Four Small Glands That Control Calcium in the Body

Four Small Glands That Control Calcium Throughout the Body

Fatigue, muscle weakness, kidney stones or loss of bone density. At first glance these problems seem unrelated. Sometimes, however, the common thread is four small glands located behind the thyroid – the parathyroid glands.

Their job is to keep the concentration of calcium in the blood within narrow limits. This matters not only for the bones. Calcium is involved in muscle contraction, the transmission of nerve signals and the normal functioning of the heart.

How does parathyroid hormone work?

When blood calcium falls, the parathyroid glands release parathyroid hormone (PTH). It sends signals to the kidneys and bones, and through the activation of vitamin D it also influences the absorption of calcium from the gut.

The kidneys retain more calcium and excrete more phosphorus. The active form of vitamin D supports the absorption of calcium from food. The bones also help maintain the balance: when PTH is elevated for a long time, calcium may be released from them.

In this way the body protects the calcium level in the blood. But if the cause of high PTH persists for a long time, the bones and kidneys may bear the consequences. That is why normal blood calcium does not always mean the whole system is working optimally.

When does PTH rise?

The cause determines how the result is interpreted. There is an important difference between a gland that produces too much hormone on its own and a gland that is responding to a problem elsewhere in the body.

In primary hyperparathyroidism, the problem starts in the parathyroid glands themselves. Most often one of them is enlarged and releases PTH even though blood calcium is already high. The condition may be discovered by chance on routine tests. Possible consequences include reduced bone density and kidney stones.

In secondary hyperparathyroidism, raised PTH is a response to another problem. Causes include vitamin D deficiency, low intake or impaired absorption of calcium, and chronic kidney disease. Here it is important to find out what keeps the hormone high, rather than looking only at its value.

And what happens when PTH is too low?

When there is not enough PTH, blood calcium can fall. This condition is called hypoparathyroidism and most often occurs after surgery in the neck area in which the parathyroid glands were affected.

Low calcium can cause tingling around the mouth and in the fingers, muscle cramps and spasms. Pronounced symptoms require urgent medical assessment. Magnesium should also be checked, because severe magnesium deficiency can impair the release and action of PTH.

Why are calcium and PTH interpreted together?

A single value on its own rarely tells the whole story. When calcium is high, the normal response of the parathyroid glands is to suppress PTH. If PTH stays high or inappropriately “normal”, that is a reason for further assessment. The diagnosis is confirmed with repeat tests and an evaluation of the whole clinical picture.

If calcium is normal but PTH is raised, possible explanations include vitamin D deficiency, a kidney problem, or insufficient calcium intake and absorption. Such a result should not automatically be labelled a parathyroid disease.

It also matters which calcium was measured. The routine test usually shows total calcium, part of which is bound to albumin in the blood. In certain circumstances the doctor may order ionised calcium to get a clearer picture.

Which tests give a fuller picture?

When a disorder in this system is suspected, an endocrinologist may consider:

• calcium and PTH, tested and interpreted together;
• albumin or ionised calcium;
• 25-OH vitamin D, phosphorus and magnesium;
• creatinine and an assessment of kidney function;
• 24-hour urine calcium where indicated;
• bone density testing and kidney assessment, if the diagnosis requires it.

These tests help not only to confirm the cause but also to assess whether the bones and kidneys are affected. In primary hyperparathyroidism, neck imaging is used mainly to locate the gland before surgery, not to make the diagnosis – that is made from blood tests.

What is Agaia's approach?

For us, the most important question is not simply “Is PTH normal?” but “Does PTH match the calcium level, and what is making it change?”

In a person with low vitamin D, a restricted diet or possibly impaired absorption, high PTH may be a sign that the body is compensating. When calcium is high and PTH is not suppressed at the same time, the direction of assessment is different and requires an endocrinologist's evaluation. Diet can support adequate calcium and vitamin D intake, but it does not replace diagnosis when a parathyroid disease is suspected.

Four small glands control a large part of the body's mineral balance. When the results for calcium, PTH, vitamin D and kidney function are looked at together, they can reveal a connection that individual symptoms do not show.