Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help for trouble breathing, severe difficulty swallowing, fainting, confusion, chest pain, or a very fast or irregular heartbeat with fever or severe agitation.
Contact the nuclear medicine team promptly if you vomit soon after taking the dose.
The vomit may contain radioactive material, so do not clean it up until you have received safety instructions.
Also arrange routine review if you later notice ongoing tiredness, feeling unusually cold, constipation or other possible signs of an underactive thyroid.
The article also says
Call your healthcare team for persistent vomiting, worsening neck swelling, significant salivary gland pain, new eye symptoms or a dry mouth that does not improve.
Every sign above is quoted from the sofidex article on Radioiodine therapy. If you are not sure, err upward — it is always reasonable to be checked.