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Why is iodine-131 useful for measuring thyroid function?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

Iodine-131 is a valuable tool in nuclear medicine for assessing thyroid gland function because of its specific biological behavior. The thyroid naturally concentrates iodine from the blood to synthesize hormones, and it does not distinguish between stable iodine and the radioactive isotope iodine-131.12

When administered to a patient, iodine-131 accumulates in the thyroid, where it acts as a gamma-ray emitter. Clinicians use a scintillation counter to measure the radiation emitted by the gland, allowing them to quantify how much of the isotope has been absorbed and retained. This process, known as the radioactive iodine uptake test, helps doctors evaluate metabolic activity and distinguish between various causes of hyperthyroidism, such as Graves disease or thyroid inflammation. With a half-life of eight days, the isotope provides a sufficient window for these diagnostic measurements.134