Factors Affecting the Estimation of Iodine Entering the Normal Thyroid Gland Using Short-Term Clearance Studies

Experiments on euthyroid subjects demonstrate that the renal clearance rate for radioiodide decreases at night by a factor of about 1.4 compared with the daytime value, whereas the thyroidal clearance rate remains approximately constant. This factor, and losses of iodide in sweat, feces and uncollected urine must be taken into account when the results of short-term radioiodine studies are used to estimate the thyroidal accumulation of stable 127iodide.