Diagnosis of hypoparathyroidism is made by measurement of serum calcium (total and ionized), serum albumin (for correction), phosphate, intact parathyroid hormone (PTH), and 25-hydroxy vitamin D levels. Normal or inappropriately low serum intact parathyroid hormone (PTH) concentration in patients with subnormal serum albumin corrected total or ionized calcium concentration diagnostic of hypoparathyroidism.

• Diagnosis of hypoparathyroidism is made by measurement of serum calcium (total and ionized), serum albumin (for correction), phosphate, intact parathyroid hormone (PTH), and 25-hydroxyvitamin D levels.^[1]
• PTH degrades rapidly at ambient temperatures and the blood sample therefore has to be transported to the laboratory on ice.
• Normal or inappropriately low serum intact parathyroid hormone (PTH) concentration in patients with subnormal serum albumin corrected total or ionized calcium concentration diagnostic of hypoparathyroidism.
• Hypomagnesemia and vitamin D deficiency should be ruled out as cause of hypocalcemia before making a diagnosis of hypoparathyroidism.
• Calculation of corrected total calcium:

• In this formula, serum calcium is measured in mg/dL and serum albumin is measured in gm/dL.

• Laboratory findings consistent with the diagnosis of hypoparathyroidism include:
  • Low parathyroid hormone
  • Low serum calcium level
  • Normal to elevated serum phosphate concentration
    

• Measurement of total serum calcium with automatic techniques has similar or even more reliability than serum ionized calcium measurement.^[2]
• An low serum calcium should be confirmed by repeat measurement.
• Serum albumin should be measured and if found low, corrected calcium should be measured.

• Method of choice for measuring intact parathyroid hormone include Immunoradiometric assay (IMRA) or Immunochemiluminescent assay (ICMA).^[3]

• Serum magnesium concentration should be measured to rule out hypomagnesemia (or sometimes hypermagnesemia) as a cause of hypocalcemia.
• Hypomagnesemia as a contributor to hypocalcemia may be difficult to rule out as serum magnesium levels may be normal even if there depletion of intracellular magnesium stores.
• Serum magnesium decreases to subnormal levels as magnesium depletion progresses.

• Serum 25-hydroxy vitamin D should be measured to rule out vitamin D deficiency as a cause of hypocalcemia.

• Serum albumin should be measured as low albumin will give falsely low total serum calcium.

• 24-Hour urinary calcium excretion is indicated by the urinary calcium:creatinine clearance ratio.
• Hypoparathyroidism and vitamin D deficiency have low urinary calcium excretion.
• Hypocalcemic patients with activating mutations in the extracellular calcium-sensing receptor have a substantially higher urinary calcium:creatinine clearance ratio.^[4]

• 24-hour urinary magnesium level measurement before initiation of treatment for hypocalcemia is useful if magnesium deficiency is detected as a cause of hypocalcemia.
• Elevated or even detectable urinary levels of magnesium suggest magnesium depletion due to renal losses since kidney should conserve magnesium in depleted body stores.