Parathyroid Gland And Calcium Metabolism Notes

Parathyroid Gland and Calcium Metabolism

Parathyroid gland

Located on the posterior aspect of the thyroid. Number of glands can range between 4 and 150 in adults (usually 4-6).

Produces parathyroid hormone.

Parathyroid hormone

• Made as a pro-hormone and cleaved twice (in liver and kidney)

• Has no serum binding protein

• Half life is 4 minutes (short acting)

Actions

• Raises serum calcium levels

• Changes in calcium concentration stimulate PTH release

• Acts on osteoblasts in bone = release of Ca2+ and PO4-

• Acts on tubular cells in kidney = increased absorption of Ca2+ and decreased absorption (increased excretion) of PO4-

• Stimulates absorption of bone and release of calcium into extracellular fluid

Also stimulates conversion of vitamin D to its active form and increases calcium uptake from the intestine = increased synthesis of active vit D

Skin UV Light

Liver

PTH (+) Kidney (+) Low PO4

Calcitrol (1,25 Vit D)

Intestine – increase Ca + PO4 absorption Bone – mobilize Ca stores

Hyperparathyroidism

Aetiology

• Primary hyperparathyroidism due to;

  • Adenoma

  • Hyperplasia

  • (Rarely) functioning carcinoma

    • Ectopic PTH secretion

  • Calcium high

Clinical features

• Slow onset, usually asymptomatic or incidental finding (see hypercalcaemia notes)

Investigations

• Biochemistry

  • U+E (raised ca, low po in primary. High Cl due to high renal clearance of phosphate and mild renal tubular acidosis)

  • LFT (raised alk. Phos indicates increased bone activity

  • Serum PTH raised

  • Secondary hyperparathyroidism = high phosphate, low calcium

• Radiology

  • Small subperiosteal bone reabsorption cysts in phalanges

  • Reduced spinal bone density

Management

• Surgery

  • Resection of diseased parathyroid glands if serum calcium persistently over 3.0mmol/L (after correction for albumin level)

• Biopsy

  • Determine whether glands are hyperplastic

• Mild symptoms may not require surgery

  • Increase fluid intake to prevent stones

  • Avoid thiazide diuretics

  • Avoid high Ca2+ and vit D intake

Secondary hyperparathyroidism

• Physiological response to hypocalcemia

  • E.g chronic renal failure or dietary vit D deficiency

• Calcium normal

Tertiary

• Chronic secondary hyperparathyroidism results in gland acting autonomously

• Calcium high

• PTH inappropriately high (unlimited by feedback control)

• Seen in chronic renal failure

Malignant hyperparathyroidism

• Some cancers e.g squamous cell carcinoma of lung, breast and renal cell carcinomas produce parathyroid-related protein (PTHrP)

• PTHrP mimics PTH = increased serum calcium

• Reduced PTH seen on assay as PTHrP not detected

Hypoparathyroidism (hypocalcaemia)

Aetiology

• Secondary to thyroid surgery

• Primary idiopathic

  • Organ-specific immune disease

  • Associated with Addisons, pernicious anaemia and malabsorption

• Di George syndrome

• Pseudohypoparathyroidism

  • End organ disease in kidneys or liver where PTH is therefore in excess amounts

Clinical features

• Acute

  • Paraesthesia (mouth, hands – bigger homunculous...

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