Aperson feels tired, gains weight, and notices changes in metabolism. which gland could be the cause?

The thyroid gland could be the cause of all those symptoms, though there are other glands that could cause a few of these symptoms

Unlike  ct scans, which use x-rays,  mri scans  use powerful magnetic fields and radio frequency pulses to produce detailed pictures of organs, soft tissues, bone and other internal body structures.  differences between  normal and abnormal tissue is often clearer on an  mri  image than a  ct.you can see more with a mri scan. for example on an mri you can see internal bleeding.