A CORRELATION BETWEEN THE SERUM PROTEINBOUND IODINE AND THE RADIOIODINE CONVERSION RATIO IN VARIOUS THYROID STATES*

Abstract

RECENTLY several tests using radioactive iodine as an indicator of the state of thyroid function have been developed. One of these utilizes a determination of the total plasma radioactive iodine and the proteinbound radioiodine twenty-four hours after the oral administration of an aqueous inorganic I¹³¹ solution (1, 2). A ratio between these two values had been found to be a useful clinical aid. It is termed the conversion ratio and has been thought to be more or less directly related to the rate at which the thyroid gland utilizes the circulating inorganic iodine of the plasma to form plasma protein-bound iodine. Regardless of how direct the relation may be, it is not a simple one. For example, the amount of inorganic iodine in the body is a factor and patients having received iodine solutions, such as Lugol's, may have a low conversion ratio. Other drugs which influence the activity of the thyroid gland will also exert an effect on this ratio. The serum or plasma protein-bound iodine as determined chemically has been used successfully to differentiate various thyroid states and apparently correlates well with thyroid activity. In an effort to evaluate the conversion ratio, in terms other than clinical impression and basal metabolic rates, both the conversion ratio and the serum protein-bound iodine have been determined in a series of patients. The series includes most levels of thyroid activity and is not restricted to those subjects who fall distinctly into one classification, such as hyperthyroidism or euthyroidism.