Mg and mcg both measure weight, while IU measures biological activity rather than mass, and that is the whole distinction between them. One bottle can use all three at once. A thyroid formula dosed at 2 mL per day makes a useful example, because a serving that small still has to account for iodine, selenium and vitamin D3 in three separate systems.
Why mg, mcg and IU on supplement labels are not one scale
Milligrams and micrograms are the same measurement at two scales. A microgram is one thousandth of a milligram, so a nutrient written in mcg will always look larger in number and smaller in substance than its neighbor written in mg. That is arithmetic, not potency.
IU is the odd one out. An international unit is a measured amount of biological effect agreed on for one specific substance, and there is no single factor that turns it into a weight. Vitamin D, vitamin A and vitamin E each carry their own conversion, and none of them transfers to the others.
What the 2 mL on a dropper does and does not tell you
Volume is not weight, and a dropper measures volume. The thyroid support drops for women describe a 2 mL daily serving, and only a small fraction of that liquid is the minerals you came for. The number tells you how far to fill the dropper, nothing about how much iodine arrives with it.
That distinction bites hardest when the ingredient is measured in micrograms. A small change in concentration shifts the delivered amount a lot, and two products with the same 2 mL serving can carry quite different quantities. The description lists iodine, selenium, zinc, L-tyrosine, ashwagandha, kelp extract, taurine, schisandra and vitamin D3, and it notes that 2 mL daily is the whole serving, but it doesn't print a per serving figure for any single one of them. If you want those numbers, the Supplement Facts panel on the physical bottle is the only place they live.
Where IU shows up and why you can't convert it
Vitamin D3 appears in that same thyroid formula, and it's the one ingredient on the list that would conventionally be expressed in IU rather than in a weight. The copy doesn't print the figure. So the honest answer to how much vitamin D3 is in a 2 mL serving is that the product description doesn't say, and no amount of unit conversion will produce it.
This is more common than shoppers expect. Marketing text is written to describe ingredients and intent, while the quantitative panel is a separate legal block, and only the second one has to carry numbers. When a description omits a number you need, read the omission as information rather than as a gap you can fill in yourself.
Milligrams on a magnesium label are not elemental magnesium
Magnesium bisglycinate is a compound, not an element. The description calls it a fully chelated form in which magnesium is paired with two glycine molecules, which means any weight given for the compound includes the weight of those glycine molecules as well. The elemental magnesium figure is a smaller number than the compound weight, and the label has to tell you which one it is printing.
The magnesium bisglycinate gummies also carry apigenin, glycine, L-theanine and vitamin B6 in the active P-5-P form, in a 60 count bottle with no melatonin. The description explains each ingredient's role and doesn't state the elemental magnesium per serving, so the panel is again the only source. The magnesium collection lists several forms side by side, and comparing them by compound weight alone will mislead you every time.
Reading a count when the panel isn't in front of you
A count is the one number on a bottle that needs no interpretation at all. The magnesium bottle is 60 gummies, and that's it. What you can do with it is arithmetic: divide by the serving size printed on the panel and you have the number of days the bottle lasts.
Counts also hide a comparison problem, because two 60 count bottles can deliver very different daily totals and the count alone won't reveal which is more concentrated. The iron gummies for teen girls are described for ages 13 and up, with vitamin C, B12, zinc and folate supplied as L-5-MTHF, and the copy describes the chelated form in detail without printing an elemental iron figure in milligrams. The same listing also carries a warm weather note, place the sealed bottle in the fridge for 15 to 20 minutes if the gummies clump, which is one of the few time based units you'll find printed anywhere on a label.
How to compare two bottles that use different units
Put everything into the smallest unit before you compare anything. Convert milligrams to micrograms first, and leave IU alone entirely, since it doesn't join the same table. Then compare per serving rather than per bottle, because bottle size is a pricing decision and not a dose decision.
The iron collection and the iodine collection show the same pattern across the catalog: forms described carefully, quantities reserved for the panel. Once you expect that split, the unit question stops being confusing and becomes a simple matter of knowing which block of the label you're reading.
Is a microgram smaller than a milligram?
Yes. A microgram is one thousandth of a milligram, which is why iodine and selenium are usually written that way. The small unit signals a small requirement, not a weak ingredient.
Why do drops list mL instead of mg?
Because a dropper measures volume, not mass. The thyroid drops use a 2 mL daily serving, and the weight of minerals inside it has to come from the Supplement Facts panel.
Can I convert IU to milligrams myself?
Not with one rule. Each nutrient has its own conversion factor, and vitamin D, A and E don't share one. Look up the factor for that specific nutrient before doing any math.
Do units matter if I just follow the serving size?
No, not for ordinary daily use. Units start to matter when you compare two bottles or stack two products, and that is exactly when conversion errors happen.