Please go to the patient to patient forum. Look for Maggiepieannie. Or just put 'Grandbaby born without thyroid" in your title. Maggie has a daughter born without a thyroid. SHe has lots of great info to share.
Replacement with thyroid hormone (levothyroxine) supplies all that is needed from a thyroid standpoint. This needs to be monitored with labs at least 1-2 x / year. Would consult with a pediatric endocrinologist to evaluate the child to ensure there are no other considerations.