Acute changes in thyroid function in patients treated with radioactive iodine.

Serum triiodothyronine (T3) and thyroxine (T4) were measured in thirteen unselected hyperthyroid patients before and at frequent intervals after iodine-131 treatment. There was an abrupt rise in T3 from a pretreatment mean of 250 ng/100 ml to a maximum of 365 ng/100 ml at 24 hours. Total T4 rose from a mean of 18.7 mug/100 ml to a maximum of 22.3 mug/100 ml at 24-48 hours. These findings may explain why an occasional patient develops acute thyroid crisis after radioiodine treatment.