Frequencies of thyroid-related diagnoses and dyslipidemia among Japanese women with menstrual-related disorders aged under 40 years: A nationwide cross-sectional study using health insurance claims data

Women with endometriosis and dysmenorrhea are increasingly recognized as having systemic comorbidities beyond pelvic disease, but large-scale data on endocrine and metabolic comorbidities across a broad menstrual-related disorders population remain limited. Using the Japan Medical Data Center Claims Database, we conducted a nationwide cross-sectional study of women aged under 40 years enrolled in 2023. Women with confirmed diagnoses of endometriosis (ICD-10: N80) and/or dysmenorrhea (N94.4 or N94.6) were classified as the menstrual-related disorders group (n = 257,897); those without such diagnoses were the control group (n = 3,183,032). The primary outcomes were the frequencies of thyroid-related diagnoses (hypothyroidism [E03], non-toxic goiter [E04], hyperthyroidism [E05], thyroiditis [E06], and other thyroid disorders [E07]) and dyslipidemia (E78). Stratified analyses were performed according to prescription for low-dose estrogen-progestin preparations (LEP) or dienogest (DNG). All thyroid-related diagnoses were recorded more frequently among women with menstrual-related disorders than among controls. Hypothyroidism was identified in 2.1% of women with menstrual-related disorders versus 0.8% of controls (prevalence ratio [PR] 2.46; 95% CI 2.39-2.54); the corresponding PR was 3.11 (95% CI 2.99-3.24) in the endometriosis subgroup (n = 96,863). Dyslipidemia was recorded in 5.2% of women with menstrual-related disorders versus 1.4% of controls (PR 3.73; 95% CI 3.66-3.80; prevalence difference +3.78 percentage points). The endometriosis subgroup consistently showed the highest frequencies. These associations remained statistically significant after adjustment for age using the Mantel-Haenszel method (adjusted PR 1.45-3.02 across all comparisons), although adjusted estimates were consistently lower than crude estimates. Among women with menstrual-related disorders, hypothyroidism was more frequent in those without LEP/DNG prescriptions (3.4%) than in those with prescriptions (1.5%), whereas dyslipidemia frequencies were similar (5.2% vs. 5.1%). These findings indicate that young Japanese women with menstrual-related disorders have substantially higher frequencies of thyroid-related diagnoses and dyslipidemia than controls, particularly those with endometriosis. Surveillance bias, age confounding, and confounding by indication cannot be excluded because of the cross-sectional claims-based design.