Perathyroid Glands

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The parathyroid glands are small endocrine glands There are usually four, and sometimes up to eight,
parathyroid glands, which are usually located close to the thyroid gland.
Parathormone (PTH), the parathyroid hormone, is important in calcium balance, acting at two sites:
The bone surface, where it stimulates with the release of calcium.
The renal tubules, where it stimulates the resorp-tion of calcium from the urine, minimizing phosphate

Increased PTH secretion is usually due to the presence of a benign parathyroid adenoma
Parathyroid adenoma is usually a solitary tumour that affects only one of the parathyroids,
the main presenting symptoms being due to excessive secretion of parathyroid hormone
(primary hyperparathyroidism), which produces the symptoms and signs of hypercalcaemia.
Variations in serum calcium level are usually kept within the normal physiological range by
limited variation in PTH secretion; inappropriate increase or decrease in PTH secretion leads to
pathological increase (hypercalcaemia) or decrease (hypocalcaemia) in serum calcium levels.
Malignant parathyroid tumours, with invasion and metastatic spread, are very rare,
although some parathyroid adenomata may show considerable pleomorphism and nuclear
and cytoplasmic atypia.
Primary parathyroid adenoma is only one of the possible causes of hypercalcaemia.
Rarely, primary hyperparathyroidism is the result of diffuse hyperplasia of the
parathyroids rather than a solitary tumour.

Diffuse hyperplasia of all parathyroid glands is usually a compensatory response to
persistently low serum calcium levels
The most common cause of compensatory parathyroid hyperplasia is in renal failure, in which
excessive urinary loss of calcium leads to a persistent serum hypocalcaemia.
The parathyroid hyperplasia produces increased PTH secretion, which mobilizes calcium from bone
by stimulating increased osteoclastic activity.
However, the calcium so released only brings the serum calcium level closer to normal,
never producing hypercalcaemia. This compensatory device is called secondary hyperparathyroidism
and is associated with a normal or low serum calcium.
Very rarely, an autonomous parathyroid adenoma may arise in one of the hyperplastic parathyroids
(tertiary hyperparathyroidism).

Hypoparathyroidism is most commonly the result of surgical removal of the parathyroid glands
The parathyroid glands may be removed inadvertently or deliberately during surgery on the thyroid gland.
If not surgically excised, they may be severely damaged by operative trauma or by
interference with their blood supply.
Much less commonly, the parathyroid glands may be damaged by an autoimmune disease process
(autoimmune parathyroid disease) associated with the presence of an auto-antibody; this usually
occurs in patients who have another autoimmune endocrine disease, e.g. Hashimoto's
disease or Addison's disease
Reduced PTH secretion leads to a reduction in the serum calcium, with a corresponding increase
in serum phosphate levels. Hypoparathyroidism is only one of the causes of hypo-calcaemia.
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Revised: 02-11-2014.