The risk of developing gynaecologic cancer is lower in women with hyperthyroidism than in those without it, a study from Taiwan has found.

Thyroid disease and altered thyroid hormone expression can affect ovulation, endometrial physiology and oestrogen levels, but studies of the association with gynaecologic cancer risk have conflicting results.

This population-based cohort study used data from the Taiwan National Health Insurance Research Database to identify women (mean age, 44 years) who were diagnosed with thyroid disease between January 2000 and December 2018.

Propensity scores were used to pair 296,872 women with hyperthyroidism and 44,852 with hypothyroidism in a 1:1 ratio with an equal number of individuals without thyroid disorders.

The cohort was followed up from the date of first diagnosis of hypothyroidism or hyperthyroidism until the diagnosis of gynaecologic cancers (endometrial cancer, uterine corpus cancer, and ovarian cancer), death, or the end of 2018.

Women with hyperthyroidism had a lower risk for all gynaecologic cancers than those without hyperthyroidism (adjusted hazard ratio [aHR], 0.86; P = .0084).

The risk of developing gynaecologic cancer was lower among women with hyperthyroidism aged 20-40 years (aHR, 0.72; P = .0043) but not among those aged > 40 years.

The reduced risk for gynaecologic cancers associated with hyperthyroidism persisted even beyond 6 years of follow-up (aHR, 0.75; P < .001).

A trend toward a slightly increased risk for gynaecologic cancer was observed among women with hypothyroidism; however, the association was not statistically significant.

The study was led by John Hang Leung from the Ditmanson Medical Foundation Chia-Yi Christian Hospital, Chiayi, Taiwan, and published online in Scientific Reports.