Iodine-131: how long does a patient stay “radioactive” after treatment?

Differentiated thyroid cancer generally has a good prognosis. After surgery, some patients receive iodine-131 treatment: a capsule or a solution to swallow, whose radioactive iodine binds to the remaining thyroid cells and destroys them. This is radioiodine therapy.

For a few days, the patient emits radiation. They are therefore admitted to a shielded room and discharged only once the dose rate measured one metre away falls below a regulatory threshold. At Mohamed Lamine Debaghine University Hospital (Bab El Oued) in Algiers, that threshold is 25 µSv/h.

The practical question is simple: how fast does this radiation decrease? It is the subject of my master’s dissertation in biophysics and radiation protection, defended on 25 September 2026 at the University of Abomey-Calavi.

What I studied

I analysed the records of 113 patients treated in the first quarter of 2026 in the nuclear medicine department of Bab El Oued University Hospital. The work had two parts: comparing the department’s radiation protection practices with the recommendations of the International Atomic Energy Agency (IAEA), and describing how each patient’s external dose rate decreased, measured several times between administration and discharge.

From these measurements, one calculates the effective half-life: the time it takes for the dose rate to halve. It combines two processes: the physical decay of iodine-131, whose half-life is about 8 days, and elimination by the body, mainly in urine, which is much faster.

What the measurements show

One hour after administration, the dose rate is proportional to the administered activity: about 0.73 µSv/h per millicurie, from 30 to 200 mCi. This consistency confirms the reliability of the measurement chain: dose calibrator, survey meter and patient positioning.

The median effective half-life is 13.2 hours. In patients who received 100 or 150 mCi, who were followed for longer, it is around 14 hours. This is close to what European teams report, for example 12.6 hours in a Finnish series published in 2024.

Variability between patients is large, and no factor available in routine practice really predicts it. Age has no effect of its own. The apparent differences by administered activity or histological type mainly reflect measurement conditions, which differ between low and high activities. The most likely factor, kidney function, was not measured systematically: it is the next question to study.

Finally, in high-activity patients, two well-timed measurements estimate the effective half-life as precisely as three, to within 0.1 hour. This supports a lighter monitoring protocol with no loss of safety.

What this means for patients and those close to them

The length of stay depends on the administered activity: a few hours for the lowest, about two days for the highest. At discharge, the patient still emits a low level of radiation, which keeps decreasing day after day.

The instructions given at discharge therefore still matter: keeping some distance from others for a few days, avoiding prolonged contact with young children and pregnant women, drinking plenty, urinating often and taking care with toilet hygiene, since unbound iodine is mainly eliminated in urine. Their exact duration is set by the department according to the activity received, and that is the one to follow.

These instructions are not excessive caution. They keep the dose received by those around the patient as low as reasonably achievable, which is the very principle of radiation protection.

Why these data matter in Africa

Published data on the effective half-life of iodine-131 come mostly from Europe, North America and Asia; African series remain rare. This one can help departments that are opening or expanding radioiodine therapy, particularly in West Africa, to plan admissions and train staff.

This dissertation was supervised by Prof. Assia Amimour, head of the nuclear medicine department at Bab El Oued University Hospital, with co-supervision by Prof. Kuassi Marcellin Amoussou-Guenou, Dr Abdelkrim Talbi and Dr Gilles David Houndétoungan. It was awarded the grade Excellent.