Hyperparathyroidism

Hyperparathyroidism is defined as an elevated secretion of PTH, of which there are three main types:
1. Primary—hypersecretion of PTH by adenoma or hyperplasia of the gland.
2. Secondary—physiological increase in PTH secretions in response to hypocalcaemia of any cause.
3. Tertiary—supervention of an autonomous hypersecreting adenoma in long-standing secondary hyperparathyroidism.

Primary hyperparathyroidism
This is the most common of the parathyroid disorders, with a prevalence of about 1 per 800 
It is an important cause of hypercalcaemia.
More than 90% of patients are over 50 years of age and the condition affects females more than males by nearly 3 : 1.

Aetiology

Adenoma 75%  -> Orange−brown, well-encapsulated tumour of various size but seldom > 1 cm diameter Tumours are usually solitary, affecting only one of the parathyroids, the others often showing atrophy; they are deep seated and rarely palpable.

Primary hyperplasia 20%  ->  Diffuse enlargement of all the parathyroid glands

Parathyroid carcinoma 5% -> Usually resembles adenoma but is poorly encapsulated and invasive locally.

Effects of hyperparathyroidism
The clinical effects are the result of hypercalcaemia and bone resorption.
 

Effects of hypercalcaemia:
- Renal stones due to hypercalcuria.
- Excessive calcification of blood vessels.
- Corneal calcification.
- General muscle weakness and tiredness.
- Exacerbation of hypertension and potential shortening of the QT interval.
- Thirst and polyuria (may be dehydrated due to impaired concentrating ability of kidney).
- Anorexia and constipation    

Effects of bone resorption:
- Osteitis fibrosa—increased bone resorption with fibrous replacement in the lacunae.
- ‘Brown tumours’—haemorrhagic and cystic tumour-like areas in the bone, containing large masses of giant osteoclastic cells.
- Osteitis fibrosa cystica (von Recklinghausen disease of bone)—multiple brown tumours combined with osteitis fibrosa.
- Changes may present clinically as bone pain, fracture or deformity.

 about 50% of patients with biochemical evidence of primary hyperparathyroidism are asymptomatic.

Investigations are:
- Biochemical—increased PTH and Ca²⁺ , and decreased PO₄^3- .
- Radiological—90% normal; 10% show evidence of bone resorption, particularly phalangeal erosions.

Management is by rehydration, medical reduction in plasma calcium using bisphosphonates and eventual surgical removal of abnormal parathyroid glands.

Secondary hyperparathyroidism 

This is compensatory hyperplasia of the parathyroid glands, occurring in response to diseases of chronic low serum calcium or increased serum phosphate.
Its causes are:
- Chronic renal failure and some renal tubular disorders (most common cause).
- Steatorrhoea and other malabsorption syndromes.
- Osteomalacia and rickets. 
- Pregnancy and lactation.

Morphological changes of the parathyroid glands are:
- Hyperplastic enlargement of all parathyroid glands, but to a lesser degree than in primary hyperplasia.
- Increase in ‘water clear’ cells and chief cells of the parathyroid glands, with loss of stromal fat cells. 

Clinical manifestations—symptoms of bone resorption are dominant.

Renal osteodystrophy
Skeletal abnormalities, arising as a result of raised PTH secondary to chronic renal disease, are known as renal osteodystrophy.

Pathogenesis

renal Disease + ↓  vit. D activation , ↓ Ca 2+  reabsorption  → ↓  serum Ca 2+ → ↑ PTH → ↓ bone absorption

Abnormalities vary widely according to the nature of the renal lesion, its duration and the age of the patient, but include:
- Osteitis fibrosa .
- Rickets or osteomalacia due to reduced activation of vitamin D.
- Osteosclerosis—increased radiodensity of certain bones, particularly the parts of vertebrae adjacent to the intervertebral discs.

The investigations are both biochemical (raised PTH and normal or lowered Ca 2+ ) and radiological (bone changes).
Management is by treatment of the underlying disease and oral calcium supplements to correct hypocalcaemia.

Tertiary hyperparathyroidism
This condition, resulting from chronic overstimulation of the parathyroid glands in renal failure, causes one or more of the glands to become an autonomous hypersecreting adenoma with resultant hypercalcaemia.