This is a randomized, double blind, mono-centric, placebo-controlled interventional study of the effects of oral vitamin D supplementation on self-reported fatigue. Overall 80 patients with proven vitamin D deficiency (20-Hydroxy-Vitamin D ≤ 20 µg/L) who fulfill all the eligibility criteria will be randomized and enrolled to receive vitamin D or placebo in 1:1 ratio. All patients must sign an informed consent prior to enrolment and meet eligibility criteria. All patients will be recruited at the Medical Outpatients Department of University Hospital Zürich.