Linking Thyroid Function, Morphology, Autoimmunity, Body Mass Index, and Reproductive Aging to Women’s Sexual Health: Evidence from a Population Study in Kaunas

<b>Background/Objectives:</b> While it is known that Hashimoto's thyroiditis (HT), goiter, thyroid nodules, and thyroid dysfunction may affect women's reproductive health through hormonal and metabolic mechanisms, data are limited regarding the specific impacts on female sexual function. This study evaluated sexual function in women with thyroid disorders and examined its associations with thyroid function, age, menopausal status, and body mass index (BMI). <b>Methods:</b> A population-based survey was conducted in Kaunas, Lithuania, within the WHO MONICA framework. A random sample of 1569 women aged 25-69 years was included in the final analysis after applying the exclusion criteria. Anthropometric measurements were taken using standardized procedures, and the BMI was calculated. Sexual function was assessed using the 19-item Female Sexual Function Index (FSFI). Thyroid structure was evaluated by a team of trained physicians using ultrasound, while thyroid function was assessed via serum analysis (ELISA-based assays for TSH, fT4, and anti-TPO antibodies). <b>Results:</b> Of the 1569 women analyzed, 64.1% had sexual dysfunction (SD) (FSFI ≤ 26.55). Age and BMI showed significant negative correlations with all FSFI domains, with the strongest associations for arousal, lubrication, and total FSFI score (<i>p</i> < 0.01). SD was more prevalent among postmenopausal (43.6%) women than in premenopausal women (22.6%, <i>p</i> < 0.001) and increased with a higher BMI (<i>p</i> < 0.001). HT was found in 28.3% of participants. Compared with the reference group, women with HT were older, had higher BMI, higher TSH levels, and more hypothyroidism (<i>p</i> < 0.001). SD was more common in the HT group (71.7% vs. 64.2%, <i>p</i> < 0.001), with significantly lower lubrication and higher pain scores. In the multivariate analysis, only goiter remained an independent predictor of SD (<i>p</i> = 0.04). <b>Conclusions:</b> In conclusion, women with HT were older; had a higher BMI; and more frequently experienced SD, particularly reduced lubrication and increased pain, compared with the reference group. Although several thyroid conditions were associated with sexual dysfunction, only goiter remained an independent predictor after adjusting for age and BMI.