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

Differentiating primary from secondary hyperparathyroidism involves assessing clinical presentation and laboratory findings.

• Primary hyperparathyroidism (PHPT) typically presents with hypercalcaemia (albumin-adjusted serum calcium ≥2.6 mmol/L) and elevated or inappropriately normal parathyroid hormone (PTH) levels. Symptoms may include thirst, frequent urination, constipation, osteoporosis, fragility fractures, and renal stones. PTH is usually elevated or above the midpoint of the reference range concurrent with hypercalcaemia NICE NG132.
• Secondary hyperparathyroidism is characterised by elevated PTH levels in response to hypocalcaemia or normal/low serum calcium, often due to chronic kidney disease or vitamin D deficiency. Serum calcium is typically low or normal, not elevated NICE CKS,NICE NG132.
• Vitamin D measurement is important: deficiency supports secondary hyperparathyroidism, whereas in PHPT vitamin D may be low but calcium remains elevated NICE NG132.
• Urinary calcium excretion helps differentiate PHPT from familial hypocalciuric hypercalcaemia but is less relevant for distinguishing primary vs secondary hyperparathyroidism NICE NG132.
• In PHPT, bone mineral density loss is often seen, especially at the distal radius, hip, and lumbar spine, and renal imaging may show stones or nephrocalcinosis; these complications are less typical in secondary hyperparathyroidism NICE CKS,NICE NG132.

Summary: Primary hyperparathyroidism presents with hypercalcaemia and elevated PTH, often with symptoms or complications of calcium excess. Secondary hyperparathyroidism presents with elevated PTH but normal or low calcium, usually due to another cause such as vitamin D deficiency or chronic kidney disease NICE CKS,NICE NG132.