What are you feeding her?? What makes you feel she is thin?? How can you say your dog is weak?? If you can see her ribs, it means the dog is weak and malnutrition-ed. In Labradors, if dogs are malnutrition ed, they are likely to have skin problems as well- they may easily get skin-infections. They may also have dull skin .
If your dog is eating well and is playful, you don't really have to worry about it. If you can't see her ribs, she is perfectly healthy.
You may have seen Labrador which looks heavier and you may wonder why YOUR dog looks so weak? The fact is those Labradors are themselves very unhealthy and they are becoming Obese. Labradors are working animal and require a lot of exercise in form of running and daily walking. If not given this mental and physical stimulation, they become obese and get succumbed to Arthritis and hips dyspepsia.
If you can see the ribs of your pet, it means she is weak. In that case you need to start feeding her Multivitamin and calcium supplement. I'm not sure what you feed her. But you can feed her variety of things like Curd, Chicken breast, rice, eggs, sweet potatoes, Paneer, etc. If you want to feed commercial diet, you can feed Royal Canine, Science Plan, Taste Of wild. RC is readily available and is good.
It will best for you if you could go for Vet check up as well.