Hypercalcemia occurs in approximately one of every five patients with thyrotoxicosis, and one of seven patients with hypercalcemia and thyrotoxicosis will have hyperparathyroidism as the cause of the serum calcium elevation. While there are no clinical features which permit easy identification of patients with hyperparathyroidism and thyrotoxicosis, determination of serum parathyroid hormone levels may help. Parathyroid hormone levels may be normal or suppressed if hypercalcemia is due to hyperthyroidism alone, and an elevated parathyroid hormone level suggest coexisting hyperparathyroidism.