PTH

PTH (parathyroid hormone) is secreted by the parathyroid glands and is the main regulator of blood calcium. The test measures PTH levels and is almost always interpreted with calcium measured at the same time. It is performed for high or low calcium, kidney stones, osteoporosis investigations, chronic kidney disease and vitamin D deficiency.

Who is this test for

• people with high or low calcium results;
• people with recurrent kidney stones;
• people with osteoporosis or an unexplained fracture, particularly at a younger age;
• people with chronic kidney disease, to monitor changes in calcium, phosphate and bones;
• people with marked vitamin D deficiency;
• people after thyroid or parathyroid surgery, and those with tingling around the mouth and muscle cramps;
• people with a known parathyroid disorder, for monitoring.

What PTH is and what it shows

The parathyroids are four small, rice-grain-sized glands behind the thyroid. When blood calcium falls, they secrete more PTH. The hormone then releases calcium from bones, reduces urinary calcium loss, increases phosphate excretion and stimulates the kidneys to produce the active form of vitamin D, which increases calcium absorption from food. When calcium is sufficient, PTH secretion falls.

Because of this feedback relationship, PTH alone says little. Only alongside calcium does it show whether the parathyroids are independently overactive, responding to calcium or vitamin D deficiency, or underactive.

Preparation and sample

PTH is measured in venous blood, and calcium is usually measured from the same draw. Fasting is unnecessary unless tests requiring it are performed that day. PTH levels vary throughout the day and night, so blood is most often collected in the morning, and follow-up measurements are best taken at the same time of day.

State which medicines and supplements you take, particularly calcium, vitamin D, diuretics, lithium or osteoporosis medicines, but do not stop them without consulting your doctor. If you take biotin (for example, hair and nail supplements), mention this when your blood is drawn.

How results are interpreted

PTH is expressed in pmol/L or pg/mL (1 pmol/L corresponds to approximately 9,4 pg/mL). In adults it is usually approximately 1,6 to 6,9 pmol/L (around 15 to 65 pg/mL), and exact limits depend on the method and are printed on your report. The doctor interprets results with calcium, vitamin D, phosphate, creatinine and magnesium, and urinary calcium if necessary.

Calcium	PTH	Most common meaning
elevated	elevated or high-normal	primary hyperparathyroidism; less commonly familial hypocalciuric hypercalcaemia or medicines such as lithium
elevated	low	the cause is outside the parathyroids, for example excess vitamin D, sarcoidosis or certain malignancies
normal or low	elevated	secondary hyperparathyroidism: vitamin D deficiency, chronic kidney disease, poor calcium absorption
low	low or low-normal	underactive parathyroids (hypoparathyroidism) or markedly low magnesium

Primary hyperparathyroidism

One or more parathyroids secrete too much PTH regardless of calcium levels, most often due to a benign adenoma in one gland. It is more common in postmenopausal women. It is often discovered incidentally through elevated calcium and may cause kidney stones, bone loss, thirst and frequent urination, fatigue, constipation and low mood. Diagnosis is established through laboratory tests, while ultrasound or scintigraphy locates the abnormal gland if the endocrinologist and surgeon plan surgery.

Secondary hyperparathyroidism

Here the glands are healthy but respond to calcium deficiency. The most common causes are vitamin D deficiency and chronic kidney disease, less commonly intestinal diseases reducing calcium absorption. The cause is treated. In kidney disease, PTH gradually rises as the disease progresses, so the nephrologist monitors it with calcium, phosphate and vitamin D and determines target levels.

Low PTH

Low PTH with elevated calcium is an expected response from healthy glands, so the cause of high calcium is sought elsewhere. Low PTH with low calcium suggests hypoparathyroidism, most often after neck surgery and less commonly autoimmune or inherited disease.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contactpage). Prices for all tests and panels are listed in the service price list. Calcium can be measured from the same draw as PTH and, as directed by your doctor, vitamin D and phosphate.

Frequently asked questions

What does elevated PTH with normal calcium mean?

Most often this is a secondary increase due to vitamin D deficiency or reduced kidney function, so the doctor checks these causes first. If excluded and PTH remains elevated, it may represent an early form of primary hyperparathyroidism, monitored with repeat results.

Can vitamin D deficiency raise PTH?

Yes. Without enough vitamin D, the intestine absorbs calcium less efficiently, so the glands secrete more PTH to maintain it. After vitamin D replacement, PTH often returns within the limits, so testing is repeated after several months.

Is elevated PTH a sign of a tumour?

In primary hyperparathyroidism, the cause is almost always benign, most often an adenoma of one gland, less commonly enlargement of several glands, while malignant parathyroid tumours are very rare. Secondary elevation is not caused by a tumour.

Why is calcium always measured with PTH?

The same PTH value may be entirely appropriate or clearly too high depending on calcium at that moment. Both results are therefore obtained from the same draw and interpreted together.