How much caffeine is in tea?

After reading Imen’s post on the subject at Tea Obsession I decided to write a bit on the subject myself. After reading an extensive amount of information from work and after hearing from other people. I have found that it really depends on a few things for how much caffeine will effect you.
One is diet. I work with vegans, one has stated that Matcha makes her jittery. Much like how I get when I have espresso or an Americano. This is not to say her diet is at fault but I believe it to be a big factor.
Two is health. I don’t have much to say on this as it should be straight forward. Some medications though, such as estrogen, can be effected by caffeine and have negative( or if your lucky positive) effects of the medication. Estrogen, according to a study, get’s reduced in the body up to 13% if you are consuming a high amount of green tea ( it stated just that one) because of the caffeine.
Three is level of activity. This should go into health but I find it separate. I have been watching my weight (not necessary but I do) and going for long long walks. This is not a high level of activity but it is something that falls into the category. This won’t necessarily effect how you take caffeine I think.

These three things count for many other things as well. But diet for me is the most important. If you consume alot of products which have caffeine in it already (energy drinks etc) then perhaps your body becomes immune to its effects. This is only a theory. We do not know.