[en] Full text: Thyrotoxicosis results from a high level of thyroid hormone in blood. ¹³¹I therapy for this is very safe treatment. Surgery is not acceptable in youngsters because of cosmetic point of view. Objective: In India most of thyrotoxicosis occurs in sea coastal region and hypothyroidism in Himalayan region. The main objective of this study to evaluate, the effect of geographical distribution and chemical pollution on thyroid. To calculate exact dose based on gland size. Materials and Methods: 160 patients of primary hyperthyroidism were selected. Age group range between 15-65 yrs. All patients from Gujarat (India) it is located in western part of India. It's sea coast is approx. 1600 km long. Here Asia's largest chemical zone is situated. Method: 5ml of blood collected from each patient. T3,T4 and TSH test done by RIA and IRMA techniques. After that ^99mTcO₄- Scintigraphy done by gamma camera (GE infinia) 15 days before administration of ¹³¹I all iodine containing food and drugs had been stopped, even iodized salts also. 20 patients got fixed dose of ¹³¹I 10 mci per patient. 140 patients got 120 micro curie per gram of thyroid tissues weight. Follow up study done after 6 months of ¹³¹I administration. Thyroid function test and scintigraphy done to evaluate pre and post therapy changes. Result: 60% of treated patients from sea coastal area, 25% from chemical and 15% from planes. The patients who got fixed dose 10 mci ¹³¹I, of them 35% became hypothyroid and 3% got 2nd dose (13-15 mci) other group who got 120 micro curie ¹³¹I per gram of thyroid tissue of them only 10% became hypothyroid but 5.4% had been treated with 20% more ¹³¹I than primary dose. In the age group of 50-65 yrs on ECG cardiac arrhythmia detected. Conclusion: In treatment of thyrotoxic patients 120 micro curie/gram group shows better result than fixed dose 10 mCi. 60% of treated patients were from sea coastal range, but 25% patients were from chemically polluted zone is guiding us to do mass survey. But in this procedure we need man power, more instruments and financial help