Are serum calcium levels high or low in hyperthyroidism?

Serum Calcium Levels in Hyperthyroidism

Serum calcium levels are typically elevated in hyperthyroidism, though usually only mildly above the normal range. 1, 2

Calcium Metabolism in Hyperthyroidism

Calcium Level Changes

• Patients with hyperthyroidism show higher serum calcium levels (2.36 ± 0.11 mmol/L) compared to euthyroid individuals (2.31 ± 0.11 mmol/L) 1
• The elevation is typically mild and often remains within the upper normal range
• In a large outpatient study, the mean serum calcium before treatment was 2.41 ± 0.21 mmol/L, which decreased to 2.36 ± 0.15 mmol/L after treatment 2

Mechanisms of Hypercalcemia in Hyperthyroidism

• Increased bone turnover with bone resorption exceeding formation 3
• Higher urinary calcium excretion occurs even in subclinical hyperthyroidism compared to euthyroid controls 3
• Reduced calcitonin activity may contribute to hypercalcemia, possibly due to exhaustion of C cells during prolonged hyperthyroidism 4

Bone Metabolism Markers in Hyperthyroidism

• Elevated markers of bone turnover are observed:

  • Increased serum osteocalcin (bone formation marker) 3, 4, 5
  • Increased urinary deoxypyridinoline (bone resorption marker) 3, 4
  • Elevated bone fraction of alkaline phosphatases 4

• These elevations indicate accelerated bone metabolism with a net increase in bone resorption 3

Clinical Implications

• Despite elevated calcium levels, symptomatic hypercalcemia is rare in hyperthyroidism 2
• Only about 5.7% of hyperthyroid patients have calcium levels above 2.65 mmol/L when using appropriate correction factors 2
• Calcium levels correlate with thyroid hormone levels - patients with higher T3 concentrations tend to have higher calcium levels 2
• Persistent increased bone turnover may contribute to accelerated bone loss and increased risk for osteoporosis in patients with Graves' disease 3

Monitoring Considerations

• Regular monitoring of serum calcium is recommended in hyperthyroid patients
• Calcium levels typically normalize after successful treatment of hyperthyroidism 2
• If hypercalcemia persists after treatment of hyperthyroidism, other causes such as primary hyperparathyroidism should be considered 1

Important Caveats

• The relationship between calcium and albumin is altered in hyperthyroidism, which can lead to overestimation of corrected calcium using conventional formulas 2
• Subclinical hyperthyroidism also shows evidence of increased bone turnover and greater calcium excretion, though serum calcium elevations are less pronounced than in overt hyperthyroidism 3
• When evaluating calcium levels in hyperthyroid patients, it's important to consider albumin levels, which are often lower in hyperthyroidism and increase after treatment 2