It was probably radioactive iodine and it was given for hyperthyroidism, not hypo. Radioactive iodine (RAI) is used for someone who has hyperthyroidism to "kill" the thyroid so it no longer produces thyroid hormones.
When RAI is first given, the person must stay away from others for a specified period of time, but once that time passes, so does the danger.
Could you explain why you're asking about this now?