title: Enhancement of radioiodine treatment of small-pool hyperthyroidism with antithyroid drugs: kinetics and dosimetry.
abstract: (131)I-Iodide is the treatment of choice in most cases of hyperthyroidism, with a standard 7,000-cGy (rad) thyroid absorbed dose generally resulting in an incidental blood absorbed dose of less than 10 cGy (rad). However, in approximately 15% of patients there is a small, rapidly secreted thyroid iodine pool (small-pool patients) and, based on theoretic calculations, an incidental blood absorbed dose of up to 150 cGy (rad) could result. In such small-pool patients, continuing antithyroid drugs (ATDs) at a reduced dosage during (131)I therapy should inhibit the formation of (131)I-labeled levothyroxine and triiodothyronine and thereby reduce the protein-bound (131)I-iodine concentration in blood and the blood absorbed dose.
  
  
  METHODS
  To test this hypothesis, thyroid and blood time-activity data were measured and absorbed doses were calculated for an (131)I tracer administered to small-pool hyperthyroid patients (n = 9) not receiving ATDs (off ATDs) and then receiving ATDs (on ATDs).
  
  
  RESULTS
  The blood absorbed dose (cGy/37 MBq [rad/mCi] administered) was reduced from 2.54 +/- 0.91 (mean +/- SD) without ATDs to 1.27 +/- 0.54 with ATDs (P < 0.0001), whereas the thyroid absorbed dose was unchanged (1,870 +/- 700 vs. 2,080 +/- 1,080). The blood absorbed dose for an administered (131)I activity required to deliver a standard prescribed absorbed dose of 7,000 cGy (rad) to the thyroid therefore was reduced by over 50% with ATDs, from 11.3 +/- 6.5 to 4.9 +/- 2.8 cGy (rad) (P < 0.001).
  
  
  CONCLUSIONS
  Continued administration of ATDs during (131)I therapy thus can effectively reduce extrathyroid radiation in small-pool patients without significantly reducing the target tissue (i.e., thyroid) dose.
authors.affiliation: Division of Nuclear Medicine, New York-Presbyterian Hospital, New York, USA. zanzonip@mskcc.org
authors.forename: Pat B
authors.initials: PB
authors.lastname: Zanzonico
authors.forename: David V
authors.initials: DV
authors.lastname: Becker
authors.forename: James R
authors.initials: JR
authors.lastname: Hurley
mesh_terms: D013956:Antithyroid Agents / Q000493:pharmacokinetics / Q000627:therapeutic use*; D006801:Humans; D006980:Hyperthyroidism / Q000000981:diagnostic imaging / Q000188:drug therapy*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics / Q000627:therapeutic use*; D011829:Radiation Dosage; D011877:Radionuclide Imaging; D013961:Thyroid Gland / Q000000981:diagnostic imaging / Q000187:drug effects*
chemical_list: D013956:Antithyroid Agents; D007457:Iodine Radioisotopes