Calcium and parathyroid hormone (PTH): bone metabolism

Calcium and parathyroid hormone (PTH) are two values that should always be read together. In isolation, each gives only a fragment of the picture: calcium within the reference range does not rule out a disorder, and an elevated PTH without calcium makes it impossible to distinguish vitamin D deficiency from a parathyroid gland problem. Together they form a map of bone metabolism and parathyroid function — and it is precisely this paired interpretation that this article addresses.

After 40, these two markers take on particular importance: a natural decline in vitamin D levels, reduced intestinal calcium absorption, and hormonal changes make bone health screening at least as relevant as monitoring cholesterol or glucose.

Total vs ionized calcium

Most laboratories measure total serum calcium. The reference range is typically 2.10–2.55 mmol/L, though the exact limits can vary slightly depending on the laboratory and measurement method — always check the reference interval printed on your own report.

Ionized calcium (~1.15–1.32 mmol/L) is the biologically active fraction: it is the form that regulates neuromuscular excitability, PTH secretion, and bone mineralisation. Total calcium reflects the sum of ionized calcium and the fraction bound to proteins, predominantly albumin.

An important nuance: in hypoalbuminaemia (low albumin, for example in liver disease, nephrotic syndrome, or severe malnutrition) total calcium will be artificially low, while ionized calcium remains normal. In such cases, the Payne correction formula is applied or ionized calcium is measured directly. For most screening situations, total calcium is sufficient.

PTH and its role

Parathyroid hormone is produced by the parathyroid glands and is the principal regulator of calcium homeostasis. It acts to raise serum calcium through three mechanisms: mobilising calcium from bone, enhancing calcium reabsorption in the kidneys, and stimulating the activation of vitamin D (conversion of 25(OH)D to active 1,25(OH)₂D), which increases intestinal calcium absorption.

The reference range for PTH is typically 15–65 pg/mL, though this is method- and laboratory-dependent. The key physiological relationship: PTH responds to calcium inversely — when calcium falls, PTH rises; when calcium rises, PTH is suppressed. This inverse relationship is the foundation for paired interpretation.

Interpretation table (4 quadrants)

Calcium	PTH	Most likely interpretation
↑ elevated	↑ elevated or high-normal	Primary hyperparathyroidism — parathyroid gland investigation warranted
↑ elevated	↓ low (appropriately suppressed)	Non-PTH-dependent hypercalcaemia (malignancy, sarcoidosis, hypervitaminosis D)
Normal	↑ elevated	Secondary hyperparathyroidism — most often vitamin D deficiency or CKD
↓ low	↑ elevated	Vitamin D deficiency + reduced calcium absorption (typical pattern)
↓ low	↓ low	Hypoparathyroidism (uncommon; post-surgical or autoimmune)

The table reflects typical clinical patterns. Final interpretation always requires clinical context and assessment by a clinician.

Secondary hyperparathyroidism from vitamin D deficiency

This is the most common finding in bone health screening of women over 40. The mechanism is straightforward: low 25(OH)D → reduced intestinal calcium absorption → PTH rises compensatorily to maintain normal serum calcium. Calcium itself remains within the reference range — which is exactly why a calcium test alone would miss this situation.

From a bone perspective, this is clinically significant: chronically elevated PTH stimulates osteoclasts and accelerates bone resorption even when calcium is normal. Treatment is correction of the vitamin D deficit (for detail on levels and approaches, see the article on vitamin D). Once 25(OH)D normalises, PTH typically returns to the normal range on its own.

The Endocrine Society (Holick et al., 2011) recommends maintaining 25(OH)D at no less than 50 nmol/L (≈ 20 ng/mL) to prevent secondary hyperparathyroidism.

Primary hyperparathyroidism

Primary hyperparathyroidism arises from autonomous PTH hypersecretion — most often due to a benign adenoma of a single parathyroid gland. It predominates in women aged 50–70 and is frequently discovered incidentally on a routine blood panel.

The hallmark finding is persistently elevated calcium combined with elevated or high-normal PTH (Bilezikian et al., 2014; NICE NG146). In asymptomatic disease, a person may have no complaints for years while bones are gradually losing density and the risk of nephrolithiasis is rising.

When primary hyperparathyroidism is suspected, the workup typically includes: 24-hour urine calcium, neck ultrasound (parathyroid glands), endocrinology referral, and assessment of kidney function — including creatinine and eGFR, since CKD is itself a cause of secondary hyperparathyroidism and must be differentiated.

Relationship to osteoporosis

The minimum laboratory set for evaluating bone metabolism includes calcium, PTH, and 25(OH)D. These three markers together allow identification of whether there is a deficiency- or hyperparathyroid-driven contribution to existing or potential bone loss.

DEXA (dual-energy X-ray absorptiometry) is a separate diagnostic procedure. According to NOF guidelines (Cosman et al., 2014), it is recommended for all women from age 65 onwards; earlier in the presence of risk factors (prior fracture, prolonged glucocorticoid use, low body weight, smoking, and others).

Persistently elevated PTH — regardless of cause — accelerates bone resorption and is an independent risk factor for osteoporosis that requires investigation and management of the underlying cause.

When to measure

The paired calcium + PTH test (usually together with 25(OH)D) is appropriate in the following situations:

Established vitamin D deficiency with suspected secondary hyperparathyroidism
Incidental finding of elevated calcium on any laboratory test
Osteoporosis or a low-trauma fracture at a relatively young age
Nephrolithiasis (kidney stones) — particularly recurrent
Chronic kidney disease (CKD) — routine protocol monitoring
Women over 40 as part of an extended bone health screening panel (optional; this is not a USPSTF recommendation — the decision is made individually with a clinician)

How HealthLab helps

Calcium and PTH are markers where the trend over time often matters more than a single result. Secondary hyperparathyroidism should normalise after vitamin D correction — and seeing that treatment response on a chart is far more useful than comparing paper printouts side by side.

HealthLab automatically recognises calcium, PTH, vitamin D, and other biomarkers from PDF lab reports of any laboratory and plots trends with reference range overlays. You see the full picture without manual data entry.

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Frequently asked questions

My PTH is elevated but calcium is normal — is that a problem?

This pattern most often indicates secondary hyperparathyroidism caused by vitamin D deficiency. The body raises PTH compensatorily to maintain normal serum calcium. The first step is to check 25(OH)D and, if needed, correct the deficiency. Once vitamin D normalises, PTH typically returns to the reference range on its own. Chronically elevated PTH accelerates bone resorption, so it is worth discussing with your doctor.

Should I test ionized calcium instead of total calcium?

For most screening situations, total calcium is sufficient. Ionized calcium is appropriate when there is reason to suspect hypoalbuminaemia (liver disease, nephrotic syndrome, malnutrition) or when total calcium and the clinical picture are discordant. In those cases, direct measurement of ionized calcium gives a more accurate result.

How should I prepare for a PTH test?

Strict fasting before a PTH blood draw is not mandatory, but a morning draw is preferred — PTH has a diurnal rhythm and is generally higher in the morning. On the day of the draw it is advisable to hold calcium and vitamin D supplements to avoid distorting the result. For any other medications, check with the doctor ordering the test.

Does elevated calcium mean cancer?

Not necessarily. Primary hyperparathyroidism is the most common cause of hypercalcaemia in outpatient practice and is benign in the vast majority of cases (a parathyroid adenoma). Malignancy-associated hypercalcaemia is a distinct clinical context that is typically accompanied by other findings and is identified in the setting of established or strongly suspected malignant disease. The exact cause of elevated calcium is always determined by a clinician based on the full clinical picture.

This material does not replace a consultation with a doctor. Interpretation of laboratory results always requires clinical context.