BUTANOL-EXTRACTABLE IODINE OF SERUM 1

It has been demonstrated chemically that the io-dine bound to proteins of the serum is predomi-nantly composed of thyroxine (1-4). The concen-tration of this protein-bound or serum precipitable iodine (SPI) appears to be closely correlated with the activity of the thyroid gland (5-7). It is prob-ably the most precise clinical criterion of such ac-tivity that has yet been extensively employed. It is, therefore, generally conceded that SPI ordinar-ily represents circulating thyroid hormone. In studies of medically treated hyperthyroid patients, however, a number of values for SPI proved un-expectedly high in comparison with other meas-ures of thyroid activity (5, 7). Danowski and associates (8-10) reported that SPI of normal in