Radionuclide Uptake by the Thyroid	

	One dynamic test of thyroid function used in clinical practice is that of radionuclide uptake using 123I, or 99mTc as pertechnetate. The proportion of a known tracer dose of such radionuclides present in the thyroid after a given time can be used as a test of thyroid function. The uptake is inversely related to the average iodine intake of a patient and is reduced by disorders of iodide trapping. Radioiodine uptake is increased in hyperthyroidism and decreased in hypothyroidism, but discrimination between normal and mildly disturbed thyroid function is poor. 123I has a shorter half-life than 131I and permits less exposure to radioactivity; technetium is trapped but not organised by the thyroid and is rarely used to assess uptake.