When you say that a mass was found, & they've told you that you have hypothyroidism, I am assuming that the "mass" is on your thyroid. If this is the case, have they told you that they are nodules? I have several nodules on both sides of my thyroid. I've never taken Lithium, so I can't say what that link could be. Have you had sonograms of the thyroid? The sonogram tells your dr. how large each nodule is. For example, if the nodule is a 2.0 or larger, the endocrinologist may want to biopsy the nodule. I've had that done, and it is an uncomfortable procedure, but a relief to find out that it is non-cancerous. If you are put on synthroid or levoxyl, it is to bring your hormone level to where it should be, and it should also serve as a way to shrink the nodule(s). I hope this has made some sense to you. Good luck!