What’s the difference between milligram (mg), microgram (mcg), and International Unit (IU)?

As metric units of measurement to quantify weight, 1000 micrograms (mcg) equal 1 milligram (mg). An IU (International Unit) is a unit of measurement for vitamins and other specific biologically active substances. The equivalency of one IU differs from substance to substance and is established by international agreement for each substance. For instance, one IU of vitamin E does not contain the same number of milligrams as one IU of vitamin D. In addition, vitamin E exists in a number of different forms having different biological activities. Rather than specifying the precise type and mass of vitamin E in a supplement, the Supplement Facts label specifies the number of International Units of vitamin E which is based upon what form of vitamin E has been used as an ingredient. The equivalents of 1 IU for selected vitamins are: Vitamin A: 1 IU is the biological equivalent of 0.3 µg retinol, or of 0.6 µg beta-carotene; Vitamin D: 1 IU is the biological equivalent of 0.025 µg ...  more