Autoimmune Thyroid Disease and Female Fertility: Does Anti-TPO Accelerate Ovarian Aging?

Background/Objectives: Thyroid autoimmunity, particularly anti-thyroid peroxidase antibodies (anti-TPO), has been implicated in reduced fertility and diminished ovarian reserve. However, the stratified effects of anti-TPO across age groups, body mass index (BMI) categories, and polycystic ovary syndrome (PCOS) status remain unclear. This study aims to investigate the association between anti-TPO positivity and ovarian reserve markers—antral follicle count (AFC), anti-Müllerian hormone (AMH), and follicle-stimulating hormone (FSH)—in euthyroid infertile women. Methods: This retrospective study included 1460 infertile women aged 18–45 years, evaluated between 2022 and 2025. Participants were categorized based on anti-TPO levels (≥9 vs. <9 IU/mL) using Beckman Coulter-DXI 800 analyzer, which uses chemiluminescent immunoassays to measure results. BMI (<30 vs. ≥30 kg/m2), and PCOS status. Age was categorized into five strata (18–25, 25–30, 30–35, 35–40, and 40–55 years), and <35 vs. ≥35 years. Linear regression models were used to assess the impact of anti-TPO on AMH and AFC within each subgroup. Additional logistic regression was performed to evaluate the odds of diminished ovarian reserve (DOR: AMH < 1 ng/mL or AFC < 5) after adjusting for age, BMI, and TSH. Results: Anti-TPO positivity (17.6% prevalence) was significantly associated with reduced AMH (1.47 ± 1.52 vs. 3.33 ± 3.03 ng/mL, p < 0.0001), reduced AFC (8.18 ± 5.06 vs. 15.88 ± 8.18, p < 0.0001), and elevated FSH (9.40 ± 6.21 vs. 8.06 ± 4.79 mIU/mL, p = 0.001). These associations remained significant in non-obese and PCOS-negative subgroups. Regression models revealed stronger associations in younger women (<35 years) and showed significant Anti-TPO × Age and Anti-TPO × BMI interactions. Logistic regression confirmed Anti-TPO ≥ 9 IU/mL as a strong predictor of diminished ovarian reserve (AMH < 1 ng/mL: OR = 3.13; AFC < 5: OR = 6.48). ROC analysis indicated modest predictive ability (AUC: 0.665–0.694), and path modeling confirmed direct effects of Anti-TPO on AMH and AFC independent of TSH or BMI. Conclusions: Elevated Anti-TPO levels are independently associated with diminished ovarian reserve in euthyroid women, particularly in younger, non-obese, and PCOS-negative individuals. Anti-TPO may serve as a useful biomarker in fertility risk assessment and personalized reproductive counseling, even in the absence of overt thyroid dysfunction.