PTH were above range, a diagnosis was missed in 28% of cases. High blood calcium can often be wrongly ascribed to other causes, such as diuretics, calcium supplements, dehydration, and impaired kidney function [10, 11].

Symptoms

Common symptoms of hyperparathyroidism include [12, 13, 14, 15]:

• Kidney stones
• Muscle weakness
• Slow digestion
• High blood pressure and other heart problems
• Fragile bones and frequent bone fractures (especially in postmenopausal women)
• Impaired cognition
• Depression
• Anxiety

If you’re above 60 years old and experiencing symptoms of hyperparathyroidism, work with your doctor to find what’s causing it and to address any underlying conditions. You may need to see an endocrinologist to further examine your parathyroid gland and to discuss your lab results [10].