Please post your latest thyroid blood tests, with reference ranges. We often see people who don't convert FT4 to FT3, well, and addition of a T3 med can help a lot.
I'm on Tirosint (T4 med), with an additional T3 med added. My T3 med is generic (cytomel is name brand) and it does well for me. Not everyone is as lucky as gimel, where the weight falls off, doing nothing. I have to work at it and sometimes, even then, have a hard time, but I'm still working on getting my FT3 up, too.
One med (T4) might work to raise your Free T3 and Free T4 high enough to relieve your symptoms, but only if your body adequately converts the T4 to T3. Frequently that is not the case and a hypo patient keeps raising med dosage, but symptoms don't go away. I was like that for well over 25 years until learning on this Forum about the importance of Free T3 and after testing and confirming mine as low in the range, my doctor switched my meds to Armour Thyroid. Now after a few bumps in the road and some tweaking of the dosage, my FT3 is 3.9 and FT4 is .84, and I feel best ever.
Also, about a year or so ago, I messed up on my meds and got back into being hypo again while waiting for Armour to be available again. During that time I gained about 2-3 pounds a week, for a total of 19. As soon as Armour was available again I switched back. Over the last 6 months or so, I lost 18 pounds without doing a thing different regarding eating and exercise. Not saying that everyone will have the same result, but at least getting your metabolism to a level that is normal for your height, weight and age at least gives you the opportunity to lose and maintain, that you don't have when hypo.
Adding cytomel may help...but you need labs on your Free T3 and Free T4 to help see if you are on the right dosage of thyroid medication.
Getting your thyroid balanced does help with weigh gain, but losing what you already grained can sometimes need a little more work with exercise etc.
mia