About calcium balance
The amount of calcium in the body and bloodstream is tightly regulated, as there can be serious consequences if the calcium levels fall too low or becomes too high. This homeostasis is controlled by hormones produced by the thyroid and parathyroid glands, with different hormones acting depending on whether the level of calcium is too high or too low therefore calcium balance is very important.
What happens when calcium levels become too high?
Hypercalcaemia is a term used to describe elevated levels of blood calcium. Hypercalcaemia is a potentially dangerous state can lead to complications such as kidney stones, gallstones, depression, anxiety, fatigue, nausea, vomiting, abdominal pain, increased urination and bone and joint pain.
When calcium levels become elevated, the thyroid gland releases a hormone called calcitonin. The role of calcitonin is to lower the level of calcium in the blood. This is achieved by acting on the bones, the kidneys and also the intestines. In the bone, calcitonin blocks the action of osteoclasts, cells which break down bone to release calcium and phosphate into the blood. When acting on the kidneys, calcitonin prevents calcium being reabsorbed back into the bloodstream, ensuring that more calcium is excreted in the urine. Calcitonin also blocks the absorption of dietary calcium in the intestines, further reducing the levels of calcium in the bloodstream. In the kidney, calcitonin has a secondary effect of increasing the amount of phosphate excreted by the kidneys. When the blood levels of calcium return to normal, the release of calcitonin is inhibited.
What happens when calcium levels become too low?
Hypocalcaemia is the term used to describe the state of low blood calcium levels. As with hypercalcaemia, there are a number of consequences which can result from hypocalcaemia. These include skin rashes, feelings of pins and needles, especially in the hands, feet and around the lips, tetany (involuntary muscle contractions), arrhythmias and convulsions. The nervous system depends on an adequate level of calcium to function properly, and hence when the levels of calcium drop, many of the above signs and symptoms are present.
Parathyroid hormone is produced by the parathyroid gland and is released when blood levels of calcium fall. Its actions are the opposite of calcitonin; acting to increase the level of calcium in the circulation. Parathyroid hormone also exerts its effects on the bone, kidney and the intestine. In the bone, parathyroid hormone indirectly stimulates the activity of osteoclasts, leading to an increase in the amount of bone being broken down and an increase in the amount of calcium and phosphate entering the bloodstream. In the kidney, parathyroid hormone increases the amount of calcium which is reabsorbed into the bloodstream, and as with calcitonin, increases the amount of phosphate which is excreted in the urine. This is important to counteract a subsequent rise in phosphate levels when calcium is released. In the intestine, parathyroid hormone acts to increase the amount of ingested calcium which is absorbed into the bloodstream. This occurs indirectly, by increasing the amount of active vitamin D produced in the body. Active vitamin D is needed to absorb the calcium we obtain from the diet. When blood levels of calcium return to normal, the secretion of parathyroid hormone is inhibited.