You don’t tell us how low, but since you say slightly I think its probably nothing to worry about. Especially, since you tell us it is always a little low. Sounds like it is your normal. If it was cancer or going to kill you somehow I think you would probably be dead by now. You certainly can get a second opinion if you’re worried. Just have your lab tests sent to another doctor.
Have you had your vitamin D and thyroid tested? I really think you should have those done the next time you get a blood test if you haven’t. Most likely they did test your thyroid with at minimum doing a TSH. Do you have a copy of your lab work? Are you tan? Or, do you protect your skin from the sun? Muscle weakness is very common related to D deficiciency. I’m not suggesting some “natural” remedy and trying to get you to pop a bunch of vitamins and herbs. I’m telling you to get a blood test to find out. Vitamin B12 would be another good test for you, especially since you have been playing with your diet. I would also say iron, but there is no way your doctor didn’t check your red blood cells, so that I would be willing to assume is normal.
So, to sum up, if you have your lab work you want to check they tested for:
Vitamin D
TSH
Vitamin B12