Previous studies have demonstrated an inverse association between subclinical hypothyroidism and osteoporosis (OP) risk in postmenopausal women. However, the precise effects of subclinical hypothyroidism on bone mineral density and its specific sites of action remain inconclusive.

To study the relationship between thyroid function indicators and bone mineral density at different sites in premenopausal and postmenopausal patients with subclinical hypothyroidism, and to explore predictive factors for osteoporosis.

A study was conducted on 591 women aged 40-60 years with subclinical hypothyroidism who underwent physical examinations at the Health Management Center of Guizhou Medical University Affiliated Hospital between 2023 and 2024. Participants were categorized by menopausal status into premenopausal women (n=189) and postmenopausal women (n=402). Serum levels of free triiodothyronine (FT₃), free thyroxine (FT₄), thyroid-stimulating hormone (TSH), aspartate aminotransferase (AST), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C). Bone mineral density (BMD) was measured at the lumbar spine, femoral neck and total hip in both groups. Pearson correlation analysis and multiple linear regression analysis were employed to investigate the correlation between thyroid indicators and bone density at various sites. Receiver operating characteristic (ROC) curves were plotted to explore the predictive value of FT₃ for osteoporosis in postmenopausal women.

Pearson correlation analysis revealed that FT₃, FT₄, AST and age were negatively correlated with lumbar spine bone mineral density (P<0.05); age, FT₄, AST, TC and LDL-C were negatively correlated with femoral neck bone mineral density (P<0.05); age, FT₄ and HDL-C were negatively correlated with total hip bone mineral density (P<0.05); height, body weight and BMI were positively correlated with lumbar spine, femoral neck and total hip bone mineral density (P<0.05). Multivariate linear regression analysis revealed that FT₃ (β=-0.120, P<0.05), body weight (β=0.267, P<0.05), and age (β=-0.401, P<0.05) were significant predictors of lumbar spine bone density in female examinees overall; FT₃ (β=-0.161, P<0.05), body weight (β=0.201, P<0.05), and age (β=-0.235, P<0.05) were factors influencing lumbar spine bone density in postmenopausal women; FT₃ was not associated with femoral neck or total hip bone mineral density in premenopausal women, postmenopausal women, or the overall female cohort (P>0.05); FT₄ was not associated with bone mineral density at any site in premenopausal women, postmenopausal women, or the overall female cohort (P>0.05). ROC curve analysis revealed that FT₃ predicted lumbar osteoporosis in postmenopausal women with an area under the curve (AUC) of 0.609 (95%CI=0.523-0.696), sensitivity of 69.2%, specificity of 51.0%, and optimal cutoff value of 4.605 pmol/L.

FT₃ is a significant factor influencing lumbar spine bone mineral density in postmenopausal women with subclinical hypothyroidism, with subclinical hypothyroidism. A level of 4.605 pmol/L represents the optimal cutoff value for FT₃ in predicting lumbar osteoporosis in this population, providing a reference indicator for preliminary screening of lumbar OP risk.