The proper answer would depend on having more in-depth information about the bookcase, as well as the TV.
How tall is the TV, or more importantly, what is the weight? How deep is the TV pedestal?
Will the bookcase be weighted with books? What will the loaded bookcase weight be?
Will this be on a carpeted floor, or hard floor?
I'll try to give you some guidelines.... hopefully it will help you understand the issues at least.
If the bookcase is mostly unloaded, then it should not serve as a stable platform for a large TV like you mentioned. Also, most bookcases are made from low-density particle board... which doesn't have much resistance to warping under load. If there are a lot of vertical dividers/supports, that can prevent the warping.
If the TV is relatively lightweight (as are many of the more recent LED-backlit LCD models - wrongly called "LED HDTV"), AND if you try to load the lowest shelves with heavy items (books, heavy ceramic items, etc), then it could be a stable platform.
Floor type matters a lot too. Carpeted floors are just not stable, which is why most LCD TV stands are rather deep (and short)... to help compensate for that. If your bookcase is on a hard floor, then all the better. Be VERY careful if you're talking about a carpeted floor with your bookcase.
I would "suggest" having at least 2-3 times the TV weight loaded in the bookcase... with most of it near the floor for maximum stability.
Final test... if it wobbles with the TV on it, if it doesn't feel strong enough, just don't trust it.
Blessings!!