Smith SM, Zwart SR, McMonigal KA, Huntoon CL. Thyroid status of Space Shuttle crewmembers: effects of iodine removal. Aviat Space Environ Med 2011; 82:49–51. Introduction: Iodine is often used for water purification and has been used throughout the U.S. space program. Because of concern about potential effects on crewmembers' thyroid function, in 1997 a system was implemented on board the Space Shuttles to remove iodine from water before it was consumed. We report here thyroid hormone data from crews flying before and after this system was implemented. Methods: Blood samples were collected and analyzed for thyroid hormone content during routine medical exams before and after Space Shuttle missions. Data are reported for 224 male and 49 female astronauts (about two-thirds of them before implementation of iodine removal). Results: Serum concentrations of total thyroxine (T4) and the free T4 index were elevated in men after flight and triiodothyronine (T3) was lower after flight, regardless of iodine removal status. T4 was higher, even before flight, in the group of men who flew after iodine removal was implemented. Conversely, T3 was lower in men who flew during that period. Before iodine removal was implemented, thyroid stimulating hormone (TSH) was elevated in male and tended to be elevated in female astronauts, with average increases of 27% and 19% after flight, respectively. After iodine removal was implemented, postflight TSH was not significantly different from preflight values. Discussion: These data provide evidence that crewmembers' increase in serum TSH on landing day after early Shuttle flights resulted from their consumption of iodinated water during spaceflight, because the same increase was not observed after implementation of the iodine removal system.