In lab results, Thyroxine (and many others) are reported as mg/dl (mass/volume). But Thyrotropin (thyroid-stimulating hormone, TSH) is reported as mIU/l. (We find IU in the specs of some vitamins.) I read that a report expressed in IU is a "function" test, in contrast to a mass/volume measurement. What exactly is an IU in this context? What is the function test? Can the TSH mIU value be converted to a mass value?
If you have an easily denaturable protein or peptide (which I’ll call "stuff"), different preparations will have different ratios of active (good) stuff to non-functional (crappy) stuff. If you define your experiments in terms of mg of the stuff, you will end up with no consistency of the activity of the stuff between batches because different batches will have different good-to-crappy ratios. However, if you test the stuff to see how well it works and measure out amounts based on activity, the you can reproduce your experiments even when you run out of stuff and have to buy another bottle (with its new and different ratio of good stuff to crappy stuff).
I don’t know how the function test for TSH is done. You could convert the activity to mass if you know the ration of good stuff to crappy stuff, but likely you don’t have that number.
I don’t have lab experience. I have lab reports of my blood and am curious why thyroxine hormone is reported as mass/volume but Thyroid Stimulating Hormone is stated in mIU/volume. If this is an international unit, why doesn’t anybody know what it is?