Could be a combination of both....I use refried beans for the constipation, and fresh fruit. The back pain may disappear when the constipation does. My dr only checks levels every six to eight weeks, he says it takes that long to level out.....hope this helps.
I can only tell you that I had severe problems with constipation on Synthroid and did everything to help prevent it. I even kept a log of everthing that went into my body to give to my doctor. My regimen included 8 oz prune juice, 2 Tablespoons EV Olive Olive, one whole apple with peeling and 3 colace, water, fiber and exercise. This seemed to do nothing. I changed to Armour thyroid and was feeling better WITHIN HOURS! Not even the medical profession was aware of this side effect. Thank God for forums like these where we can get answers instead of living in misery. God Bless.