Female Reproductive System and Thyroid Dysfunction: Findings from a 12-Year Follow-Up in the Tehran Thyroid Study

<b><i>Background:</i></b> The impact of thyroid dysfunction (TD) on the female reproductive system has been extensively documented. While there is evidence suggesting that alteration in female reproductive status may affect thyroid function, conflicting results have prevented definitive conclusions. This study aimed to investigate the associations of parity, spontaneous abortion (mentioned as abortion throughout this study), and menopause status with the prevalence and incidence of TD. <b><i>Methods:</i></b> From the Tehran thyroid study population, 2711 participants were included in the cross-sectional analysis to explore associations between female reproductive status and TD. Overall, 2191 participants with euthyroid were included in the survival study and followed up in 3-year intervals. Multinomial logistic regression was adopted in cross-sectional analysis and multivariable Cox proportional hazard model was used to determine associations between the incidence of TD with parity, abortion, and menopause status, adjusting for age, smoking, body mass index, and thyroid peroxidase antibodies positivity. <b><i>Results:</i></b> At the baseline, multiple parities (≥4) were significantly associated with overt hypothyroidism (odds ratio [OR] = 1.12; confidence interval [CI] 1.0-1.26) and subclinical hyperthyroidism (OR = 1.11 [CI 1.03-1.21]). Furthermore, multiple abortions were associated with overt hyperthyroidism (OR = 2.09 [CI 1.02-4.26]). Over the course of the study, multiple parities were significantly associated with the incident subclinical and clinical hypothyroidism. Conversely, a history of abortion was associated with a reduced risk of incident overt hypothyroidism. We found no significant association between menopause status and the prevalence or incidence of either hypothyroidism or hyperthyroidism. <b><i>Conclusions:</i></b> Our results suggest that the female reproductive system may be associated with thyroid function. Parity and abortion are associated with the occurrence of TD. A deeper understanding of the underlying mechanisms of the cellular and molecular alterations in signaling cascades during pregnancy is necessary to fully elucidate these associations.