Mrs Carter who is 45 year old Caucasian lady was seen last week as she was reporting symptoms of tiredness. Her symptoms had been ongoing for a month. She denies any other systemic symptoms or mood disturbance. Mrs Carter is normally fit and well, with no significant past medical history and is not on any regular medication. She is a non-smoker and drinks three units of alcohol a week. Mrs Carter is a vegetarian and works full-time in a cinema. You did a thorough systems examination and agreed to do blood tests including vitamin D. Results of the blood tests are all normal except the vitamin D level which is 33.
Which one of the following is the most appropriate next step?