Radioiodine Turnover Studies as a Means to Predict Stable Intrathyroidal Iodine Stores and Comments upon Its Use in the Diagnosis and Treatment of Hyperthyroidism

Stable intrathyroidal iodine pool (ITI) is known to affect both diagnosis and treatment of hyperthyroidism. Very few laboratories have facilities to measure ITI. In our department x-ray fluorescence was routinely used for more than 15 years. We report here that it is possible to predict the ITI by means of classic ^l3lI turnover studies and at least distinguish hyperthyroid patients with a small ITI pool ("small pool" patients) from those with a large ITI. It could be shown from a retrospective study (selected hyperthyroid patients, n = 118) that (1) in our area the small pool patients represent the majority as opposed to the situation in the United States, (2) that there was a highly significant negative correlation (p < 0.001) between the PB¹³¹,I at 24 h and ITI, and (3) that the non-small pool patients were more resistant to treatment than the others. In a prospective study of 91 consecutive patients with a thyroid problem, it was be shown that in the euthyroid group no correlation could be found between ITI on the one hand and ^l3lI uptake and PB¹³¹I at 24 h or urinary iodine on the other. In the hyperthyroid patients a strong negative correlation was again found between ITI and PB¹³¹I (p < 0.001), stronger than with ¹³¹I uptake p = 0.093). No correlation existed with urinary iodine. In a second prospective study of hyperthyroid patients (n = 56), it was confirmed that measuring the PB¹³¹I could classify hyperthyroid patients into non-small pool and small pool subjects. It is suggested that the PB¹³¹I at 24 h should be reintroduced routinely in the workup of hyperthyroid patients.