1,000 IU = 25ug
Vitamin D: 1 IU is the biological equivalent of 0.025 μg cholecalciferol/ergocalciferol 1000iu=25ug
mcg and ug are both abbreviations for the same unit: micrograms. The "u" in ug should actually be the letter mu, but when you have trouble typing it, you can use "u" instead.
There are a few different ways to read this question. I drop is typically estimated to be 25ul so has a mass of 25ug. Lugol's iodein is made by adding 5g of iodien and 10g of possasium iodide and making up the final volume to 100ml with distilled water. This would mean 15g of solid per 100ml or 0.15 g/ml. 0.15/4 gives 0.0375g of stuff other than the distilled water in a drop.