The first thing you need to do is check all your meds even over the counter things and see if any have the side effect of diarrhea. Vitamins will cause this like vitamin a c e and the magnesium in them. If you take an antiacid lots of them also have magnesium and this will cause you more diarrhea. I think you said you take thyroid meds and you have to take the thyroid one hour before you take calcium or 4 hours after.
Other meds too may be affected by the calcium and it will soak them up if you take them together and you will not get the benefits from them in that case. You need to check with the pharmacy or your doctor on that.
Then stop whatever you do not have to take with the doctors permission then give the calcium a try. Start with 1/2 tablet with each of your 3 daily meals at least 4 or 5 hours apart. Then you can adjust up or down according to if you are getting some relief or still having loose BM's. Always take the calcium with food.
I take synthroid also so I take that as soon as I get up and by the time I get ready for work it has the hour it need to get into my system and then I eat breakfast with my first calcium dose.
Email me if you need help I would be happy to assist you or anyone wanting to give this a try.