Multivitamins may help you with nutrition, but if you have thyroid issues, they will not cure or deminish its weight loss or other Symptoms, You should be tested for Hyperthyroid and Hypothyroid, just to eliminate these issues as reasons for weight issues, Some thyroid sufferers bounce back and forth between Hyper and Hypo, so to be on the safe side, get tested for both, For trouble gaining weight, that would be Hyper, and you should get thyroid hormone levels, Free T3, Free T4 and TSH tests done, for autoimmune Thyroid disease with Hypothyroid, Tgab and TPOab, This would be for Hashimoto's Disease,
For Hyperthyroid you would need a TSI test for Graves Disease.
Free T3 being the most important thyroid hormone test, many MD's fail to do this test and you should insist on it. When your test results are in, request a copy, and post them here with all levels and reference ranges provided for each. Ranges vary from lab to lab, so the ranges must be included, then members can explain them to you and answer any other questions you may have. Best Wishes FTB4
Your symptoms seem to be related to hyperthyroidism, so you should certainly have the TSH, Free T3, Free T4 and TSI antibody tests.
You should also get tested for diabetes, as this, too is a common cause for weight loss..
If you have either Graves Disease or Diabetes, multivitamins will not help you gain weight.