Objective: Healthcare costs, including costs for laboratory tests, are increasing worldwide. One example is the measurement of vitamin D. General practitioners in the Capital Region of Denmark include a vitamin D status in approximately 20% of all laboratory requisitions. This study intended to examine the effect of a compulsory pop-up form in the electronic request system on the number of vitamin D tests and to monitor the indications. Design: From 1 January 2017, we introduced a compulsory pop-up form in which the general practitioners had to state the indication for measuring vitamin D, choosing from a predefined set of indications. Intervention practitioners were compared with control practitioners before and after the intervention. Setting: General practices in the Capital Region of Denmark. Subjects: In total, 572 general practitioners and 383,964 patients were included in the period from 1 January 2016 to 31 December 2018. Main outcome measures: Number of vitamin D tests and distribution of indications. Results: We observed a drop in number of vitamin D requisitions to 70% (in 2017) and 75% (in 2018) relative to 2016. During the same period, the number of requisitions increased by 33% in a non-intervention group of practitioners. The indication ‘Monitoring of treatment with vitamin D’ was the most frequently used indication, recorded in 121,475 patients. Conclusion: A compulsory pop-up form reduces the number of vitamin D requests from general practitioners by 25%. The implication is that pop-up forms can be used to decrease healthcare costs.