Secondary hyperparathyroidism - Disorders of the parathyroids

Secondary hyperparathyroidism

Definition

This is a syndrome of appropriately raised parathyroid hormone (PTH) in response to hypocalcaemia.

Incidence/prevalence

Increasing because of survival of renal patients on dialysis.

Aetiology

Common causes of chronic hypocalcaemia are chronic renal failure and vitamin D deficiency.

Pathophysiology

1. Chronic renal failure leads to reduced hydroxylation of inactive vitamin D (25-hydroxycholecalciferol 25(OH)D3) to the active vitamin D (calcitriol or 1,25(OH)2D3) in the kidney, and hence a functional vitamin D deficiency.

2. Chronic hypocalcaemia caused by vitamin D deficiency stimulates chronically increased parathyroid hormone secretion, which may cause some restoration of serum calcium. The failing kidney also retains phosphate which binds calcium, reducing serum levels further.

3. The pathological effects are due to raised PTH levels which cause loss of calcium from the skeleton.

Clinical features

This condition is usually asymptomatic and chronic, although hyperparathyroidism may cause vague bone pains. Hypocalcaemia is rarely severe.

Complications

Tertiary hyperparathyroidism (hypercalcaemia due to autonomous parathyroids).

Investigations

Low or normal serum calcium, with a raised PTH. Phosphate is high. Skeletal X-ray classically shows subperiosteal erosions, ‘brown tumours’ which are areas of radiolucency which mimic lytic bone lesions and a ground-glass appearance of the skull.

Management

Dietary calcium and Vitamin D supplements. For renal patients alfacalcidol and calcitriol are suitable forms of vitamin D, as they do not require hydroxylation by the kidney to become active.