Fatigue is a vague but common complaint that is poorly characterized by physicians as well as patients. While fatigue may result from a number of different etiologies, at the present time, a comprehensive approach to each patient with fatigue does not include routine measurement of serum vitamin D levels. A 61-year-old man was evaluated for excessive daytime fatigue. No features characteristic for depression, sleep apnea, or narcolepsy were present. A comprehensive work-up, including thyroid function tests and testosterone levels, did not reveal any abnormalities. However, serum 25-hydroxyvitamin D level was low, at 18.4 ng/mL. Vitamin D supplementation was initiated. At follow-up in 3 and 12 months, the patient reported complete resolution of daytime fatigue, corresponding to an increase in his vitamin D levels. Possible mechanisms for clinical improvement include effects of vitamin D on components of inflammatory cascades, including tumor necrosis factor-alpha and prostaglandin D2, which result in decrease in central nervous system homeostatic sleep pressure. While more research is needed to determine if patients presenting with fatigue should be routinely screened for vitamin D deficiency, clinicians should consider obtaining vitamin D levels in patients with unexplained fatigue, nonspecific musculoskeletal pain, and risk factors for vitamin D deficiency.
Keywords: Excessive daytime sleepiness; Fatigue; Vitamin D deficiency.