Hypoparathyroidism

Hypoparathyroidism may be due to a variety of mechanisms, including destruction of parathyroid glands (autoimmune or surgical), abnormal parathyroid gland development, altered regulation of parathyroid hormone (PTH), or impaired PTH action on end organs. The resulting hypocalcemia can trigger a variety of symptoms, ranging from muscle cramps to seizures or heart failure. Manifestations of chronic hypoparathyroidism, however, are quite specific, and include basal ganglia calcifications (resulting in movement disorders), cataracts, and skeletal and dental abnormalities. Laboratory findings in hypoparathyroidism include hypocalcemia with low or inappropriately normal PTH, hyperphosphatemia, and normal renal function. Treatment usually includes correcting the hypocalcemia through calcium and vitamin D supplementation and treatment of the underlying cause.

• Postoperative: most commonly occurs as the result of accidental injury to parathyroids (or their blood supply) during thyroidectomy, parathyroidectomy, or radical neck dissection
• Autoimmune: second most common cause
• Congenital:
  • Parathyroid aplasia or hypoplasia (DiGeorge syndrome)
  • PTH gene mutation
  • Autosomal dominant hypocalcemia
• Nonautoimmune destruction:
  • Infiltration of parathyroid gland (Wilson's disease, hemochromatosis, granulomas, metastases)
  • Radiation-induced destruction
  • Gram-negative sepsis
  • Toxic shock syndrome
  • HIV infection

References:^[1][2]

• Acute manifestations
  • Symptoms of hypocalcemia (e.g., tetany)
  • Chvostek sign: hypocalcemia → hyperexcitable nerves → tapping the facial nerve on the cheek → contraction of facial muscles
  • Trousseau sign: inflate BP cuff → allow for occlusion of brachial artery for a few minutes → carpal spasm
• Chronic manifestations
  • Extrapyramidal disorders : symptoms include parkinsonism, dystonia, hemiballismus, choreoathetosis, oculogyric crises, or dementia
  • Ocular disease: cataracts, keratoconjunctivitis
  • Skeletal: increased bone mineral density, osteosclerosis
  • Dental abnormalities: dental hypoplasia, failure of tooth eruption, defective root formation
  • Cutaneous manifestations: dry, puffy, coarse skin

References:^[2][3][4]

• Hypocalcemia with low or inappropriately normal PTH
• Hyperphosphatemia
• Normal 25-hydroxyvitamin D (25[OH]D)
• Normal or low 1,25-dihydroxyvitamin D (1,25D): low concentration of PTH cannot stimulate renal production of 1,25D
• Normal magnesium
• Normal creatinine

References:^[2][3]

• See differential diagnosis of hypocalcemia.
• Pseudohypoparathyroidism type 1A: end-organ (i.e., bones and kidneys) resistance to parathyroid hormone (PTH) despite sufficient PTH synthesis due to a defective G_s protein alpha subunit
  • Inheritance: autosomal dominant; gene defect inherited from the mother (GNAS gene imprinting)
  • Pathophysiology: mutations in GNAS1 → α subunit is not encoded → impaired activation of adenylate cyclase when PTH binds to G_s → resistance to PTH in kidney and bone tissue
  • Clinical features: Albright hereditary osteodystrophy
    • Round face
    • Short stature
    • Obesity
    • Brachydactyly of the 4^th and 5^th fingers
    • Subcutaneous ossifications
    • Intellectual disability
  • Diagnostics
    • Persistent hypocalcemia despite increased levels of PTH
    • Hyperphosphatemia
• Pseudopseudohypoparathyroidism: extremely rare condition that mimics PHP type 1a but without end-organ resistance to PTH
  • Inheritance: autosomal dominant, defective G_s protein alpha subunit is inherited from the father (GNAS gene imprinting). The normal allele from the mother allows for maintaining the responsiveness of the kidneys to PTH.
  • Clinical features: Albright hereditary osteodystrophy
  • Diagnostics: normal calcium, PTH, and phosphate

The differential diagnoses listed here are not exhaustive.