Treatment of hyperthyroidism with radioactive iodine is associated with increased cancer mortality later in life, including breast cancers, latest findings show.

US researchers extended the follow-up of the Cooperative Thyrotoxicosis Therapy Follow-up Study of more than 35,000 patients with hyperthyroidism enrolled between 1946 and 1964 by a maximum of 68 years.

The initial study found no raised total or site-specific cancer mortality rates through to 1990 in patients treated with radioactive iodine compared with the general population.

However, the latest findings of the radiation dose-response relationships for site-specific cancer deaths among 18,805 radioactive iodine-treated patients with hyperthyroidism found a positive relationship for most cancers at 26 years of follow-up.