Menopausal transition and thyroid function trajectories: a large-scale longitudinal study of healthy women

CONTEXT: Thyroid dysfunction and menopause both affect cardiovascular risk, but the specific influence of the menopausal transition on thyroid function independent of aging remains unclear. OBJECTIVE: To examine longitudinal changes in thyroid function across the menopausal transition among a large cohort of healthy midlife women. DESIGN: Longitudinal cohort study (2004-2024) using annual health checkup data and interrupted time-series analyses within mixed-effects linear models. SETTING: Annual health screening program at St. Luke's International Hospital, Tokyo, Japan. PARTICIPANTS: 7,644 Japanese women who experienced natural menopause and had at least one health checkup both before and after menopause. Those with hormone therapy or surgical menopause were excluded. MAIN OUTCOME MEASURES: Serum TSH and free thyroxine (fT4) levels. Secondary outcomes included abnormal TSH categories (>4.5, >10.0,