There is clinical uncertainty as to the testing of serum 25-Hydroxy vitamin D (25[OH]D) concentrations and when to use high-dose supplementation. Data show that there has been a rapid increase in the number of tests performed within the Northumbria Healthcare NHS Foundation Trust over the past 8 years and an increase in high-dose supplementation over the past 5 years. We performed a retrospective analysis of the 25(OH)D test requests over the period from January to October 2017. A total of 17,405 tests were performed in this time period. The overall average concentration was 57.5 nmol/L and this figure was similar across age groups, although a larger proportion of patients aged over 75 had a concentration <25 nmol/L. Test requests were classified into ‘appropriate’, ‘inappropriate’ and ‘uncertain’ categories based on current expert opinion. We found that between 70.4% and 77.5% of tests could be inappropriate, depending on whether the ‘uncertain’ categories of falls and osteoporosis are considered to be justified. Tiredness, fatigue or exhaustion was the reason for testing in 22.4% of requests. We suggest that a more rational approach to testing, and subsequent treating, could lead to reductions in costs to the healthcare system and patients.