Name: Radioactive Iodine
Class: Antithyroid Drug
Mechanism: Trapped by thyroid. b particles cause highly localized tissue destruction. X-rays are diagnostically useful. I131 generally used due to short t½ (8 d).
Absorption: 90% ends up in thyroid or urine. 10% absorbed by the rest of the organ systems.
Dist.: Metabolism.:
Excretion, t½: Urine. 8 d.
Toxicity/S.E.s: High incidence of hypothyroidism (5-10% in 1-2 yr., eventually 50%). May require multiple exposures—up to a year (thioamides usu. used to cover hyperthyroidism in the interim). C/i in pregnant E and kids.
Utility: Treat hyperthyroidism w/o surgery. Indicated for old folk (esp. w/heart disease), when subtotal thyroidectomy or thioamides have not worked, and especially for some metastatic thyroid cancers where cells continue to take up iodide and respond to TSH. Used to evaluate thyroid fxn (e.g., hypo/hyperthyroidism, goiter types, response to TSH/TRF.).
Special Features: Min. dose = 80-150 mCi/g thyroid or 4-10 mCi total. Estimate w/tracer dose to determine uptake.