PTH (parathyroid hormone)

Behind your thyroid lie four glands the size of a grain of rice, the parathyroid glands. They do one thing: measure the calcium in your blood and adjust it. If it falls, they release PTH, parathyroid hormone, and that works in three places at once. It pulls calcium out of your bones, makes your kidneys excrete less calcium and more phosphate, and sets in motion the conversion of vitamin D in the kidneys, so your gut absorbs more calcium. If calcium rises, the glands switch off again. PTH is therefore not a value that says something about calcium itself, but about the effort your body puts in to keep it in place. That is also why it can never be read on its own: the same PTH means something different beside a low calcium than beside a high one. Your report shows PTH in pmol/L. Normal is about 1.6 to 4.0 pmol/L where the labs overlap, but the range differs per lab more than for most values, because the assays differ and the hormone breaks down fast; the range on your own report is the most useful here. It is drawn in a chilled tube and processed quickly.

The most common reason PTH sits high is a vitamin D deficiency, and that is exactly why it is interesting for anyone who wants to stay healthy. Without enough vitamin D your gut absorbs too little calcium, the calcium in your blood dips a fraction, and the parathyroid glands pull harder: PTH rises and takes the difference from your bone. You do not notice it, but it costs bone for years. A slightly raised PTH with a normal calcium is therefore often the first sign that your vitamin D is too low for your bones, even before the vitamin D number itself raises the alarm. More sun, oily fish and where needed a supplement let PTH fall again. Little calcium in the diet, especially in anyone who does not use dairy, does the same. The second reason is a kidney function that is declining. The kidney converts vitamin D into its active form and excretes phosphate; if it does that less, PTH rises to compensate, and that is one of the ways chronic kidney disease damages bone. The third reason is rarer and different: a parathyroid gland that makes too much by itself, usually from a benign growth. Then PTH sits high while calcium is high too, because the gland no longer listens to the thermostat. That gives osteoporosis, kidney stones, tiredness and low mood, and it is very treatable with a small operation. The distinction from the ordinary, responding rise sits in the calcium beside it, and that is the whole art of reading PTH.

A high PTH is, in order of how often it occurs: a vitamin D deficiency, reduced kidney function, too little calcium in the diet or poor absorption of it, a parathyroid gland making too much by itself, and rarely lithium or an inherited cause. The key is the calcium beside it. High PTH with a low or normal calcium is a gland correcting, and then you look for the cause in vitamin D, the kidneys and the diet. High PTH with a high calcium is a gland that is itself the source, and then investigation of the parathyroid glands belongs to it. A low PTH means the glands are switching off or no longer can: a high calcium coming from elsewhere, from too much vitamin D or calcium as a supplement, or rarely from a malignant condition; damage to the glands after surgery in the neck; or a severe magnesium deficiency, because without magnesium they cannot release PTH. Low PTH with a high calcium is therefore a reason to look further, low PTH with a normal calcium usually not. Always read PTH beside calcium, and beyond that beside vitamin D, phosphate, magnesium and your kidney function. And when comparing over time, keep to one and the same lab, because the ranges differ strongly here.