Hyperthyroidism and Hypercalcemia: Pathophysiology

• Increased Bone Turnover (Primary Mechanism)

• Thyroid hormones (T3 and T4):
  • Stimulate bone remodeling:
    • But they disproportionately increase:
      • Osteoclastic bone resorption
    • Mechanism:
      • Thyroid hormone increases osteoblast activity:
        • Which in turn stimulates osteoclasts:
          • Via the RANKL pathway
    • This results in:
      • ↑ osteoclast-mediated bone resorption
      • ↑ release of calcium and phosphate from bone
    • Key physiologic effects:
      • Accelerated bone turnover
      • Net bone loss
      • Calcium release into circulation
    • This explains why hyperthyroidism
    • is associated with:
      • Hypercalcemia (usually mild)
      • Hypercalciuria
      • Osteopenia / osteoporosis
  • Increased Sensitivity to Catecholamines:
    • Thyroid hormone enhances β-adrenergic activity:
      • Which further stimulates bone turnover
    • This contributes to:
      • Increased osteoclast activity
      • Further calcium mobilization from bone
  • Suppressed Parathyroid Hormone (PTH):
    • Because calcium increases, the body responds physiologically:
      • Serum calcium rises
      • PTH becomes suppressed
  • Increased Renal Calcium Excretion:
    • Because of the higher filtered calcium load:
      • Hypercalciuria develops
      • Patients may occasionally develop nephrolithiasis:
        • However, stones are much less common than in primary hyperparathyroidism.
  • Increased IL-6 and Cytokine Activity:
    • Hyperthyroidism may increase cytokine signaling such as:
      • IL-6
      • TNF-α
    • These cytokines stimulate osteoclast differentiation and further promote bone resorption
• Typical laboratory pattern:

Test	Finding
Calcium	Mildly ↑
PTH	Suppressed
Phosphate	Normal or mildly ↑
1,25-Vitamin D	Normal
Urinary calcium	↑

This helps differentiate thyrotoxicosis-related hypercalcemia from primary hyperparathyroidism.

• Clinical Characteristics:
  • Typical hypercalcemia seen in hyperthyroidism:
    • Once thyrotoxicosis is treated, calcium levels usually normalize

Characteristic	Feature
Frequency	~15–20% of patients with hyperthyroidism
Severity	Usually mild (Ca 10.5–11.5 mg/dL)
Mechanism	Increased bone resorption
PTH	Suppressed
Treatment	Correction of hyperthyroidism

• Important Surgical Teaching Point:
  • When evaluating hypercalcemia with suppressed PTH, consider:
    • Differential diagnosis
      • Malignancy
      • Hyperthyroidism
      • Vitamin D intoxication
      • Granulomatous disease (sarcoidosis)
      • Medications (thiazides)
  • Thus, thyroid function tests should be obtained in unexplained hypercalcemia
• Key References:
  • Mosekilde L. Hyperthyroidism and bone metabolism. Endocrinol Metab Clin North Am. 1990.
  • Ross DS et al. 2022 American Thyroid Association Guidelines for Thyrotoxicosis. Thyroid. 2022.
  • Mundy GR, Martin TJ. The hypercalcemia of malignancy and endocrine disorders. Metabolism. 1982.
  • Bilezikian JP et al. Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. ASBMR.
• Teaching pearl for residents:
  • If you see hypercalcemia with suppressed PTH and symptoms of thyrotoxicosis, always check TSH before assuming malignancy