A longer reproductive span and prolonged use of hormone replacement therapy may be linked to a higher risk of thyroid cancer, according to a large-scale study presented at ENDO 2026 in Chicago.
New research indicates that longer lifetime exposure to female hormones could increase the risk of thyroid cancer.
Researchers from the College of Medicine at The Catholic University of Korea analyzed data from approximately 5.7 million women aged 40 or older, drawn from the Korean National Health Insurance Service database. All participants had undergone breast and cervical cancer screening between 2010 and 2011 and provided detailed reproductive health information. The study followed these women until a thyroid cancer diagnosis, death, or the end of the follow-up period in 2023.
Using Cox proportional hazards models adjusted for age, smoking, alcohol use, physical activity, BMI, hypertension, diabetes, dyslipidemia, cancer history, and income, the team found that a longer reproductive span—the years between a woman's first menstrual period and menopause—was associated with a progressively increased risk of thyroid cancer.
Hormone replacement therapy (HRT) use, particularly for five years or more, was also linked to a higher risk. The overall incidence rate was approximately 2.4 per 1,000 person-years.
"A woman's reproductive history could help assess individual thyroid cancer risk," stated Dr. Jinyoung Kim, assistant professor at the Catholic University of Korea.