Hypoparathyroidism for USMLE Step 2

Hypoparathyroidism for the USMLE Step 2 Exam

Hypoparathyroidism results from inadequate secretion of parathyroid hormone (PTH), leading to hypocalcemia and hyperphosphatemia. PTH normally regulates serum calcium by promoting bone resorption, increasing renal calcium reabsorption, and enhancing intestinal absorption of calcium via activation of vitamin D.
Key Mechanisms:
Decreased PTH Secretion: Commonly from parathyroid gland damage or destruction.
PTH Resistance: Seen in pseudohypoparathyroidism, where tissues are resistant to PTH, leading to hypocalcemia despite elevated PTH levels.

Post-Surgical Hypoparathyroidism: The most common cause, typically occurring after thyroidectomy or neck surgery where the parathyroid glands are inadvertently damaged or removed.
Autoimmune Hypoparathyroidism: Results from autoimmune destruction of the parathyroid glands, commonly part of polyglandular autoimmune syndrome.
Congenital Causes:
DiGeorge Syndrome: A genetic disorder leading to the congenital absence or underdevelopment of the parathyroid glands.
Familial Hypoparathyroidism: Rare genetic mutations affecting PTH production or action.
Radiation Therapy: Damage to the parathyroid glands due to radiation exposure during cancer treatment in the head or neck.
Hypomagnesemia: Severe magnesium deficiency impairs PTH secretion or function.

Neuromuscular Symptoms:
Tetany: Muscle spasms and cramping, especially in the hands and face.
Chvostek’s Sign: Twitching of the facial muscles upon tapping the facial nerve.
Trousseau’s Sign: Carpal spasm triggered by inflating a blood pressure cuff above systolic pressure.
Paresthesia: Numbness and tingling around the mouth, hands, and feet.
Seizures: Severe hypocalcemia can lower the seizure threshold.

Cardiovascular Symptoms:
Prolonged QT Interval: Hypocalcemia can prolong the QT interval, increasing the risk of arrhythmias.
Hypotension: Low calcium levels may impair smooth muscle function and reduce vascular tone.
Other Features:
Mental Status Changes: Confusion, anxiety, and irritability.
Laryngospasm or Bronchospasm: Life-threatening airway muscle spasms may occur with severe hypocalcemia.

Serum Calcium: Low total and ionized calcium levels.
Serum Phosphate: Elevated due to reduced renal excretion in the absence of PTH.
Serum PTH: Low or inappropriately normal in the presence of hypocalcemia.
Magnesium Levels: Hypomagnesemia can contribute to hypocalcemia and should be corrected if present.
Vitamin D Levels: Should be evaluated to rule out deficiency.
ECG: May show a prolonged QT interval due to hypocalcemia.

Acute Hypocalcemia:
Intravenous Calcium: Calcium gluconate is used for symptomatic hypocalcemia with tetany, seizures, or cardiac complications.
Chronic Hypocalcemia:
Oral Calcium Supplements: Long-term calcium supplementation (calcium carbonate or calcium citrate) to maintain serum calcium within the low-normal range.
Vitamin D Supplementation:
Calcitriol (Active Vitamin D): Necessary for patients with hypoparathyroidism due to impaired endogenous vitamin D activation.
Thiazide Diuretics: May be used to decrease urinary calcium excretion, reducing the risk of nephrolithiasis.
Magnesium Supplementation: Correct hypomagnesemia if present to improve PTH function.
Recombinant Human PTH (rhPTH):
Natpara (rhPTH 1-84): Can be used in patients with refractory hypocalcemia or those requiring high doses of calcium and vitamin D.

Hypocalcemic Crisis: Severe hypocalcemia may result in life-threatening symptoms, such as tetany, laryngospasm, or arrhythmias, requiring urgent calcium replacement.
Nephrocalcinosis: Can occur with chronic high-dose calcium and vitamin D supplementation, leading to calcium deposition in the kidneys.
Ectopic Calcifications: Chronic hypoparathyroidism can cause calcium deposits in soft tissues, such as the basal ganglia, leading to movement disorders.

Key Points

Hypoparathyroidism is characterized by low PTH secretion or action, most often following neck surgery, leading to hypocalcemia and hyperphosphatemia.
Clinical manifestations include tetany, muscle spasms, prolonged QT interval, and mental status changes. Severe hypocalcemia can cause life-threatening complications like laryngospasm and cardiac arrhythmias.
Diagnosis is confirmed by low serum calcium, elevated phosphate, and low PTH levels. Magnesium deficiency should be corrected as it can worsen hypocalcemia.
Treatment includes acute IV calcium replacement for severe symptoms and long-term management with oral calcium, calcitriol, and magnesium supplementation. Recombinant PTH can be used for patients with refractory disease.