Longer exposure to hormone replacement therapy and longer reproductive lifespan showed potential associations with increased thyroid cancer risk in women, according to findings presented at ENDO 2026 (Abstract ORF12-08).
“These results show how common life events in women, such as the timing of menopause and use of hormone therapy, may influence cancer risk,” said Jinyoung Kim, MD, PhD, Assistant Professor in the College of Medicine at The Catholic University of Korea. “A woman’s reproductive history could be considered in assessing her individual risk to developing thyroid cancer.”
Background and Study Methods
The greater proportion of thyroid cancer incidence in women than men has suggested a potential role of reproductive and hormonal factors in thyroid carcinogenesis that has yet to be backed up by consistent evidence.
Researchers in Korea conducted a population-based retrospective cohort study of women aged 40 years or older who participated in the national breast and cervical cancer screening programs between 2010 and 2011 (n = 5,659,432). They then searched for incident thyroid cancers from Korean National Health Insurance Service claims data. Patients were followed until thyroid cancer diagnosis, death, or the end of follow-up in 2023.
The study authors reviewed associations between reproductive factors and the incidence of thyroid cancer with Cox proportional hazard regression models and adjustments for age, smoking status, alcohol consumption, regular physical activity, body mass index, hypertension, diabetes mellitus, dyslipidemia, history of cancer, and household income levels.
Key Findings
The total incidence of thyroid cancer in the retrospective cohort study was 2.4 per 1,000 person-years.
Those with a longer reproductive span had an increased risk of thyroid cancer, which progressively increased, from an adjusted hazard ratio (aHR) of 1.09 (95% confidence interval [CI] = 1.06–1.12) for a reproductive span of ≥ 30 years, 1.19 (95% CI = 1.16–1.22) for ≥ 35 years, and 1.30 (95% CI = 1.25–1.35) for ≥ 40 years.
An increased risk of thyroid cancer was also seen in women who used hormone replacement therapy, with a greater association noted in women who received it for at least 5 years (HR = 1.25; 95% CI = 1.19–1.31).
A history of lactation was associated with a reduced risk, but parity did not show a clear association with thyroid cancer risk.
The researchers suggested that cumulative endogenous hormonal exposure had an impact on thyroid carcinogenesis, which could explain the predominance of female thyroid cancer diagnoses.
DISCLOSURES: This research was supported by the Basic Science Research Program through a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education. For full disclosures of the study authors, visit endo2026.endocrine.org.
