Parathyroid hormone (PTH): what it is, what it tells you and when it’s tested

Parathyroid hormone, or PTH, is made by four small glands in your neck, the parathyroids. It keeps blood calcium steady: when calcium falls, PTH rises and brings it back up, drawing on your bones, kidneys and gut. That is why PTH is always read together with calcium and vitamin D.

When it’s tested

Doctors mostly order it when blood calcium is high or low, to see whether the parathyroids are the cause. Also with repeated kidney stones, osteoporosis or very low vitamin D.

It is often tested in people with chronic kidney disease, because kidneys that work less well push PTH up, and after thyroid or parathyroid surgery.

On its own it is not a prevention test: it has to be read with calcium from the same sample. Rosa does not test it. It does measure calcium, vitamin D, creatinine and eGFR, which are the first tests to look at and often show whether PTH needs checking.

What it tells you, and what it does not

High PTH and high calcium: a parathyroid gland is overactive. This is primary hyperparathyroidism, usually from a small benign growth.

High PTH with normal or low calcium: often your body is compensating, for low vitamin D, too little calcium in your diet or kidneys that work less well. This is secondary hyperparathyroidism.

Low PTH and high calcium: the parathyroids have held back, and the cause of the high calcium lies elsewhere. Low PTH and low calcium: the parathyroids work too little, for example after neck surgery.

That is why it is read together with other tests:

• Calcium: Without calcium, PTH cannot be read
• Vitamin D: If it is low, PTH rises to compensate
• Creatinine: Kidneys that work less well push PTH up
• Phosphate: PTH makes your kidneys lose it

Normal PTH levels

The lab decides what is normal. Many lab reports give 15 to 65 pg/mL for intact PTH. Other methods use slightly different limits: the range on your own report is what counts.

In people with enough vitamin D, PTH is lower on average: in one study, leaving out people with low vitamin D brought the upper limit down to about 46. That is why a PTH near the top of the range is always read with vitamin D.

PTH is a fragile hormone: the tube must be processed soon, and it is best to repeat the test at the same lab.

What your result means

Find the row for your result, in pg/mL, and read it with the calcium from the same sample. Which values need a doctor? Above 65, or any value if your calcium is out of range.

• Below 15: Low. With high calcium, your body holds PTH back and the cause lies elsewhere. With low calcium, the parathyroids work too little. Talk to your doctor, soon if your calcium is out of range too.
• 15 to 65: Normal. Usually your parathyroids work well. But with high calcium, even a normal PTH can be too much. Usually nothing to do. If your calcium is high, talk to your doctor.
• Above 65: High. Often from low vitamin D or kidneys that work less well. If calcium is high too, a parathyroid may be overactive. Talk to your doctor: they look at calcium, vitamin D and kidneys from the same time.

If your PTH is high or low

• High PTH: Often from low vitamin D or kidneys that work less well. With high calcium, a parathyroid may be overactive.
• Low PTH: With high calcium, the cause lies elsewhere. With low calcium, the parathyroids work too little.

What causes high PTH

The most common come first. Calcium decides where to start.

• Low vitamin D: The most common cause. Your body absorbs less calcium and PTH rises to compensate.
• Kidneys working less well: In chronic kidney disease PTH rises, sometimes a lot.
• Too little calcium in your diet, or a gut that absorbs it poorly: For example with coeliac disease or after stomach surgery.
• Some medicines: Thiazide diuretics, lithium and some bone medicines can raise it. Do not stop them on your own.
• Primary hyperparathyroidism: A parathyroid gland is overactive, usually from a benign growth. Calcium is often high, sometimes normal.

What can skew the result

A few things shift PTH without any parathyroid problem.

• A tube left standing: PTH breaks down. If the sample is not processed soon, the value can fall.
• Time of day and meals: It changes a little through the day and after eating. That is why a fasting morning test is preferred.
• The lab’s method: Different labs give different numbers. Compare results done in the same place.
• Vitamin D or calcium supplements: They can lower it. Tell your doctor what you take and for how long.

When to worry

Talk to your doctor if:

• Your PTH is above 65 pg/mL, or outside the range on your report.
• Your calcium is high too, even slightly: the reason needs finding.
• You have had kidney stones, or have been found to have osteoporosis.
• You have extreme thirst, vomiting, confusion or great weakness, or cramps and spasms in your hands with tingling around your mouth: call 112 or go to the emergency department.

Frequently asked questions

What are normal PTH levels? Many lab reports give 15 to 65 pg/mL for intact PTH. The range on your own report is what counts, and the result is read with calcium.

What do high PTH and low vitamin D mean? Usually that your body is making up for low vitamin D. Your doctor usually corrects the vitamin D and then checks PTH again.

Is high PTH with normal calcium serious? Often not: it is usually down to low vitamin D or the kidneys. If those causes are ruled out and it stays high, your doctor considers primary hyperparathyroidism with normal calcium.

Do I need to fast for a PTH test? Labs often ask for a morning test, fasting. Follow your lab’s instructions.

Does Rosa test PTH? No. Rosa measures calcium, vitamin D, creatinine and eGFR, the first tests to look at. If your doctor wants PTH, they order it separately.

Sources

• Bilezikian and colleagues, evaluation and management of primary hyperparathyroidism: summary statement and guidelines from the Fifth International Workshop. Journal of Bone and Mineral Research, 2022. PMID 36245251
• Souberbielle and colleagues, the use in clinical practice of parathyroid hormone normative values established in vitamin D-sufficient subjects. Journal of Clinical Endocrinology and Metabolism, 2003. PMID 12915625
• Holick and colleagues, evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2011. PMID 21646368
• ASST Grande Ospedale Metropolitano Niguarda, Milan, laboratory: test sheet for intact parathyroid hormone, with reference values. Ospedale Niguarda, esami di laboratorio 227