SAT-382 Thyroid Hormone Overreplacement as a Potential Cause of Elevated Bone Turnover Markers: A Case of Central Hypothyroidism

Abstract Disclosure: T. Aggarwal: None. S. Azad: None. A.T. Drincic: None. Background: The management of central hypothyroidism typically aims to maintain free thyroxine (FT4) levels in the upper half of the reference range, as recommended by current guidelines. However, optimal FT4 targets may vary among individuals, and strict adherence to these guidelines can occasionally lead to subtle signs of overtreatment. Managing central hypothyroidism becomes particularly challenging due to limited utility of thyroid stimulating hormone (TSH), as levels may be inappropriately normal or low despite varying FT4 levels. Case Presentation: We report the case of a 27-year-old male with central hypothyroidism secondary to a pure germinoma of the pineal region with disseminated neuroaxial disease. The patient underwent chemotherapy and radiotherapy and has since been on levothyroxine replacement therapy. His FT4 levels were maintained between 1.2-1.6 ng/dL (0.6 -1.6 ng/dL) per standard recommendations and the patient showed no clinical signs of hyperthyroidismRoutine blood work up showed elevated alkaline phosphatase (ALP) on multiple occasions, with a highest value of 202 U/L (reference range 32-91 U/L). His bone-specific alkaline phosphatase (BSAP) was 70.3 µg/L (reference range 10-28.8 µg/L) indicating bone origin of elevated ALP. Given his history of malignancy, an extensive workup was performed, including a whole-body bone scan, which ruled out metastatic disease. As the cause of the elevated ALP remained unclear, we decided to reduce his levothyroxine dose. This led to gradual normalization of both ALP and BSAP levels in a linear fashion, alongside decreasing FT4 levels, without the emergence of hypothyroid symptoms. Conclusion: This case highlights the importance of considering thyroid hormone overreplacement as a potential cause of elevated ALP after excluding other etiologies. Elevated biochemical bone markers (ALP and BSAP) may manifest in some patients on levothyroxine therapy when FT4 levels are maintained in the upper-normal range. Our findings underscore the need to individualize FT4 targets based on the patient’s clinical and biochemical response, rather than rigidly adhering to guideline-based recommendations. Presentation: Saturday, July 12, 2025