Wa'alaikum assalam! Honestly macam rare sikit kat baby, so I'd really suggest you check with the paediatrician or endocrinologist first, they can properly assess baby's condition & advise on diet, sebab depends on what's causing it & how severe. A couple of my mum friends ada kids with thyroid issues & their doctors mostly focused on medication compliance & regular monitoring rather than drastic diet changes for the baby, tapi setiap case lain. If you're breastfeeding, the doctor will guide you too on whether you need to adjust anything on your end. Hang in there, thyroid conditions in babies are usually very manageable once diagnosed!