Calcium

Your body contains about a kilo of calcium, and more than ninety-nine percent of it sits in your bones and teeth. The small part in your blood is the working stock: nerve cells fire with it, muscle fibres contract with it, your heart keeps its rhythm with it and your blood clots with it. Your body holds that part within a narrow band, and it uses your bones as reserve. If the calcium in your blood falls, your parathyroid glands release PTH, which takes calcium from the bones, makes the kidneys hold on to calcium and activates vitamin D so the gut absorbs more. If it rises, the PTH stops and the kidneys excrete the surplus. That is why the calcium in your blood says little about how much calcium you eat: with too little calcium in the diet the blood value stays normal and your bones pay the bill. What does shift the blood value is the regulating loop itself, the parathyroid glands, vitamin D and the kidneys, and the protein the calcium is bound to. About half the calcium in your blood travels bound to albumin. A low albumin gives a low total calcium while the free, working part is normal, and that is the most common reason for a low value. Your report shows calcium in mmol/L. Normal from age 19 is about 2.20 to 2.55 mmol/L, where the labs overlap, one of the narrowest bands on this site. Below 1.80 mmol/L and above 3.00 mmol/L the lab phones the requester the same day.

Calcium is a value with a quiet lower end and a sharp upper end. A slightly lowered total calcium is usually a low albumin or a vitamin D deficiency, and the latter is very common in the Netherlands, with little sun from October to April: the gut then absorbs too little calcium, PTH takes it from the bones and the blood value stays just up at the cost of bone density. Anyone who tops up vitamin D, eats enough calcium and loads their bones with strength training and walking keeps the regulating loop at rest and the bones strong. The upper end is the side to take seriously, even without symptoms. A high calcium in someone who feels well, often found by chance, is usually a parathyroid gland releasing too much PTH; that slowly wears down the bones, gives kidney stones and tiredness, and is curable with a small operation. A high calcium in someone who is ill, with thirst, passing a lot of urine, confusion or bone pain, can be a malignancy releasing calcium from the bones, and that should be investigated the same week. Dehydration and a tight tourniquet also lift the value, which is why a repeat with a calm draw is the first step with a high value that does not fit you. And calcium is a value you read with its neighbours. Albumin says whether the total can be trusted, PTH says whether the parathyroid glands are the driver, vitamin D says whether the gut can absorb enough, and creatinine says whether the kidneys are involved. Without those four a calcium value is half an answer.

A low calcium usually means binding or regulation, and almost never diet. In order of how often it occurs: a low albumin, in which the total falls while the free part is normal, and then corrected calcium or free calcium is the more honest measure; a vitamin D deficiency, in which the gut absorbs too little and PTH takes it from the bones; a kidney disease, in which the kidney no longer activates vitamin D and holds on to phosphate; and a low magnesium, without which the parathyroid glands release no PTH. Rare are parathyroid glands that work too little, usually after a neck operation. Tingling around the mouth and in the fingers, cramps and a cramped hand are the symptoms of a really low free calcium. A high calcium you take seriously. Overactive parathyroid glands are the most common reason in anyone who feels well, and you recognise them by a PTH that is not low beside a high calcium. Dehydration concentrates the blood and lifts the total; a tight tourniquet or a clenched fist does the same during the draw, and both recover with a calm repeat. Medicines such as thiazide diuretics, lithium, too much vitamin D or calcium tablets lift the value. A malignancy, usually with symptoms and a low PTH, is the reason a high calcium in someone who is ill is investigated the same week. Prolonged bed rest lets unloaded bones release calcium. Always read calcium beside albumin, because that says whether the total can be trusted, beside PTH with every deviating value, beside vitamin D and magnesium with a low value, and beside creatinine and the eGFR, which say whether the kidneys are involved. ALP shows whether the bones are remodelling.