The authors have developed a method to estimate the 24-hour sodium iodide thyroid uptake based on a 5-minute Tc-99m pertechnetate thyroid uptake using the equation: Estimated Iodide Uptake = 17.72*In(Pertechnetate Uptake) + 30.40. This estimation has a correlation coefficient of 0.90. It is based on a data pool of 44 patients who underwent I-131 and Tc-99m studies within 2 weeks of each other from 1978-1988, with established diagnoses as follows: 12 euthyroid, 6 hyperthyroid with multinodular goiters, 15 hyperthyroid with diffuse goiters, 4 with subacute thyroiditis, and 7 unknown. The population consisted of 30 women and 14 men with a mean age of 52.0 +/- 17.5 years; this sample was screened for use of thyroid hormone, propylthiouracil, and radiographic contrast. The authors believe this estimation method is of value whenever a 24-hour iodide uptake is desired, and where speed and minimizing radiation dose are factors. This method is strongly recommended for thyroid uptake evaluation before I-131 therapy.