COTI (collar therapy indicator) has been recently introduced for the detection of gamma rays with emphasis on thyroid investigations. The aim of this study was to test the feasibility of a prototype version of COTI including activity detectors with low sensitivity in performing thyroid uptake measurements for a large group of patients. Consequently, thyroid uptake tests were carried out for a total of 89 patients (22 males and 67 females; age: 44 +/- 13 years) with thyroid cancer (n = 74), hyperthyroidism (n = 16) at 2 and 24 h after administration of 0.44-2 MBq of I-131. Eight individuals among the thyroid cancer patients were monitored up to 96 h after administration. The COTI device was equipped with two CsI (Tl) detectors, known as LoHi type, sensitive to activity ranges from 0.02 to 30 MBq of I-131. The uptake values from COTI were compared with those measured with a standard probe. It was found that the mean uptake of thyroid activity in thyroid cancer patients was 2.1 +/- 1.3% at 2 h when measured with the standard probe, while it was 2.2 +/- 1.2% when measured with COTI. In addition, the average uptake at 24 h after administration was 2.5 +/- 3.2% and 3.2 +/- 3.8% measured with COTI and the standard probe, respectively. A strong correlation was found at 24 h between the results obtained with COTI and the standard probe, while a weaker correlation was seen at 2 h. Overall, there was no significant difference between the results obtained with the standard probe and those obtained with COTI at both 2 and 24 h (P-value >= 0.05). Besides, 85% of the uptake values measured with COTI were less than those measured with the standard probe at the 24 h after administration. The average uptake value was 0.9 +/- 0.8% after 96 h by COTI, and 1.4 +/- 1.3% by the standard probe. Pertaining to the hyperthyroidism patients, COTI showed mean uptake values of 20 +/- 16% and 23 +/- 18% at 2 and 24 h, respectively. In contrast, the standard probe suggested higher mean uptake values of 26 +/- 18% and 30 +/- 22%, respectively. It is concluded that the prototype of COTI used in the present study has been proved to be a feasible and promising tool in thyroid investigations. It is noted, however, that the next COTI generation should include detectors equipped with collimator and energy discrimination.