HI!
The yorkie standard, as I've always understood it - is under 7 pounds. Therefore, there is no such thing as teacup - it is truly a marketing term that is often used by breeders trying to sell you a puppy with an inflated price - because it's a puppy that has a projected smaller adult weight. Hypoglycemia is a risk associated with all yorkie puppies - so you really have to be aware of that in any case.
Smaller yorkies are NOT guaranteed to have more health problems than larger yorkies, but sometimes do IF they are bred to BE smaller. That said, runts sometimes do have health problems bc of issues in the womb (and after birth). There are healthy 2-3 pound yorkies and non-healthy 2-3 pound yorkies. Same w/ 7-8 pound yorkies.
Smaller yorkies do have their own unique risks - as in, open fontanels are more common, liver shunt may be more common, risk of injury, surgery is riskier -etc etc - but their size in and of itself does not just mean an unhealthy dog. A healthy dog, in my mind - is created by a healthy mom and dad - and a great, conscientious, top-notch breeder.