This patient with refractory vitamin D deficiency, despite aggressive attempts at repletion, should be screened for celiac disease. The fact that supplementation with a therapeutic dose of vitamin D has failed to replete her body stores should raise concern for a malabsorption disorder. Based on her history of another autoimmune disorder (vitiligo), lower range BMI, and nonspecific signs and symptoms such as fatigue, subclinical celiac disease would be a reasonable cause of her malabsorption. Celiac disease may present with classic symptoms of diarrhea, overt malabsorption, and weight loss, but may also exist in a very mild form and may go largely undetected since patients have only nonspecific symptoms and subclinical malabsorption. If this patient tests positive for celiac disease, removing gluten from her diet will improve her intestinal lining and improve absorption of vitamin D. Even with therapy, patients with malabsorption will likely require increased doses of vitamin D supplementation.

Parathyroid imaging with a sestamibi scan is not indicated because the patient does not have primary hyperparathyroidism. Her parathyroid hormone level is elevated as an appropriate physiologic response to her markedly low vitamin D levels. Once her vitamin D levels are sufficient (>30 ng/dL [75 nmol/L]), her parathyroid hormone level should be remeasured to ensure that it has returned to the normal range. The parathyroid hormone level should be remeasured in approximately 4 weeks.

Referral for parathyroidectomy would also not be indicated in this patient without an established diagnosis of primary hyperparathyroidism.

A person her age with normal diet and minimal sun exposure should require about 1000 U daily of vitamin D to maintain adequate vitamin D stores. The choice to use cholecalciferol versus ergocalciferol is often based on the level of vitamin D deficit. Since the ergocalciferol is more readily available in the 50,000 U form and has a shorter half-life, it is recommended when a patient's vitamin D level is less than 10 ng/mL (25 nmol/L). Cholecalciferol is often used when the level is between 20 and 30 ng/mL (50-75 nmol/L) or for maintenance and therefore would not be ideal to replete this patient. Clinical discretion can be used for levels between 10 and 20 ng/mL (25-50 nmol/L).

Although vitamin D₃ (cholecalciferol) is a reasonable option for treatment of vitamin D deficiency, as mentioned above, cholecalciferol is best used for maintenance of vitamin D levels or repletion when the 25-hydroxyvitamin D level is between 20 ng/mL (50 nmol/L) and 30 ng/mL (75 nmol/L). Neither form of repletion, however, will be effective in the presence of significant malabsorption.