Iodine-131: Half-Life

In the United States, patients treated with ¹³¹I ablation for hyperthyroidism or thyroid carcinoma are sent home with instructions on hygiene and limitations on social interactions. The instructions vary a bit among different centers, but are based on a balance between insurance reimbursement for inpatient isolation, patient comfort, and public safety.

The duration of isolation is usually decided empirically based on the administered dose, ranging from 2-7 days. This is based on the effective half-life of ¹³¹I, which is eliminated mainly through urine, but also in stool.

The physical half-life of ¹³¹I is fixed by nature at approximately 8 days. The effective half-life, however, depends on a number of patient factors. In healthy subjects, the effective half-life for the clearance of ¹³¹I is between 5-7 days. The effective half-life is similar in patients being treated for thyrotoxicosis. In patients with thyroid carcinoma who have been treated with total thyroidectomy, ¹³¹I clears faster because of the absence of significant thyroid tissue. The effective half-life in these patients is between 10 hours - 15 hours.

References

• Greenlee C, Burmeister LA, Butler RS, Edinboro CH, Morrison SM, Milas M; American Thyroid Association Radiation Safety Precautions Survey Task Force. Current safety practices relating to I-131 administration for diseases of the thyroid: a survey of physicians and allied practitioners. Thyroid. 2011 Feb;21(2):151-60.
• Ravichandran R, Binukumar J, Saadi AA. Estimation of effective half life of clearance of radioactive Iodine (I) in patients treated for hyperthyroidism and carcinoma thyroid. Indian J Nucl Med. 2010 Apr;25(2):49-52.