Hyperparathyroidism and bone

Hyperparathyroidism is a condition of excessive secretion of parathyroid hormones, either by the parathyroid glands or an ectopic focus (usually a malignant tumor such as breast, lung or pancreatic cancer) leading to multisystem manifestations. Bone disease occurs in about half of patients with the disease.

• The parathyroid glands produce parathyroid hormone.
• This hormone helps maintain a proper balance of calcium in the bloodstream and in tissues that depend on calcium to function properly.
• Parathyroid hormone
  • Increases the concentration of calcium in the serum.
  • Decreases the level of phosphate in the serum
• To achieve this goal, this hormone:
  • Increases intestinal absorption of calcium
  • Decreasing renal tubular excretion of calcium

OSTEOCLASTS (BONE RESORPTION CELLS) HAVE RANK RECEPTORS ON THEIR SURFACE WHILE

OSTEOBLASTS (BONE DEPOSITION CELLS) HAVE COMPLEMENTARY RANKL RECEPTORS TO THEIR SURFACE.

• PTH binds to osteoblasts via its own receptors and dissociates RANKL, which goes to and binds to its corresponding RANK receptor on osteoclasts, thereby activating them and causing bone resorption.
• Thus, for bone resorption, PTH acts via osteoblasts which further stimulate bone resorption cells, i.e. osteoclasts.

What happens when there is too much parathyroid hormone circulating in the body?

• Too much PTH tells the body to make more calcium available.
• The body responds with:
  • Release more calcium into the blood from the bone (where most of your calcium is stored).
    • The loss of calcium in the bone weakens them and increases the risk of fractures.
  • The digestive tract absorbs more calcium from the foods you eat.
  • The kidneys hold calcium back into the bloodstream instead of flushing it out in the urine.
    • Too much calcium in your kidneys can cause kidney stones to form.

Presentation

Generalized osteopenia secondary to diffuse bone resorption

• Generalized bone pain: The bones may even be tender to palpation.
• Pathological fractures: Usually involve the dorsolumbar spine, neck and stem of the femur, and pubic branches.
• Brown tumors: In areas of particularly rapid bone loss, hemorrhage, restorative granulation tissue, and vascular fibrous tissue replace normal bone marrow, resulting in a brown tumor.

Symptoms

• Osteoporosis
• Kidney stones
• Excessive urination
• Abdominal pain
• Tired
• Depression
• Bone and joint pain
• Nausea, vomiting or loss of appetite

Radiography

• Generalized osteopenia
  • leading to diffuse bone loss
• Brown tumors:
  • present as expansive lytic cystic lesions seen primarily in:
    • Mandible
    • Maxillary
    • Ribs
    • Clavicle
    • Pelvis
• Subperiosteal resorption of the phalanges.
• Skull with salt and pepper appearance (diffuse dotted lines).
• Horizontal striped vertebrae resulting from alternating bands of bone loss and osteosclerosis in patients with renal osteodystrophy.