Friday, January 27, 2012

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.


  • 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.

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