Levothyroxine use and the risk of breast cancer: a nation-wide population-based case-control study.

Purpose: To investigate whether the use of levothyroxine was associated with breast cancer risk.

Methods: We conducted a population-based case-control study in Taiwan. Cases consisted of all patients who were aged 20 years and older, and had a first-time diagnosis of breast cancer for the period between 2001 and 2011.