I am dieting and walking 3 miles a week is it normal to lose 3-5 lbs a week. i have had thyroid checked. or could there be something wrong?

Weight loss: The amount of weight that you are losing with your activity is well within normal but you also must be dieting. You must walk roughly about 35 pounds to burn off the calories found in 1 pound of fat. Exercise alone is not enough to lose that much weight so good luck with your diet and continue to watch your intake as that is really the long-term solution along with exercise on a regular basis.

Yes, it can be.: Depending how little you eat, yes, you can lose that match weight in a week. It is important, though, that you stay hydrated with enough water, that you eat some vegetables every day and that you take a simple multivitamin while you're dieting. Exercise is important, but it's almost all about your food intake. Pay attention when you get close to your goal weight and/or size. Level off slowly.

That is normal : You are fine , if you feel you are happy with your weight now then increase you calories to 1600-1800 kcal a day high protein low carbs and you will maintain the weight if you continue working out. God luck.