Recently 123 Iodine (123I) has become available for commercial use in Brazil. It has several advantages over the commonly used 131 Iodine (131I) for thyroidal studies. The aim of this study was to establish the reference values for thyroid 123I uptake at 6 and 24h in a sample of healthy individuals from Porto Alegre, RS. An equation for estimation of iodine uptake at 24h using the values of 6h was also evaluated. Thirty nine healthy volunteers (29 females, 10 males; mean age 34 years) without previous thyroid disease and normal thyroid function tests were submitted to thyroid 123I uptake test as follows: 123I (CNEN - IEN, Rio de Janeiro) 0.037MBq was administered orally and the activity in the thyroid gland was determined after 6 and 24h by counting over the neck during 60 seconds. The uptake was expressed as a percentage of administered dose corrected for extrathyroid (thigh) activity. The 24h 123I uptake was estimated from the equation: 24h % Uptake = 73.2 x Log10 (% Uptake 6h) - 55.7. The mean value at 6h was 15.8% ± 3.3 (95 % C.I 9.4-22.2 %) and at 24h was 24.6 % ± 6.4 (95 % C.I. 12.7 - 36.1 %). Significant correlation between estimated and measured values at 24h was found (r = 0.55, p < 0.01), but with significant individual variation. In summary, the values of reference for our population were defined and we suggest that the thyroid uptake should be done at 6h instead of 24h.
123Iodine; Thyroid uptake; Thyroid function test; Standard
