8259 Hypercalcemia as an Unusual Presentation of Hyperthyroidism

Disclosure: F. Mohammadrezaei: None. A. Mubashir: None. K. Chen: None. Background: Asymptomatic Non-Parathyroid dependent hypercalcemia can occur in 20% of patients with hyperthyroidism [1]. It is uncommon for hyperthyroidism to present with hypercalcemia, and it is rare for calcium levels to exceed 12 mg/dl [2]. Case Presentation: A 65-year-old man was referred to Endocrinology for hypercalcemia with complaint of poor appetite and gradual 10–15-pound weight loss over the past year. Patient was under the care of a rheumatologist for joint pain who noted his calcium levels to be elevated at 12.1 mg/dl (8.2-10.1). Further investigation into hypercalcemia revealed an alkaline phosphatase level of 129 IU/L (36-112), 25OH Vitamin D level of 60.6 NG/ML (30-100), ACE level of 79.8 U/L (9-67), PTH of 18 PG/ML (15-65) and PTHrP levels < 2.0 pmol/L (< 2.0). A CT scan of the chest, abdomen, and pelvis was performed due to hypercalcemia and mildly elevated ACE level and did not reveal any evidence of sarcoidosis; however, it showed enlargement of the thymus. On our initial examination, his vital signs were within normal range and physical examination revealed fine tremors in his upper extremities. On further evaluation, his thyroid function tests (TFTs) showed suppressed TSH < 0.005 MICU/ML (0.39-4.08) and elevated FT4 7.34 NG/DL (0.58-1.64) and patient was initiated on methimazole and propranolol. Thyroid Stimulating Immunoglobulin (TSI) was positive 1.96 IU/L (0.00-0.55) consistent with Graves' disease. Thyroid ultrasound revealed a heterogeneous thyroid with multiple nodules and radioactive iodine uptake and scan displayed multiple "hot and cold" areas consistent with a multinodular goiter (MNG) and elevated uptake of 78% at 24 hours (10-30%). Subsequent fine-needle aspiration (FNA) findings were benign. Following initiation of treatment for hyperthyroidism and normalization of FT4, the patient's unintentional weight loss improved with weight regain and more notably, hypercalcemia resolved, as indicated by calcium levels of 10 and 9.5 at 3 months and 1 year respectively, supporting the diagnosis of hyperthyroidism-related hypercalcemia. Conclusion: The key takeaway from this case is the identification of hypercalcemia as a potential complication of hyperthyroidism, though the precise mechanism remains unclear. It may be related to the stimulation of bone turnover by thyroid hormones. Further studies are needed to uncover the exact underlying pathophysiological mechanism. References: (1) Baxter JD, Bondy PK: Hypercalcemia of thyrotoxicosis. Ann Intern Med. 1966, 65:429-42. 10.7326/0003- 4819-65-3-429. (2) Khan H, Nawaz M, Schofield J, Soran H. Severe hypercalcaemia secondary to relapsed Graves' disease. BMJ Case Rep. 2021 Jan 18;14(1): e238898. Presentation: 6/1/2024