MON-735 A Case Of Vitamin D Deficiency Resistant To Treatment: A Diagnostic Challenge

Abstract Disclosure: A. Bassatne: None. S. Tobolsky: None. A.O. Malabanan: None. Background: Vitamin D deficiency is prevalent worldwide due to inadequate sun exposure, limited dietary sources, and malabsorption disorders. Deficiency is associated with impaired calcium homeostasis, osteoporosis, increased fracture risk, and muscle weakness, as well as a higher susceptibility to infections, autoimmune and cardiovascular diseases. Therefore, management through supplementation, dietary intake, and ultraviolet B (UVB) exposure is essential. Clinical Case: A 55-year-old female patient with history of ovarian tumor s/p hysterectomy and oophorectomy at age 38 presented to clinic for management of vitamin D deficiency and osteoporosis. She reports long lasting body aches and myalgias. On evaluation, she was found to have undetectable 25(OH)D levels (<4 ng/ml; Ref >20 ng/ml). Other labs were significant for elevated PTH levels 242 pg/ml (Ref: 11-90 pg/ml) as well as elevated Alkaline Phosphatase (168 U/L; Ref 25-100 U/L). Her calcium and phosphorus levels were normal. DXA scan revealed osteoporosis at the femoral neck (T-score -4.5) and total hip (T-score -3.8). She was trialed on different doses of ergocalciferol (D2) and cholecalciferol (D3) up to 50,000 IU/day in addition to improving UVB exposure with tanning beds and UVB lamps. However, her 25(OH)D levels did not respond and remained suboptimal. As part of a research study, she had an absorption test which revealed undetectable D2 levels after receiving 100,000 IU of D2 the night prior. We did also try calcifediol which improved 25(OH)D levels slightly to 10 ng/ml (Ref >20 ng/ml). Due to suboptimal response, we underwent evaluation for underlying conditions causing resistance to treatment. Screening for celiac disease was negative. We did screen for pancreatic insufficiency by measuring other fat-soluble vitamins which revealed low vitamin A (21 mcg/dl; Ref: 38-98 mcg/dl), vitamin E and vitamin K levels were normal. In the absence of gastrointestinal symptoms, our gastroenterology colleagues advised against further workup with fecal elastase. She did not have evidence of liver or kidney disease. Our workup did not reveal any specific cause for vitamin D deficiency resistant to treatment, we therefore referred the patient to genetics to evaluate for underlying CYP2R1 mutations or vitamin D binding protein polymorphisms. Conclusion: We present a case of vitamin D deficiency resistant to treatment and discuss the thorough workup for underlying causes. Although malabsorption is possible in this patient as evident by her absorption test, deficiency of other fat-soluble vitamins and mild response to calcifediol which is hydrophilic; we would expect patient to have some degree of gastrointestinal symptoms and would expect 25(OH)D levels to improve with UVB exposure as this would bypass the absorption pathway. However, this was not the case in our patient and the cause of her resistance to treatment remains a challenge. Presentation: Monday, July 14, 2025