How can I differentiate between primary and secondary hyperparathyroidism based on laboratory findings?

To differentiate between primary and secondary hyperparathyroidism based on laboratory findings:

• In primary hyperparathyroidism, laboratory tests typically show elevated or inappropriately normal parathyroid hormone (PTH) levels in the presence of elevated albumin-adjusted serum calcium (usually ≥2.6 mmol/L on repeated measurements).
• In contrast, secondary hyperparathyroidism is characterised by elevated PTH levels but with normal or low serum calcium levels, as the elevated PTH is a compensatory response to hypocalcaemia or vitamin D deficiency.
• Measurement of vitamin D levels is important to identify vitamin D deficiency, a common cause of secondary hyperparathyroidism.
• Urinary calcium excretion tests (such as 24-hour urinary calcium or calcium:creatinine ratio) can help exclude familial hypocalciuric hypercalcaemia, which can mimic primary hyperparathyroidism.
• In primary hyperparathyroidism, PTH is elevated or inappropriately normal despite hypercalcaemia, whereas in secondary hyperparathyroidism, PTH elevation occurs in response to low or normal calcium levels.

Therefore, the key laboratory distinction is the combination of serum calcium and PTH levels: primary hyperparathyroidism shows high calcium with high or inappropriately normal PTH, while secondary hyperparathyroidism shows normal/low calcium with elevated PTH.

Additional tests such as vitamin D measurement and urinary calcium excretion assist in confirming the diagnosis and excluding other causes.

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