She seems healthy but has always been underweight.

I'm wondering if any of you out there have a similar problem?

Scaramouche is 3 years old and has always been underweight (fluctuates between 4.8 and 5.6 pounds). She has no problem eating, is playful and active, and shows no signs whatsoever of being in bad health.  Vet checkups all came back negative for thyroid issues and worms, but I'm at a loss as to why she's so thin. You can't feel her ribs or backbone, but her hip bones are noticeable when petting her. Her mom was a very tiny girl as well, and I her sister Sox is a small girl too, but is up around 8.5 - 9lbs.

Any ideas as to what may be causing this or if there's anything I can do to help her gain some weight? She's a picky eater as well, and I keep switching food in the hopes she'll find one she really likes, but she treats most dry food the same (however LOVES treats and wet food).

Sox on the left, Scaramouche on the right:

They are both SO beautiful!  Grain-free wet food is best, it's what they would eat in the wild.  There are high protein/high calorie foods the vet can prescribe which would probably put some weight on her, but kitten food would probably do the same thing.  As far as being a picky eater, so many of them are.  There are several things you can use as food toppings to tempt her - slightly warming her food before serving so that the smell entices her,  nutritional yeast (NOT brewer's yeast), Fortiflora feline probiotics which most cats adore, bone broth (I make mine in a slow cooker).  There are also broths in envelopes made by Fancy Feast, and the Fancy Feast appetizers can be used as a food topping.  They have no fillers or preservatives, just solid meat and my cats adore them. And last but not least, a little sprinkle of Parmesan cheese can work wonders - not too much, just a slight sprinkle.  Also, cats in the wild eat 6-7 small meals a day, and she may be the type of housecat that prefers that routine.

Once you can get her to start eating regularly (which may actually be the only problem here), Animal Essentials Plant Enzymes and Probiotics is a great additive to aid healthy digestion and give good immune support (if you use this with Fortiflora, cut the dose).  I highly recommend either NutraCal or EnerCal, which are high calorie nutritious gels you put inside her cheek.  Also, Nu-Pet Feline Granular is an excellent source of vitamins, minerals and antioxidants. I also give mine Nordic Naturals Omega-3 oil, which must be added to food.  Never give a cat an oil via oral syringe, very dangerous.   All of these products are available online at Only Natural Pet or Chewy.  You can get very good prices on Fortiflora feline on eBay, but be sure you find out the expiration date.  

My daughter's cats are small like Scaramouche, in the 5-6 pound range, but they are all in their teens.  If you try adding calories and get no results, especially since she is so young, I would take her back to the vet for more help.  Sometimes a perfect blood panel hides an underlying problem, and since the blood work is so good, her thyroid is fine and she is parasite-free, you may need to have more tests done.  But definitely try the above tips first as long as she is playful, is eating some, drinking filtered water, and is grooming.  She is a little doll and I wish you both the best of luck with putting some meat on her little bones.  

Being a picky eater and underweight usually go along with IBD.  There usually are other symptoms though, vomiting, diarrhea or constipation.  My IBD kitty was thin and a picky eater for he had his first flare when he was only 18 months old.  I think the pickiness comes from a food not agreeing with them so they won't eat it again.

There is a blood test that checks B12 levels in the blood  If that hasn't been done I would get that one done.  If the level is low then it is some type of malabsorption issue probably caused by IBD.

I agree about a possible B-12 deficiency.   I meant to include a link re the bone broth, which contains lots of B vitamins.  Here it is:  http://healthypets.mercola.com/sites/healthypets/archive/2013/12/02/pet-bone-broth.aspx  

Thanks so much for those tips, that's a great help!  I shall give those a try before I consider taking her back to the vet.  I never even thought about warming the food up - great idea! I've used the fancy feast packets before, and both of them love that as an additive

She does the munch 6-7 times a day thing since she was a kitten so I'm guessing that's her eating routine.  She has a ton of energy and no problem grooming so no major worries there

If she LOVES canned food, then I would look for the highest calorie canned foods out there...things like Hounds and Gatos, Natures, Logic, certain of the Wellness Cores.  Also, you could try adding an egg yolk to her canned food.  Loaded with nutrients and calories.  Cooked or raw, doesn't matter, but if raw, make sure you don't include the whites.

BTW, they are both adorable