Did you know that vitamin D is unique amongst vitamin supplements?

It is oil/fat soluble (as opposed to water soluble like most of the others)
It is measured in IU (international units). All other food supplements are measured in milligrams (mg, where 1000 mg = 1 gram).
When supplementing with large doses of vitamin D, a symptom most commonly observed was reduced appetite and nausea. In an effort to avoid these symptoms, early researchers into RDAs (recommended daily allowance) and safe dosages were dealing with very small amounts – less than a milligram.
They established the minimum amount required to avoid developing Ricketts to be 400 – 600 IU and the International Unit (IU) such that 40 IU = 1 microgram (1 ug, and 1000 ug = 1 mg. So the maximum RDA for vitamin D was initially set to 10 – 15 micrograms, or 0,01 – 0,015 mg. Other supplements are typically measured in 10s or 100s of milligrams.

References:

https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

https://youtu.be/TZ0Dm6ULLAw?si=4mz8kFr1CGJPpzby