Abstract 1. 1. Total I 131 in serum was determined, and resin separation of I 131 into inorganic and organic fractions in the serum was carried out. A consecutive daily survey was performed on patients with varying thyroid metabolic activity. Six and 24-hour I 131 thyroid uptake and total 24- and 48-hour urinary excretion tests were performed on all patients studied, in addition to other clinical and laboratory tests of thyroid function. 2. 2. Inspection of the curves from the data obtained showed characteristic slopes of ascent and descent depending on the clinical status of the patient. At any particular time interval, the direction of the curve for the various conditions studied may be different; nevertheless, absolute values can be approximated. From this it is concluded that single determinations of total serum I 131, or protein-bound I 131, may lead to serious misinterpretations of the metabolic thyroid state of the patient under study. 3. 3. In studying the results of daily serial total serum I 131 and conversion ratio curves on a group of patients, we have noted a leveling off after the fourth day following the ingestion of the test dose. This is demonstrated in our curves for euthyroid patients. 4. 4. We have presented 4-day curves for determinations of total serum I 131 and conversion ratios for a group of euthyroid subjects of various ages. These curves represent basic normal criteria for the study of dynamic distribution of I 131 in the blood. They were used as standards for comparison with curves from hypothyroid and hyperthyroid subjects and individual variants. 5. 5. Curves for subjects formerly hyperthyroid, who had been rendered euthyroid after treatment with surgery as well as after treatment with radioactive I 131, have been presented and it was noted that the total serum I 131 curves reverted to the type representing the euthyroid, while the conversion ratio curves continued to resemble their original shape characteristic of hyperthyroidism. This confirms the fact that there is continued increased metabolism of the thyroid gland remnant after both types of treatment. 6. 6. In contrast, curves from groups of subjects remaining hyperthyroid following surgery and after radioactive I 131 treatment, retain the contour of the hyperthyroid patient. 7. 7. Individual curves illustrating partial hypothyroidism following surgery, athyreosis, secondary hypothyroidism as differentiated by the administration of TSH, and the effect of previous iodine administration are all presented and analyzed as illustrative cases. 8. 8. Our experience with the determination of minute amounts of I 127 in the serum are presented. 9. 9. It is concluded that total blood serum I 131 and conversion ratio curves here described and consecutively determined for 4 days allowed evaluation of the status of the individual patients and the metabolic state of their thyroid gland. 10. 10. The curves presented offer a practical and sensitive method for the evaluation of the clinical thyroid status of the individual patient, particularly in obscure cases, or where prior therapy has clouded the clinical status and other laboratory findings.