2. To obtain sequential biochemical and kinetic data on the behafrior of therapeutic Isl, for 3 or 4 weeks in selected cases of hyperthyroidism. 3. With the above extensive data (not commonly available with routine treatment) in hand, to follow the clinical results in order to determine the causes for failure of therapy, for overtreatment and for the development of nodularity; and, in addition, to find out what pretreatment characteristics will alter the reterfrion and therapeuti: behavior of the dose to determine the need for adjustments df the therapeutic dose upward or downward. 4. To study in detail (functionally as well as morphologically) [any irradiated thyr tissue that becomes available from whatever the source may be: 1$1 I therapy, x-ray therapy to the head or neck, or exposure to fallout in the case of the Marshall islar The objectives of the animal research were concerned primarily with the producti of neoplasms with 131 I in rats: 131 1. To develop a model for readily producing thyroid tumors solefy with "I. 2. To use the model to study the circumstances that lead to neoplasms in the ir- radiated gland. 3. To study the development of neoplasms from their earliest irnfeption and to see how different histologic patterns evolve. This may be seen in rat thyroids of certa age destined to develop neoplasms at a given time after irradiatjon. 4. To determine whether a hypothyroid state and/or TSH play a fart in the inductic of such neoplasms by using radioimmmoassay for TSH. S. To see whether the predictable development of neoplasms may|be inhibited. 6. To appraise the nature of the failure to produce neoplasms beter somewhat large doses than the "critical'' dose which produces such neoplasms but} causes no obvious evidence of histologic damage.