We report a case of a 27-year-old woman who presented to our endocrinology clinic with two episodes of severe and symptomatic hypercalcemia associated with thyrotoxicosis in Graves' disease. The degree of hypercalcemia (3.34 mmol/L) is the most significant we have seen documented in the literature. On both occasions, we found that it was responsive to fluids and antithyroid medication. Unusually, we also noted that her severe hypercalcemia was associated with an unsuppressed parathyroid hormone level in the absence of demonstrable parathyroid disease. We conclude that thyrotoxicosis can be associated with a severe and symptomatic hypercalcemia and unsuppressed parathyroid hormone. Our case would suggest that it is important to screen for thyrotoxicosis in a new presentation of hypercalcemia, and that a diagnosis of primary hyperparathyroidism should not be made in the context of thyrotoxicosis.