As you have said that your husband had radio imaging of thyroid ( radioiodine), had oyesters .
I think either he has a chinese resturant syndrome due to MSA ingredient in most of their dishes , that cause allergic reaction
, sweating , chest tightness
Or it could be a reaction of MSA with the high level of radioiodine in his body causing the allergic reaction.
If his symptoms of chest tightness , low blood pressure , fainting
is there you need to take him to hospital for adrenaline
shot if not then these symptoms will go away as soon as he defecate or vomit out.
Hope this helps